Increasing dietary palmitic acid decreases fat oxidation and daily energy expenditure

BACKGROUND

Oleic acid (OA) is oxidized more rapidly than is palmitic acid (PA).

OBJECTIVE

We hypothesized that changing the dietary intakes of PA and OA would affect fatty acid oxidation and energy expenditure.

DESIGN

A double-masked trial was conducted in 43 healthy young adults, who, after a 28-d, baseline, solid-food diet (41% of energy as fat, 8.4% as PA, and 13.1% as OA), were randomly assigned to one of two 28-d formula diets: high PA (40% of energy as fat, 16.8% as PA, and 16.4% as OA; n = 21) or high OA (40% of energy as fat, 1.7% as PA, and 31.4% as OA; n = 22). Differences in the change from baseline were evaluated by analysis of covariance.

RESULTS

In the fed state, the respiratory quotient was lower (P = 0.01) with the high OA (0.86 +/- 0.01) than with the high-PA (0.89 +/- 0.01) diet, and the rate of fat oxidation was higher (P = 0.03) with the high-OA (0.0008 +/- 0.0001) than with the high-PA (0.0005 +/- 0.0001 mg . kg fat-free mass(-1) . min(-1)) diet. Resting energy expenditure in the fed and fasting states was not significantly different between groups. Change in daily energy expenditure in the high-OA group (9 +/- 60 kcal/d) was significantly different from that in the high-PA group (-214 +/- 69 kcal/d; P = 0.02 or 0.04 when expressed per fat-free mass).

CONCLUSIONS

Increases in dietary PA decrease fat oxidation and daily energy expenditure, whereas decreases in PA and increases in OA had the opposite effect. Increases in dietary PA may increase the risk of obesity and insulin resistance.

Diarrhea in Severely Burned Children

BACKGROUND

Diarrhea is a common problem in critically ill patients. Our patients are fed a high-carbohydrate enteral formula. We hypothesized that diarrhea in our patients may be related to the osmotic effects of unabsorbed carbohydrate in the small intestine and colon.

METHODS

We studied 19 patients, 3 months to 17 years, with burns >40% total body surface area. Each subject was studied weekly for up to 4 weeks postburn. Breath H2 concentration was measured. For the 24-hour period before the breath H2 measurement, the enteral carbohydrate intake, stool volume, and total enteral fluid volume were recorded. At each of several weekly intervals for each subject, the times when stool volume and enteral carbohydrate intake were each maximal were noted.

RESULTS

Maximal stool volume ranged from 12 to 69 mL/kg/d. At the time point of maximal carbohydrate intake, diarrhea (stool volume >10 mL/kg/d) occurred in 18 of 19 patients, and maximal stool volume occurred in 10 of 19. Breath H2 concentration (ppm/5% CO2; mean +/- SEM) was 5.5 +/- 3.5 at the time of maximal carbohydrate intake, and was 25 +/- 20 at maximal stool volume. There were no correlations among breath H2 concentration, stool volume, enteral fluid intake, and enteral carbohydrate intake.

CONCLUSIONS

Almost all the subjects had diarrhea over several weeks postburn. The lