Calibration and validation of a carbon oxidation system and determination of the bicarbonate retention factor and the dietary phenylalanine requirement, in the presence of excess tyrosine, of adult, female, mixed-breed dogs,

Carbon oxidation methods have been used as rapid and sensitive methods to determine whole-body AA requirements in multiple species. The objectives of the current studies were to validate complete CO recovery, determine the bicarbonate retention factor, and estimate the Phe requirement, in the presence of excess Tyr, in adult dogs using the direct oxidation technique. In this series of studies, 2 oxidation chambers were constructed and calibrated to ensure accurate collection of breath CO. First, 104.6 ± 7.1% CO was recovered from chambers and suggests that the chambers were appropriately designed for complete and efficient CO recovery. Second, we determined bicarbonate retention in 5 dogs using repeated oral dosing of a bicarbonate tracer (NaHCO) with small meals. At isotopic and physiological steady state, 102.5 ± 2.6% of the delivered NaHCO was recovered in breath. Third, the Phe requirement, when Tyr was supplied in excess, was determined by the rate of appearance of CO in the breath (CO). Dogs ( = 5) were fed test diets with different concentrations of Phe ranging from deficient to excessive for 2 d prior to conducting the tracer studies. The mean Phe requirement (when Tyr was supplied in excess) was 0.535% of diet (upper 95% confidence interval = 0.645% diet) on an as-fed basis or 0.575% of diet (upper 95% confidence interval = 0.694% of diet) on a DM basis and was based on a calculated (modified Atwater calculation) dietary ME density of 3.73 Mcal/kg DM. These data support the use of carbon oxidation methods and oral dosing of isotope to measure whole-body requirements of indispensable AA in adult dogs and suggest the current recommendations may be low.

Isoleucine requirement of pregnant sows

The objective of this study was to determine the Ile requirement in early (d 39 to 61) and late (d 89 to 109) pregnancy using the indicator AA oxidation method. The same 7 Large White × Landrace sows in their fourth parity were used in early and late pregnancy. Each sow received 6 diets based on corn, corn starch, and sugar in both early and late pregnancy at constant feed allowances (2.5 kg/d). Diets provided Ile at 20, 40, 60, 80, 100, and 120% of the Ile requirement (6.2 g/d based on the 1998 NRC) in early and 60, 80, 100, 140, 160, and 180% in late pregnancy. After determination of (13)C background in expired CO2 and plasma free Phe for 1.5 h when confined in respiration chambers, sows were fed the tracer, L[1-(13)C]Phe, a rate of 2.0 mg/(kg BW·h) over 4 h divided into eight 30-min meals. Expired CO2 and plasma free Phe were analyzed for (13)C enrichment above background. Requirements were determined as the breakpoint in 2-phase nonlinear models. Sow BW was 246.5 kg in early and 271.6 kg in late pregnancy. Daily gain of the 6 sows was similar in early (344 g/d) and late pregnancy (543 g/d). During pregnancy, sow maternal gain was 19.1 ± 4.4 kg and litters of 17.7 ± 0.8 piglets weighed 22.6 ± 0.9 kg at birth. The Ile requirement was 3.6 ± 1.2 g/d (P = 0.001) in early pregnancy with a Phe retention (-0.59 g/d) and energy retention (-0.31 MJ/d) that were not different from 0. This indicates that the fourth parity sows had requirements close to maintenance in early pregnancy. The Ile requirement in late pregnancy was 9.7 ± 1.9 g/d (P = 0.001) when sows retained 3.30 g/d of Phe and -1.45 MJ/d of energy. The greater Ile requirement in late pregnancy was probably caused by the increased conceptus growth after d 70 of pregnancy. Phenylalanine flux, oxidation, and nonoxidative disposal increased (P < 0.1) from early to late pregnancy, but body protein breakdown did not. Phenylalanine oxidation, nonoxidative disposal, and retention increased (P < 0.01) with increasing Ile intake in early pregnancy but were not affected by Ile intake in late pregnancy. Body protein breakdown did not respond to Ile intake in early or late pregnancy. Although energy retention was similar in early and late pregnancy, the respiratory quotient decreased (P = 0.047) from early (1.05) to late pregnancy (0.98), indicating lipid mobilization in late pregnancy when Ile was at or above the requirement. The results of this study show that the Ile requirement of sows increases from early to late pregnancy.

What is the current role for parenteral lipid emulsions containing omega-3 fatty acids in infants with short bowel syndrome?

Parenteral nutrition associated liver disease is the most common complication of pediatric short bowel syndrome (SBS). There is emerging evidence that the disease may be reversed with the use of parenteral lipid emulsions derived from fish-oils, which contain significant concentrations of omega-3 fatty acids (w3FA). This paper will review the rationale for the use of parenteral lipid emulsions containing w3FA in SBS and the evidence for their efficacy. Given the promising results and apparent safety of these emulsions, we shall also consider what the current role for PN lipid emulsions containing w3FA in children with SBS should be.

A comparison of short-term appetite and energy intakes in normal weight and obese boys following glucose and whey-protein drinks

OBJECTIVE

To compare the effect of glucose and whey-protein preloads on satiety and food intake (FI) as affected by time to the next meal and body composition in normal weight (NW) and obese (OB) boys.

DESIGN

Cross-sectional clinical intervention study of the effect of caloric preloads on FI control in boys.

SUBJECTS

Seventeen NW (body mass index (BMI)=18.9+/-0.5 kg/m(2); age=12.2+/-0.3 years) and 17 OB boys (BMI=25.8+/-0.9 kg/m(2); age=11.4+/-0.3 years) (Experiment 1) and 12 NW boys (BMI=18.6+/-0.6 kg/m(2); age=12.1+/-0.3 years) (Experiment 2).

MEASUREMENTS

On three separate mornings and in random order each of the boys consumed a noncaloric sweetened preload, glucose (837 kJ) or whey protein (837 kJ) (Experiment 1) or noncaloric preload, glucose (1.0 g/kg) or whey protein (1.0 g/kg) (Experiment 2) made up to 250 ml with water 2 h after the consumption of a fixed breakfast. Food intake from a pizza meal was measured 30 min (Experiment 1) or 60 min (Experiment 2) later.

RESULTS

In Experiment 1, glucose suppressed FI (mean kJ+/-s.e.m.) in NW (3126+/-304) and OB boys (3116+/-286) compared with the control (NW, 4015+/-337 and OB, 3791+/-255). Whey protein suppressed FI in NW, but not in OB boys. Body weight, fat-mass and fat-free mass were positively associated with FI after all treatments in NW, but was not related to FI in OB boys. In Experiment 2, FI was suppressed by whey protein (2683+/-367) more than by glucose (3107+/-294) or the control (3585+/-361).

CONCLUSION

NW and OB boys respond differently to whey-protein preloads, with time to next meal a factor in the response to both glucose and protein preloads.

Estimate of the variability of the lysine requirement of growing pigs using the indicator amino acid oxidation technique1

Although AA requirements for the mean in a population of growing pigs are well established, there are no direct estimates of their variability within the population. The indicator AA oxidation method allows repeated measurements in a short period of time so that the AA requirement can be determined for individual pigs. The objective was to determine the Lys requirement in individual pigs to derive a first estimate of the population mean requirement and its variability. Nine individually housed barrows (15 to 18 kg) were surgically implanted with venous catheters for isotope infusion. Pigs were offered, in random order, isonitrogenous and isoenergetic diets with one of seven Lys concentrations (4.8 to 15.5 g of Lys/kg diet, as-fed basis). The pigs were fed twice daily, except for study days when they received one-half of the daily allowance in eight equal hourly meals. After a validated minimum adaptation period, indicator (Phe) oxidation was determined for each dietary Lys level during a 4-h primed, constant infusion of L-[1-(14C)]Phe at a rate of 464 kBq/h. The Lys requirement was calculated using a two-phase linear regression crossover analysis within individual pigs. For each pig, Phe oxidation decreased linearly (P < 0.02) as the dietary Lys concentration increased until the requirement was reached; thereafter, Phe oxidation was not different. The true ileal digestible Lys requirement ranged from 7.5 to 10.6 g/kg of diet (as-fed basis) for the nine animals. The mean requirement for all pigs was 9.1 g/d (CV, 11.6%) or 93.9% (CV, 9.8%) of the predicted (NRC, 1998) requirement based on each pig's mean BW and energy intake. The measured and predicted requirements did not differ. The indicator AA oxidation method gave values for Lys requirement similar to conventional methods. The short (< 3 wk) experimental period allows, for the first time, the estimate of population variability, which provides for more accurate calculation of the effect of altering Lys intake on herd performance and production economics. This method is suitable to use with all dietary indispensable AA.

A comparison of the metabolism of eighteen-carbon 13C-unsaturated fatty acids in healthy women

Altered use of different dietary fatty acids may contribute to several chronic diseases, including obesity, noninsulin-dependent diabetes mellitus, and cardiovascular disease. However, few comparative data are available to support this link, so the goal of the present study was to compare the metabolism of [(13)C]oleate, [(13)C]alpha-linolenate, [(13)C]elaidate, and [(13)C]linoleate through oxidation and incorporation into plasma lipid fractions and adipose tissue. Each tracer was given as a single oral bolus to six healthy women. Samples were collected over 8 days, and (13)C was analyzed using isotope ratio mass spectrometry. At 9 h postdose, cumulative oxidation was similar for [(13)C]elaidate, [(13)C]oleate, and [(13)C]alpha-linolenate (19 +/- 1%, 20 +/- 4%, and 19 +/- 3% dose, respectively). Significantly lower oxidation of [(13)C]linoleate (12 +/- 4% dose; P < 0.05) was accompanied by its higher incorporation into plasma phospholipids and cholesteryl esters. Abdominal adipose tissue was enriched with [(13)C]alpha-linolenate, [(13)C]elaidate, or [(13)C]linoleate within 6 h. The percentage linoleate in plasma phospholipids correlated positively with [(13)C]linoleate and [(13)C]elaidate oxidation, indicating a potential role of background diet. Conversion of [(13)C]linoleate and [(13)C]alpha-linolenate to longer chain polyunsaturates was a quantitatively minor route of utilization.

Total sulfur amino acid requirement in young men as determined by indicator amino acid oxidation with L-[1-13C]phenylalanine

BACKGROUND

Determining the sulfur amino acid (SAA) requirements of humans has remained elusive because of the complex nature of SAA metabolism. Current recommendations are based on nitrogen balance studies.

OBJECTIVE

The goal of the present study was to determine the methionine requirement of men fed a diet devoid of cysteine (total SAA requirement).

DESIGN

Six men were randomly assigned to receive 6 graded intakes of methionine: 0, 6.5, 13.0, 19.5, 26.0, and 32.0 mg x kg(-1) x d(-1). The total SAA requirement was determined by measuring the oxidation of L-[1-13C]phenylalanine to 13CO2 (F(13)CO2)). The mean total SAA requirement was estimated with use of a linear regression crossover analysis, which identified a breakpoint of the F(13)CO2 response to methionine intake.

RESULTS

On the basis of the mean measures of F(13)CO2, the mean requirement and population-safe intake (upper limit of the 95% CI) of total SAAs were found to be 12.6 and 21 mg x kg(-1) x d(-1), respectively.

CONCLUSION

Although the mean SAA requirement is consistent with current guidelines for the total SAA intake, the population-safe intake is substantially higher than the currently recommended total SAA intake.

Dietary cysteine reduces the methionine requirement in men

BACKGROUND

Despite early evidence suggesting that dietary cysteine has a sparing effect on methionine requirements, some recent reports question the existence of a measurable sparing capacity.

OBJECTIVE

The goal of the present study was to determine whether dietary cysteine could reduce the requirement for methionine in men consuming diets with and without cysteine.

DESIGN

Six men were randomly assigned to receive graded intakes of methionine while fed a diet containing either no exogenous cysteine or an excess of cysteine (21 mg x kg(-1) x d(-1)). The methionine requirement was determined by measuring the oxidation of L-[1-13C]phenylalanine to 13CO2 and estimated by using a linear regression crossover analysis.

RESULTS

The mean and population-safe (upper limit of the 95% CI) methionine requirements in the absence of exogenous cysteine were found to be 12.6 and 21 mg x kg(-1) x d(-1), respectively. The mean and population-safe methionine requirements in the presence of excess dietary cysteine were found to be 4.5 and 10.1 mg x kg(-1) x d(-1), respectively, representing a cysteine sparing effect of 64% in a comparison of mean methionine requirements and of 52% in a comparison of population-safe methionine intakes. Furthermore, the difference between population-safe intakes with and without dietary cysteine establishes a safe cysteine intake of 10.9 mg x kg(-1) x d(-1) in the presence of adequate methionine intakes.

CONCLUSION

Our data suggest that dietary cysteine can reduce the exogenous requirement for methionine in men. These results strongly support the existence of a cysteine sparing effect in humans.

Is resting metabolic rate different between men and women?

A low resting metabolic rate (RMR) has been proposed as a possible cause for the increased body fat commonly seen in women compared with men. Absolute RMR is higher in men, but whether RMR adjusted for lean body mass (LBM) remains higher is unresolved. The objective of the present study was to determine whether RMR adjusted for various body composition factors differed between healthy adult men and women. Thirty men years, BMI and twenty-eight women years, BMI were included in the analyses. RMR was measured by open-circuit indirect calorimetry for 60 min. Extracellular water (ECW) was measured by corrected Br(-) space and total body water (TBW) by 2H dilution. LBM was estimated as TBW/0.732. Intracellular water (ICW) was calculated as TBW-ECW, and body cell mass (BCM) as ICW/0.732. Men were heavier and had higher BMI, LBM, BCM and ECW, but less fat mass. Absolute RMR was higher in men than women v. P<0.0001). This difference became non-significant when RMR was adjusted for LBM by ANCOVA v. P=0.2191), but remained significant when adjusted for BCM v. P=0.0249). Fat mass explained a significant amount of variation in RMR in women (r(2) 0.28, P=0.0038), but not in men (r(2) 0.03, P=0.3301). The relationships between body fat and the various subcompartments of BCM and RMR require further elucidation.

Treatment of vitamin K deficiency in cystic fibrosis: Effectiveness of a daily fat-soluble vitamin combination

OBJECTIVE

Patients with cystic fibrosis (CF) and pancreatic insufficiency (PI) commonly have vitamin K deficiency, and those with CF-associated liver disease (CFLD) have universal vitamin K deficiency. We evaluated the effectiveness of an oral fat-soluble vitamin combination (ADEKs) to treat patients with vitamin K deficiency.

STUDY DESIGN

Patients with PI and CF (mean age, 15 years; range, 0.6 to 46 years) including 6 with advanced CFLD were prospectively enrolled in a study of a fat-soluble vitamin combination taken on a daily basis. None had received vitamin K supplementation for at least 4 months before the study. Fat-soluble vitamin combination supplementation was given for a minimum of 4 months; the mean vitamin K intake was 0.18 mg/d (SD = 0.1, range, 0 to 0.3). The primary outcome was change in plasma PIVKA-II (prothrombin in vitamin K absence).

RESULTS

Before supplementation 58 (81%) of 72 patients had abnormal PIVKA-II levels (>2.9 ng/mL). After supplementation 29 (40%) had abnormal PIVKA-II levels (P =.001). All 6 patients with advanced CFLD had abnormal PIVKA-II levels (median, range of 20.8, 5.5 to 55 ng/mL) before treatment, which corrected to normal in 50% (4.1, 2.1 to 65 ng/mL). Four patients, 2 with CFLD, had a prolonged prothrombin time (>13.5 seconds) at both time periods.

CONCLUSIONS

An oral fat-soluble vitamin combination with a modest amount of vitamin K can, as a daily supplement, improve the PIVKA-II levels in patients with PI and CF.

Tyrosine requirement of healthy men receiving a fixed phenylalanine intake determined by using indicator amino acid oxidation

BACKGROUND

The currently accepted total aromatic amino acid requirement for adults is based on nitrogen balance measurements in individuals who received their intake of aromatic amino acids solely as phenylalanine.

OBJECTIVE

The objective of this study was to determine the requirement for the amino acid tyrosine in healthy men receiving an adequate, but not excessive, intake of phenylalanine (9 mg x kg(-1) x d(-1)).

DESIGN

The effect of a graded intake of tyrosine was determined in 6 healthy men consuming energy-sufficient diets containing 1 g protein x kg(-1) x d(-1). The tyrosine requirement was determined by using indicator amino acid oxidation methodology with L-[1-13C]lysine as the indicator. Subjects were studied at each of 7 tyrosine intakes.

RESULTS

A graded intake of tyrosine had no effect on lysine flux. The mean tyrosine requirement was determined from the response of the oxidation of L-[1-13C]lysine to breath 13CO2. A 2-phase linear regression crossover analysis of breath 13CO2 identified the breakpoint and upper 95% confidence limit, which represents the mean and safe intakes, to be 6.0 and 7.0 mg x kg(-1) x d(-1), respectively.

CONCLUSIONS

The safe intake of total aromatic amino acids calculated from the present results for tyrosine and our previous estimate for phenylalanine is estimated to be 21 mg x kg(-1) x d(-1). This intake is 1.5 times the currently recommended total aromatic amino acid intake of the FAO/WHO/UNU (1985), 14 mg x kg(-1) x d(-1). Furthermore, the absolute aromatic amino acid requirement may be dependent on the proportional balance of these amino acids in the diet.

The effect of graded intake of glycyl-L-tyrosine on phenylalanine and tyrosine metabolism in parenterally fed neonates with an estimation of tyrosine requirement

Although tyrosine is considered indispensable during the neonatal period, its poor solubility has limited its inclusion in parenteral amino acid solutions to less than 1% of total amino acids. Dipeptides of tyrosine are highly soluble, have been shown to be well used and safe in animal models and humans, and, therefore, may be used as an effective means of providing tyrosine in the parenterally fed neonate. The goal of the present study was to determine the tyrosine requirement of the parenterally fed neonate receiving graded intakes of glycyl-L-tyrosine as a source of tyrosine. Thirteen infants receiving adequate energy (340 +/- 38 kJ. kg(-1).d(-1)) and protein (2.4 +/- 0.4 g.kg(-1).d(-1)) were randomized to receive parenteral nutrition with one of five graded levels of glycyl-L-tyrosine. The mean requirement and safe level of intake were estimated using a 1-(13)C-phenylalanine tracer and linear regression cross-over analysis that identified a break point in the response of label appearance in breath CO(2) (F(13)CO(2)) and phenylalanine oxidation to graded tyrosine intake. Based on the mean estimates of whole-body phenylalanine oxidation, the tyrosine mean requirement and safe level of intake were found to be 74 mg.kg(-1). d(-1) and 94 mg.kg(-1).d(-1), respectively. This represents 3.1 and 3.9% of total amino acids, respectively, considerably higher than levels found in present commercially available pediatric amino acid solutions. These data raise concern regarding the adequacy of aromatic amino acid intake in the parenterally fed neonate.

Pathogenesis of malnutrition in cystic fibrosis, and its treatment

PATHOGENESIS

We have developed a model of the pathogenesis of malnutrition in cystic fibrosis. It consists of the relationship between nutrient balance and nutrient requirement. The validation has been conducted with respect to energy, but the same general principals can be applied to any nutrient. A patient with CF either loses weight or fails to grow normally if their absorbed energy intake is less than their total daily energy expenditure. Multiple factors have the potential to contribute to reduced energy intake including, anorexia, gastroeosophageal (GE) reflux leading to vomiting and hence food loss, as well as maldigestion. Another more recently recognized source of energy loss, is glucosuria as a result of CF related diabetes (CFRD). Conversely, lung inflammation appears to be related to increases in resting metabolic rate (RMR). Acute exacerbations of the chronic lung disease increases RMR which returns to a basal level some weeks after the inflammation is treated. In clinically stable patients with CF, RMR rises in a quadratic fashion as lung function falls. When FEV(1)is >85% predicted RMR is not different from controls, but it rises in a curvilinear fashion as FEV(1)falls. Initially it appears that patients adapt to their increased RMR by reducing their activity so their total daily energy expenditure (TDEE) is often no higher than controls. But this is by no means always the case. Furthermore good lung care requires CF patients to be involved in aerobic activities, hence their TDEE would rise. Although there has been considerable interest as to whether the genetic defect has an energy wasting effect, it appears genetic factors have little or no effect on RMR.

TREATMENT

This starts with making an energy diagnosis. First, a 3 day faecal fat balance study is conducted. This provides information with regard to intake as well as to maldigestion. In addition a history of GE reflux is sought, since it can readily be treated with H(2)-blockers. If significant fat malabsorption exists, efforts are made to improve pancreatic enzyme dose and function. The possibility of CFRD also needs to be considered. We measure the RMR of the patient using open circuit indirect calorimetry. Recommendations for diet therapy are based on estimated TDEE, which is determined from RMR taking into account faecal losses. Diet therapy places the emphasis on increasing the fat content of the diet. We have conducted a study to determine whether or not oral supplements help increase TDEE and they did not; they merely replaced food energy. Conversely, nocturnal gastrostomy supplemental feeding, while reducing voluntary food energy intake by about 20%, does result in a significant increase in total daily energy intake. Our target is to achieve a completely normal nutritional status. Long term follow-up of these patients has shown significantly better survival in patients who achieve normal nutritional status. The advent of lung transplantation has added another dimension. In our experience, following a successful lung transplant, most patients no longer need their supplemental gastrostomy feeding.

SUMMARY

Our clinic policy is to encourage a high fat diet (35-40% total energy) and our patients grow normally in height and weight until their lung disease deteriorates significantly. Patients who develop a negative energy balance seldom if ever respond to diet therapy and hence are candidates for supplemental nocturnal gastrostomy feeds. Gastrostomy fed patients constitute 3 to 5% of our total CF population of approximately 590 patients.

Steady state is not achieved for most plasma amino acids during 12 hours of fasting in the neonatal piglet

Kinetics studies in neonates are important to establish the requirement for amino acids and to understand the mechanisms of normal and altered metabolism. During kinetics experiments, plasma amino acid concentrations should be in steady state. Our objective was to determine whether 12 h of fasting, after parenteral or enteral feeding, resulted in a steady state in concentrations of amino acids. Two-day-old piglets were implanted with catheters (d 0), and randomly assigned to either intragastric (i.g., n = 6) or i.v. (n = 6) feeding. On d 5, piglets were fasted for 12 h. During the first 2 h, plasma concentrations of almost all amino acids declined except asparagine (i.g. and i.v.), tyrosine (i.v.), and glycine (i.v.), which increased. Only i.g. glycine did not change. Between 2 and 12 h, the only indispensable amino acids that did not change were phenylalanine (i.v.) and histidine (i.g. and i.v.). The branched-chain amino acids increased during this period (i.v. and i.g.). The greatest change was tyrosine, increasing 13% (i.v.) and 32% (i.g.) per hour. After 12 h of refeeding, glycine, serine, threonine, and asparagine concentrations were lower than baseline (p<0.05) in the i.v. group. In i.g. fed piglets, only threonine remained below baseline (p<0.05), and arginine was greater than baseline (p<0.05). Differences between i.v. and i.g. may be the result of impaired small intestinal metabolism secondary to parenteral feeding. In neonatal pigs, most plasma amino acids were unstable during 12 h of fasting. Thus, kinetics studies that require a steady state must be conducted in the fed state.

Current total parenteral nutrition solutions for the neonate are inadequate

The amino acid requirements of the parenterally fed neonate are poorly defined. Newborn infants are at risk for amino acid deficiency and toxicity, due to lack of small intestinal metabolism and metabolic immaturity. We discuss recent evidence that identifies inadequacies of commercial amino acid solutions with respect to the balance and quantity of aromatic amino acids, and sulphur amino acids. We present data demonstrating that impaired small intestinal metabolism (or lack of first pass metabolism) alters the whole body requirement for methionine, threonine, and arginine, and discuss the potential adverse effects of excess or inadequate parenteral amino acid intake.

Threonine dehydrogenase is a minor degradative pathway of threonine catabolism in adult humans

The threonine dehydrogenase (TDG) pathway is a significant route of threonine degradation, yielding glycine in experimental animals, but has not been accurately quantitated in humans. Therefore, the effect of a large excess of dietary threonine, given either as free amino acid (+Thr) or as a constituent of protein (+P-Thr), on threonine catabolism to CO(2) and to glycine was studied in six healthy adult males using a 4-h constant infusion of L-[1-(13)C]threonine and [(15)N]glycine. Gas chromatography-combustion isotope ratio mass spectrometry was used to determine [(13)C]glycine produced from labeled threonine. Threonine intakes were higher on +Thr and +P-Thr diets compared with control (126, 126, and 50 micromol x kg(-1) x h(-1), SD 8, P < 0.0001). Threonine oxidation to CO(2) increased threefold in subjects on +Thr and +P-Thr vs. control (49, 45, and 15 micromol x kg(-1) x h(-1), SD 6, P < 0.0001). Threonine conversion to glycine tended to be higher on +Thr and +P-Thr vs. control (3.5, 3.4, and 1.6 micromol x kg(-1) x h(-1), SD 1.3, P = 0.06). The TDG pathway accounted for only 7-11% of total threonine catabolism and therefore is a minor pathway in the human adult.

Organ and plasma amino acid concentrations are profoundly different in piglets fed identical diets via gastric, central venous or portal venous routes

In a previous study in piglets fed identical diets intravenously, intraportally or intragastrically, we determined that small intestinal atrophy affects nitrogen metabolism to a greater extent than liver by-pass. Because whole-body amino acid homeostasis is dependent on interorgan metabolism, we also measured the free amino acid concentrations in liver, small intestinal mucosa and kidney in order to study alterations in amino acid metabolism within these organs. Piglets (n = 15; 2-4 d old) were fed identical diets continuously for 8 d via gastric (IG), portal (IP) or central venous (IV) catheters. Concentrations of all measured amino acids were affected by route of feeding in one or more organs. In particular, urea cycle amino acid concentrations were altered in plasma and mucosa of IV and IP pigs, suggesting that arginine synthesis by an atrophied gut may have been limited. Furthermore, most indispensable amino acid concentrations were lower in IP pigs for all organs vs. IG pigs; however, except for phenylalanine, plasma concentrations of these amino acids were not different, demonstrating the liver's "smoothing" capability. Gut atrophy in both IV and IP pigs resulted in significantly lower concentrations of all indispensable amino acids compared with IG pigs. Alterations of all amino acids in various organs due to route of feeding suggest that more detailed analyses of regulatory mechanisms and amino acid interactions on interorgan amino acid metabolism are necessary for all amino acids.

Challenges in the dietary treatment of cystic fibrosis related diabetes mellitus

Cystic fibrosis related diabetes mellitus is an increasingly recognized problem as survival in patients with cystic fibrosis improves. In a 5 year retrospective study of 627 children and adults attending Toronto cystic fibrosis clinics, we identified 57 (9%) patients with cystic fibrosis related diabetes mellitus; four (1.3%) of 301 children (<18 years) and 53 (16%) of 326 adults. The development of this complication of cystic fibrosis is associated with increased mortality, deteriorations in both respiratory and nutritional status, and the development of late microvascular, but not macrovascular, diabetic complications. Unfortunately, systematic review of the literature provides few well designed studies that provide sound evidence for clinical practice. Recommendations are therefore often based on anecdote, rather than physiological or outcomes research. Dietary therapy combines the principles of the dietary management of both cystic fibrosis and diabetes mellitus, but emphasizes the need for a high energy diet (> 100% of recommended daily intake) in patients with cystic fibrosis related diabetes mellitus. The importance of calories from fat is emphasized, with no restriction on total carbohydrate intake. Insulin intake mirrors carbohydrate intake. Routine dietary therapy is straightforward, but challenges occur due to both complications of cystic fibrosis and advancing disease. If a patient with cystic fibrosis related diabetes mellitus is malnourished, overnight enteral tube feeding is often used, with an adjusted insulin regimen. There is a great need for both physiological and outcomes research to provide sound scientific evidence for the dietary treatment of cystic fibrosis related diabetes mellitus.

Effect of tyrosine intake on the rate of phenylalanine hydroxylation in adult males

This study evaluated the effect of varying levels of tyrosine intake on the estimation of phenylalanine hydroxylation. Healthy men were fed 1 g protein kg(-1) x d(-1) for a 2-day period. On the third day, subjects consumed a formula diet containing 1 g protein kg(-1) x d(-1) hourly over 10 hours, and primed hourly oral doses of L-[15N]phenylalanine and L-[3,3-2H2]tyrosine for the last 6 hours. Each subject was studied at 7 levels of tyrosine intake (3.0, 4.5, 6.0, 7.5, 9.0, 10.5, and 12.0 mg x kg(-1) x d(-1)) at a constant intake of phenylalanine (9 mg x kg(-1) x d(-1), 4.55 micromol x kg(-1) x h(-1)). Phenylalanine hydroxylation was estimated from the ratio of plasma amino acid isotope enrichment of [15N]phenylalanine and [15N]tyrosine and the tyrosine flux estimated from [2H2]tyrosine enrichment. Phenylalanine and tyrosine fluxes showed no significant response to alterations in the intake of tyrosine. Linear regression analysis showed a significant response such that the rate of phenylalanine hydroxylation decreased as tyrosine intake increased (R2 = .21; P = .003). The mean rates of phenylalanine hydroxylation were 3.89 to 8.06 micromol x kg(-1) x h(-1). Given model uncertainties, the apparent protein breakdown observed at tyrosine intake levels less than 10.5 mg x kg(-1) x d(-1), and the significant differences observed between the present data and our prior data, we cannot estimate the tyrosine requirement with any degree of certainty with the present hydroxylation results.

Distribution of protein turnover changes with age in humans as assessed by whole-body magnetic resonance image analysis to quantify tissue volumes

We tested the hypothesis that nonmuscle lean tissue mass and its rate of protein catabolism remain constant with aging despite changes in the proportional contribution of these tissues to whole-body protein metabolism. Whole-body protein kinetics, using the 60-h oral [(15) N]glycine method, and muscle and nonmuscle protein catabolism, based on protein kinetic data, urinary N(tau)-methylhistine excretion and lean tissue volumes defined by whole-body magnetic resonance imaging, from eight healthy elderly subjects (5 females and 3 males, mean age 71.5 y) were compared with those of seven young persons (3 females and 4 males, mean age 28 y). There were no significant age or gender effects on rates of protein kinetics per L total lean tissue. There was a lower (P < 0.004) rate of muscle protein catabolism in the elderly (1.8 +/- 0.2 vs. 2.6 +/- 0.1 g. L(-1). d(-1)) and a trend (P = 0.08) for lower muscle volume (19.7 +/- 1.5 vs. 25.0 +/- 2.4 L). This contrasted with intraabdominal lean tissue, where the rate of protein catabolism (13. 8 +/- 0.6 vs. 13.2 +/- 0.9 g. L(-1 ). d(-1)) and volume (7.5 +/- 0.3 vs 8.0 +/- 0.5 L) did not differ between age groups. Thus, the decrease in the contribution by muscle to whole-body protein metabolism with age is associated with an increase from 62 to 74% (P < 0.001) in the contribution by nonmuscle lean tissues. These findings have potential implications for the nutrition of both normal and sick elderly persons.

Threonine requirement of young men determined by indicator amino acid oxidation with use of L-[1-(13)C]phenylalanine

BACKGROUND

Threonine is an indispensable amino acid with a complex degradative pathway. Use of the indicator amino acid oxidation technique should provide an estimate of the threonine requirement that is not affected by its metabolic pathway.

OBJECTIVE

Our objective was to determine the requirement for threonine in men by using the indicator amino acid oxidation method and to provide statistical estimates of the population mean and 95% CIs of the threonine requirement. We hypothesized that the current World Health Organization estimate of the threonine requirement, 7 mg*kg(-)(1)*d(-)(1) (based on nitrogen balance studies), is too low.

DESIGN

Six healthy men each received 6 different threonine intakes while consuming an energy-sufficient diet with 1.0 g L-amino acid mixture*kg(-)(1)*d(-)(1). The effect of graded alterations in dietary threonine intake on phenylalanine flux and oxidation was studied by using L-[1-(13)C]phenylalanine as the indicator amino acid.

RESULTS

The results of two-phase linear regression crossover analysis showed that the mean threonine requirement, based on indicator oxidation, was 19.0 mg*kg(-)(1)*d(-)(1) with an upper safe intake of 26.2 mg*kg(-)(1)*d(-)(1).

CONCLUSIONS

This is the first application of the indicator amino acid oxidation technique in humans to study the requirement for an indispensable amino acid with a complex degradative pathway. We found that the upper safe intake for 95% of the population is almost 4-fold higher than the current World Health Organization estimate.

Tyrosine requirements in children with classical PKU determined by indicator amino acid oxidation

Tyrosine (Tyr) is an essential amino acid in phenylketonuria (PKU) because of the limited hydroxylation of phenylalanine (Phe) to Tyr. The recommended intakes for Tyr in PKU are at least five times the recommended phenylalanine intakes. This suggests that Phe and Tyr contribute approximately 20 and 80%, respectively, of the aromatic amino acid (AAA) requirement (REQ). In animals and normal humans, dietary Tyr was shown to spare 40-50% of the Phe requirement, proportions that reflect dietary and tissue protein composition. We tested the hypothesis that the Tyr REQ in PKU would account for 45% of the total AAA REQ by indicator amino acid oxidation (IAAO). Tyr REQ was determined in five children with PKU by examining the effect of varying dietary Tyr intake on lysine oxidation and the appearance of (13)CO(2) in breath (F(13)CO(2)) under dietary conditions of adequate energy, protein (1.5 g x kg(-1) x day(-1)), and phenylalanine (25 mg x kg(-1) x day(-1)). Lysine oxidation and F(13)CO(2) were determined using a primed 4-h oral equal-dose infusion of L-[1-(13)C]lysine. Lysine oxidation and F(13)CO(2) decreased linearly as Tyr intake increased, to a break point that was interpreted as the mean dietary Tyr requirement (16.3 and 19.2 mg x kg(-1) x day(-1), respectively). At Tyr intakes of >16.3 and 19.2 mg x kg(-1) x day(-1), lysine oxidation and F(13)CO(2), respectively, were low and constant. This represents 40.4 and 44.4%, respectively, of the total AAA intake. The current recommendations for Tyr intake in PKU patients appear to be overestimated by a factor of approximately 5. This study is the first application of the IAAO technique in a pediatric population and in humans with an inborn error of metabolism.

Prevalence of vitamin K deficiency in cystic fibrosis

BACKGROUND

Patients with cystic fibrosis (CF) are at risk of developing vitamin K deficiency because of pancreatic insufficiency, hepatobiliary disease, or both.

OBJECTIVE

Our objective was to determine the prevalence of vitamin K deficiency in unsupplemented patients with CF and to identify risk factors that might be associated with the deficiency.

DESIGN

Ninety-eight patients with CF-83 who were pancreatic insufficient (age: 15.2 +/- 10.7 y; range: 0.6-45.8 y), 15 who were pancreatic sufficient (age: 26.2 +/- 11.6 y; range: 6.5-45.3 y), and 62 healthy individuals (age: 16.2 +/- 12. 8 y; range: 1-45 y)-were studied prospectively. None had taken vitamin K supplements. Eight pancreatic-insufficient patients had advanced CF-associated liver disease. Plasma prothrombin in vitamin K absence (PIVKA-II) was measured by immunoassay. All control subjects had PIVKA-II concentrations <3 microg/L.

RESULTS

Seventy-eight percent of pancreatic-insufficient patients had PIVKA-II concentrations >/=3 microg/L (22.8 +/- 35.7 microg/L). All patients with CF-associated liver disease had abnormal PIVKA-II concentrations. The mean PIVKA-II concentration of pancreatic-insufficient patients with liver disease was greater than that of those without liver disease (46.6 +/- 65.3 compared with 15. 3 +/- 26.1 microg/L; P < 0.05). Five pancreatic-sufficient patients had mildly elevated PIVKA-II concentrations. Six (7%) pancreatic insufficient patients (3 with CF-associated liver disease) had mildly prolonged prothrombin time but no clinical bleeding. There was no correlation between PIVKA-II concentrations and severity of fat malabsorption or antibiotic use.

CONCLUSIONS

Vitamin K deficiency is common in unsupplemented patients with CF and pancreatic insufficiency and routine supplementation should be considered in all of these patients.

Proline ameliorates arginine deficiency during enteral but not parenteral feeding in neonatal piglets

The indispensability of arginine has not been conclusively established in newborns. Because parenteral feeding bypasses the gut (where de novo synthesis of arginine occurs from proline), a dietary supply of arginine that is sufficient to maintain urea cycle function may be of greater importance during intravenous compared with enteral feeding. Two-day-old piglets (n = 12) were fed nutritionally complete diets for 5 days via either a central vein catheter (IV pigs, n = 6) or a gastric catheter (IG pigs, n = 6). Subsequently, each piglet received three incomplete test diets [arginine free (-ARG/+PRO), proline free (-PRO/+ARG), or arginine and proline free (-ARG/-PRO)] in a randomized crossover design. Plasma ammonia was assayed every 30 min for 8 h or until hyperammonemia was observed. Ammonia increased rapidly in IV pigs receiving -ARG/+PRO and -ARG/-PRO (84 +/- 36 and 74 +/- 37 micromol. l(-1). h(-1), respectively), requiring early diet cessation. A rapid increase was also exhibited by IG pigs receiving the -ARG/-PRO, but not the -ARG/+PRO diet (31 +/- 15 vs. 11 +/- 7 micromol. l(-1). h(-1), respectively, P < 0.05). Plasma arginine and proline were indicative of deficiency (IG and IV groups) when deplete diets were infused. Arginine is indispensable in parenteral and enteral nutrition, independent of dietary proline.

Isotopic enrichment of amino acids in urine following oral infusions of L-[1-(13)C]phenylalanine and L-[1-(13)C]lysine in humans: confounding effect of D-[13C]amino acids

Urine sampling of the free amino acid pool serves to reflect plasma enrichment and is used as a noninvasive means to determine isotope enrichment in studies of amino acid metabolism. We determined the effect of D-[13C]phenylalanine and D-[13C]lysine content of tracers on urinary amino acid enrichment following oral infusion of L-[13C]phenylalanine in 18 preterm infants and L-[1-(13)C]lysine in seven healthy adult females. Urinary [13C]phenylalanine enrichment was higher (P < .0001) for L-[13C]phenylalanine containing 0.4% D-[13C]phenylalanine (28.6 +/- 7.1) versus L-[1-(13)C]phenylalanine that contained undetectable D-[13C]phenylalanine (10.2 +/- 1.5). D-[13C]phenylalanine, measured by chiral column gas chromatography-mass spectrometry (GC-MS), accounted for 10% to 30% (20.5% +/- 7%) of total phenylalanine in the urine of infants who received 0.4% D-[13C]phenylalanine, and was absent from the urine of infants receiving tracer with undetectable [13C]phenylalanine. Urinary L-[13C]phenylalanine enrichment did not differ between tracer groups (9.8 +/- 1.5 and 9.8 +/- 2.5). In adult females, the use of L-[1-(13)C]lysine (1.6% D-lysine) resulted in a higher (P < .02) urine total L,D-[13C]lysine enrichment compared with plasma enrichment (40.8 +/- 4.1 v 11.1 +/- 0.7). This study demonstrates the significant presence of D-[13C]amino acids in urine that originate as contaminants from commercially manufactured tracers, as a result of renal tubular discrimination of D-amino acids. A tracer containing detectable amounts of D-[13C]isomer cannot be recommended for any study in which urine will be used to reflect enrichment in the arterial plasma pool.

Intestinal atrophy has a greater impact on nitrogen metabolism than liver by-pass in piglets fed identical diets via gastric, central venous or portal venous routes

Whole-body nitrogen metabolism is altered during parenteral feeding as a result of gut atrophy and/or lack of splanchnic first-pass metabolism. We developed in vivo models to describe the metabolic and physiologic effects of first-pass metabolism by the small intestine/liver, liver or non-splanchnic tissues. Fifteen 2- to 4-d-old piglets were fed identical diets continuously for 8 d via gastric (IG), portal (IP) or central venous (IV) catheters. Despite similar weight gain, IV and IP pigs had higher nitrogen output and hence lower nitrogen retention (80%) compared with IG pigs (87%) (P = 0.002). Body protein content was also higher in IG pigs (583 mg/g dry matter) compared with IV (550) and IP pigs (534) (P = 0.003). Despite similar intestinal lengths, total small intestinal and mucosal weights were approximately 40% lower in IV and IP pigs than in IG pigs. Free urea cycle amino acids were altered in plasma and mucosa, suggesting that limited arginine synthesis by an atrophied gut may have limited protein deposition. Although villous atrophy was observed in the duodena and jejuna of IV and IP pigs, reduced crypt depth was observed only in IV pigs. Crypt depth was similar in all four gut sections from IG and IP pigs, suggesting that nutrient flux through the liver affects gut growth. Overall, metabolic responses to IV (non-splanchnic) and IP (liver) feeding were similar as a result of gut atrophy, whereas responses to IG (small intestine + liver) and IP (liver) feeding were different, suggesting that small intestinal atrophy affects nitrogen metabolism to a greater extent than liver by-pass.

A comparison of parenteral and enteral feeding in neonatal piglets, including an assessment of the utilization of a glutamine-rich, pediatric elemental diet

BACKGROUND

The amino acid requirement profile for infants is different than that for adults and needs to be established; this profile also is different for infants receiving total parenteral nutrition. We used the neonatal piglet as a model for the infant to address (1) the metabolic and physiologic changes due to route of feeding and (2) the adequacy of the amino acid pattern in a pediatric elemental diet.

METHODS

Diets differed only in their amino acid pattern (modified human milk [MHM] and a commercial pediatric elemental diet [PED]) and were fed continuously for 8 days. Control piglets were fed MHM diet via gastric catheters (IG) and were compared with pigs fed MHM diet via venous catheters (IV) or to pigs IG-fed PED.

RESULTS

MHM-IV piglets experienced enlarged livers and gut atrophy, and lower nitrogen retention and body protein content. Higher glutamine (and lower glutamate) in PED-IG, compared with MHM-IG, produced no apparent advantage with respect to gut growth or histology. Proline, histidine, and lysine may have been limiting, and isoleucine and valine excessive, in the PED-IG diet as indicated by plasma concentrations, compared with sow-fed piglets; however, imbalances in the amino acid profile were not excessive because nitrogen retention was not different between MHM-IG or PED-IG pigs.

CONCLUSIONS

Therefore, the amino acid profile of MHM needs to be modified to improve nitrogen retention during parenteral feeding and the profile of oral PED could be improved to normalize plasma amino acid concentrations.

Prior protein intake may affect phenylalanine kinetics measured in healthy adult volunteers consuming 1 g protein. kg-1. d-1

Study of the amino acid metabolism of vulnerable groups, such as pregnant women, children and patients, is needed. Our existing protocol is preceded by 2 d of adaptation to a low 13C formula diet at a protein intake of 1 g. kg-1. d-1 to minimize variations in breath 13CO2 enrichment and protein metabolism. To expand on our potential study populations, a less invasive protocol needs to be developed. We have already established that a stable background 13CO2 enrichment can be achieved on the study day without prior adaptation to the low 13C formula. Therefore, this study investigates phenylalanine kinetics in response to variations in prior protein intake. Healthy adult subjects were each fed nutritionally adequate mixed diets containing 0.8, 1.4 and 2.0 g protein. kg-1. d-1 for 2 d. On d 3, subjects consumed an amino acid-based formula diet containing the equivalent of 1 g protein. kg-1. d-1 hourly for 10 h and primed hourly oral doses of L-[1-13C]phenylalanine for the final 6 h. Phenylalanine kinetics were calculated from plasma-free phenylalanine enrichment and breath 13CO2 excretion. A significant quadratic response of prior protein intake on phenylalanine flux (P = 0.012) and oxidation (P = 0.009) was identified, such that both variables were lower following adaptation to a protein intake of 1.4 g. kg-1. d-1. We conclude that variations in protein intake, between 0.8 and 2.0 g. kg-1. d-1, prior to the study day may affect amino acid kinetics and; therefore, it is prudent to continue to control protein intake prior to an amino acid kinetics study.

Aggressive oral, enteral or parenteral nutrition: prescriptive decisions in children with cancer

Over the past 18 years, our laboratory has been interested in the pathogenesis of energy imbalance caused by a variety of diseases. Our view is that a clear understanding of the various factors causing negative energy balance, which in turn results in malnutrition, is the most effective way of designing preventive and therapeutic nutritional strategies. Thus, in cancer, one of the common factors is anorexia, due either to the primary tumor or to the effects of cancer therapy. Currently there is little evidence of increased resting energy expenditure in children with cancer, except in cases with very high tumor burden. Conversely, there are suggestions of a failure to down-regulate resting energy expenditure in the presence of reduced food intake in patients with cancer. Damage to the gastrointestinal tract, due to the effects either of the tumor or of tumor therapy, may result in maldigestion and/or malabsorption. Thus, as a result of a combination of reduced intake, reduced absorption and increased needs, the child with cancer may become malnourished. Prevention and treatment are dependent on the type of cancer and the pathogenesis of the negative energy balance. In broad terms, we try as far as possible to use external routes. With the advent of percutaneously placed gastrostomies and gastrojejunal tubes, we use these methods increasingly to provide nutritional support. Only in patients whose gastrointestinal tract cannot be used do we turn to i.v. feeding. In these patients, the placement of a central venous line is required, but great care must be taken to avoid infection. Whatever form of nutritional support is used, whether enteral or parenteral, we measure the body composition and energy expenditure in the patient, so that the nutritional therapy can be tailored to the child's specific needs. Using these approaches, we are having significant success in preventing and reversing malnutrition in children with cancer and those undergoing bone-marrow transplantation.

Threonine kinetics in preterm infants fed their mothers' milk or formula with various ratios of whey to casein

BACKGROUND

Plasma threonine concentrations are elevated in infants fed formula containing a whey-to-casein protein ratio of 60:40 compared with concentrations in infants fed formula containing a ratio of 20:80 or human milk (60:40).

OBJECTIVE

We studied whether degradation of excess threonine was lower in formula-fed infants than in infants fed their mothers' milk.

DESIGN

Threonine kinetics were examined in 17 preterm infants (gestational age: 31+/-2 wk: birth weight: 1720+/-330 g) by using an 18-h oral infusion of [1-13C]threonine at a postnatal age of 21+/-11 d and weight of 1971+/-270 g. Five infants received breast milk. Formula-fed infants (n = 12) were randomly assigned to receive 1 of 3 formulas (5.3 g protein/MJ) that differed only in the whey-to-casein ratio (20:80, 40:60, and 60:40).

RESULTS

Threonine intake increased significantly in formula-fed infants with increasing whey content of the formula (48.5, 56.4, and 63.2 micromol.kg(-1).h(-1), respectively; pooled SD: 2.2; P = 0.0001), as did plasma threonine concentrations (228, 344, and 419 micromol/L, respectively; pooled SD: 75; P = 0.03). Despite a generous threonine intake by infants fed breast milk (58.0+/-16.0 micromol.kg(-1).h(-1), plasma threonine concentrations remained low (208+/-41 micromol/L). Fecal threonine excretion and net threonine tissue gain, estimated by nitrogen balance, did not differ significantly among groups. Threonine oxidation did not differ significantly among formula-fed infants but was significantly lower in formula-fed infants fed than in infants fed breast milk (17.1% compared with 24.3% of threonine intake, respectively).

CONCLUSION

Formula-fed infants have a lower capacity to oxidize threonine than do infants fed breast milk.

Phenylalanine and tyrosine metabolism in neonates receiving parenteral nutrition differing in pattern of amino acids

Tyrosine is considered to be an indispensable dietary amino acid in the neonate, yet achieving adequate parenteral tyrosine intake is difficult due to its poor solubility. Increasing the supply of phenylalanine is the most common means of compensating for low tyrosine levels. Unfortunately, plasma phenylalanine concentrations are sometimes elevated in infants receiving high phenylalanine intake. This led us to study the phenylalanine and tyrosine metabolism in 16 neonates randomized to receive total parenteral nutrition with either a high or a moderate phenylalanine-containing amino acid solution. A primed, 24-h continuous stable isotope infusion of L-[1-13C]phenylalanine and L-[3,3-2H2]tyrosine was given to enable the measurement of phenylalanine and tyrosine kinetics. Results demonstrated that 1) phenylalanine hydroxylation was significantly greater in infants receiving high phenylalanine, 2) phenylalanine oxidation and percent dose oxidized was also significantly greater in infants receiving high phenylalanine, 3) apparent phenylalanine retention was greater in neonates receiving high phenylalanine, and 4) alternate catabolites of phenylalanine and tyrosine metabolism were significantly greater in infants receiving high phenylalanine compared with moderate phenylalanine. We conclude that neonates respond to increased parenteral phenylalanine intake by increasing their hydroxylation and oxidation rates. The greater oxidation of phenylalanine in infants receiving high phenylalanine in conjunction with the urinary excretion of alternate catabolites of phenylalanine and tyrosine suggests that the high phenylalanine intake may be in excess of needs. However, the lower apparent phenylalanine retention observed in infants receiving moderate phenylalanine suggests that the total aromatic amino acid level of moderate phenylalanine may be deficient for neonatal needs.

Development of a minimally invasive protocol for the determination of phenylalanine and lysine kinetics in humans during the fed state

The primed, continuous intravenous infusion of amino acids labeled with 13C together with measurement of isotopic enrichment in plasma is commonly used to study amino acid metabolism. However, a less invasive, oral infusion that also produces an isotopic steady state in CO2 and urine would be useful, particularly for pediatric studies. We measured the 13C enrichments of expired CO2, plasma and urine free phenylalanine and lysine and estimated flux and oxidation rates in adult humans (n = 12) who received a 4-h oral, primed, equal dose infusion of either L-[1-13C]phenylalanine, L-[1-13C]lysine (D-lysine = 1.6%) or L-[1-13C]lysine (D-lysine </= 0.2%). Steady fed state conditions were established by feeding subjects eight hourly meals beginning 4 h before the start of the oral infusion protocol. Isotopic plateau in CO2, plasma and urine was achieved within 120 min of phenylalanine or lysine infusion. At isotopic plateau, the mean ratio of plasma to urine enrichment was 1.0 +/- 0.04 (SEM), 0. 39 +/- 0.03 and 0.97 +/- 0.02 for L-[1-13C]phenylalanine, L-[1-13C]lysine (D-lysine = 1.6%) and L-[1-13C]lysine (D-lysine</= 0. 2%), respectively. There was good agreement between isotopic enrichment in plasma and urine for L-[1-13C]phenylalanine and L-[1-13C]lysine (D-lysine </= 0.2%). However, use of L-[1-13C]lysine (D-lysine = 1.6%) resulted in significantly higher enrichment in urine, probably due to renal tubular discrimination of D-lysine. Mean flux rates for phenylalanine and lysine were consistent with the literature. Thus, the oral, primed, equal dose infusion produces the isotopic steady-state condition required for amino acid flux and oxidation determination within an 8-h period.

Threonine requirement of neonatal piglets receiving total parenteral nutrition is considerably lower than that of piglets receiving an identical diet intragastrically

Evidence is accumulating that the amino acid requirements for neonates receiving total parenteral nutrition (TPN) are significantly different than those for oral feeding and need to be determined. The parenteral threonine requirement was determined in 3-d-old male Yorkshire piglets (n = 25) by examining the effect of varying dietary threonine intakes [0.05-0.6 g/(kg.d)] on phenylalanine oxidation. The diet included adequate energy, total amino acids and phenylalanine, with excess tyrosine. Phenylalanine kinetics were determined from a primed, continuous intravenous infusion of L-[1-14C]phenylalanine. Phenylalanine oxidation, estimated from the rate of 14CO2 released in expired air during isotope infusion, decreased (P < 0.05) as threonine intake increased from 0.05 to 0.15 g/(kg.d) and was low and constant for threonine intakes >0.15 g/(kg.d). Using breakpoint analysis with 95% confidence interval (CI), mean requirement and safe level of parenteral threonine intake were estimated to be 0.19 and 0.21 g/(kg. d), respectively (equivalent to 13 and 14 mg/g amino acids, respectively). To compare these data with those of orally fed controls, we then repeated the experiment by infusing identical diets intragastrically to piglets (n = 25); the varying dietary threonine intakes were 0.1-1.2 g/(kg.d). Employing identical kinetics and analyses, the mean requirement and safe level of oral threonine intake were estimated to be 0.42 and 0.51 g/(kg.d), respectively (equivalent to 28 and 34 mg/g amino acids, respectively). These data demonstrate that the threonine requirement of neonates during TPN is approximately 45% of the mean oral requirement.

Nutrition and cystic fibrosis

Cystic fibrosis is commonly associated with energy deficiency in children and adults. Chronic undernutrition will lead to failure to thrive, wasting, and stunting of linear growth; nutrition and survival are intimately related in cystic fibrosis. These problems can simply be considered as energy imbalance, and management centers on restoration of energy balance. Specific nutrient deficiencies, such as fat soluble vitamins, are common in cystic fibrosis. Recent work has highlighted problems with bone density and also the prevalence of vitamin K deficiency. Management of nutritional problems can be complex, and injudicious treatment can further worsen the situation, as the relationship between high daily doses of pancreatic enzymes and the development of fibrosing colonopathy illustrates.

Determination of amino acid requirements by indicator amino acid oxidation: applications in health and disease

Indispensable amino acid requirements for children, patients and pregnant women are largely unknown. We describe a new, non-invasive technique that can estimate requirements in vulnerable populations. Future applications will lead to optimal nutritional care for populations in which an appropriate balance of amino acids is essential for metabolic health and growth.

Tryptophan requirement in young adult women as determined by indicator amino acid oxidation with L-[13C]phenylalanine

To determine tryptophan requirements with use of the indicator amino acid oxidation technique, we studied 5 healthy, premenopausal women who each consumed 6 different intakes of tryptophan. The energy-sufficient diet contained 1.0 g x kg(-1) x d(-1) of an L-amino acid mixture patterned after egg protein, with adequate phenylalanine (14 mg x kg(-1) x d(-1)) and excess tyrosine (40 mg x kg(-1) x d(-1)). The effect of graded alterations in dietary tryptophan on phenylalanine flux and oxidation was studied by using L-[1-(13)C]phenylalanine as the indicator amino acid. Graded increases in tryptophan had no effect on phenylalanine flux. Determined by two-phase linear regression crossover analysis, the mean tryptophan requirement based on indicator (phenylalanine) oxidation was 4.01 mg x kg(-1) x d(-1), with a safe intake at 5.02 mg x kg(-1) x d(-1). These results show that the indicator amino acid oxidation technique can be used to determine the requirement for amino acids such as tryptophan that cannot be successfully studied by direct oxidation because of their characteristic metabolism. Our value of safe intake for 95% of the population was found to be 43% higher than that reported previously for adults.

Evidence that phenylalanine hydroxylation rates are overestimated in neonatal subjects receiving total parenteral nutrition with a high phenylalanine content

Recent publications have indicated that the parenterally fed neonate has a substantial ability to hydroxylate phenylalanine. Examination of these data suggests that, at high phenylalanine intakes, estimated rates of hydroxylation exceed rates of intake. This implies significant net tissue breakdown. However, the quantitative validity of the estimates of phenylalanine hydroxylation cannot be assessed without nitrogen balance data. We have recently developed a parenterally fed neonatal piglet model and have used this to study aromatic amino acid metabolism in piglets fed different amino acid solutions. Reappraisal of the data from these studies has allowed us to estimate both phenylalanine hydroxylation and tissue protein accretion. Piglets were parenterally fed Vamin [292 micromol of Phe x kg(-1) x h(-1), 26 micromol of Tyr x kg(-1) x h(-1)], Vaminolact + Phe [VLP, 277 micromol of Phe x kg(-1) x h(-1), 26 micromol Tyr x kg(-1) x h(-1)], or Vaminolact + glycyl-L-tyrosine [VLGT, 152 micromol of Phe x kg(-1) x h(-1), 159 micromol of Tyr x kg(-1) x h(-1)] for 8 d. Nitrogen balance was measured over the last 5 study d, and aromatic amino acid kinetics were determined using a primed continuous infusion of L-[1-4C]phenylalanine on d 8. Average body protein gain, derived from nitrogen balance, was 11 g x kg(-1) x d(-1). For the Vamin and VLP groups, the rates of phenylalanine hydroxylation were estimated to be 139 and 90% of intake, respectively. However, phenylalanine hydroxylation was only 16% of intake for the VLGT group. In view of the tissue protein accretion data, it appears that the rate of phenylalanine hydroxylation may be overestimated in neonates fed high phenylalanine parenteral nutrition. The extent to which the parenterally fed neonate can adapt to a high phenylalanine intake, by increasing the rate of phenylalanine hydroxylation, remains to be determined.

Effect of exogenous insulin on protein metabolism with differing nonprotein energy intakes in Type 2 diabetes mellitus

OBJECTIVE

To determine if insulin treatment combined with a generous protein intake would normalize whole-body protein kinetics and nitrogen balance in obese subjects with Type 2 diabetes mellitus when compared to obese nondiabetic subjects: 1) during weight-maintenance and 2) after a very low energy diet (VLED).

DESIGN

Clinical intervention study of iso- followed by hypoenergetic feedings with or without exogenous insulin.

SUBJECTS

Sixteen obese subjects with a body mass index (BMI) of 39+/-4 kg/m2, with Type 2 diabetes mellitus (three men, six women) or without (one man, six women).

MEASUREMENTS

Nitrogen flux rate calculated from the urine 15N-urea enrichment by using the 60 h oral 15N-glycine method, rates of protein synthesis and breakdown calculated from nitrogen flux on days 6-8 (and 13-15 in the diabetic subjects) of isoenergetic feeding and days 24-26 of a 1.9 MJ diet.

RESULTS

With insulin therapy: 1) during isoenergetic feeding, in the hyperglycaemic diabetic subjects, nitrogen balance was significantly less than in the obese controls (-0.6+/-0.6 compared with +1.8+/-0.9 g N/d, P = 0.037) but became positive (+2.6+/-0.6 g N/d, P < 0.05); nitrogen flux decreased and net protein synthesis increased from values different from those of the obese controls to values no longer different; 2) during the VLED, plasma glucose concentrations < 7 mmol/L were achieved and maintained in all diabetic subjects. Nitrogen equilibrium observed in five out of seven obese nondiabetic and four out of nine diabetic subjects was associated with no change in nitrogen flux from the euglycaemic isoenergetic studies, but with 17% and 23% lower rates of synthesis (P < 0.05) and 7% and 15% lower rates of breakdown (NS) in nondiabetic and diabetic subjects, respectively.

CONCLUSION

Sufficient exogenous insulin to near-normalize glycaemia improves the altered protein metabolism in hyperglycaemic diabetic subjects during isoenergetic feeding, and restores nitrogen equilibrium better than with VLED alone. Protein metabolism is more sensitive to the state of diabetes control than is generally appreciated 'clinically'.

Free amino acids in milks of human subjects, other primates and non-primates

Preterm and term transitional milks of human subjects and mature milks of human subjects, non-human primates and non-primates were analysed for free amino acids (AA) using precolumn phenylisothiocyanate derivatization and liquid chromatography. Differences in free AA between three types of human milk were small. Milks of pinnipeds (seals and sea lions) contained the highest levels of total free AA (8634-20,862 mumol/l), while the milks of cows and sheep had the lowest levels of total free AA (1061-1357 mumol/l). The milks of human subjects, chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla), elephants (Elephas maximus), horses and pigs had intermediate levels of total free AA (3069-7381 mumol/l). Glutamic acid was the most abundant free AA in milks of human subjects (1339-2157 mumol/l), non-human primates (423-2528 mumol/l), elephants (1332 mumol/l), horses (1119 mumol/l), and cows (349 mumol/l). Taurine was the most abundant free AA in milks of pinnipeds (5776-13,643 mumol/l), pigs (1238 mumol/l), goats (1150 mumol/l) and sheep (341 mumol/l). Taurine was the second most abundant free AA in milks of human subjects and non-human primates, while histidine was the second most abundant free AA in milks of pinnipeds. Milks of each species had a distinctive free AA pattern which may reflect the relative importance of the free AA during early postnatal development.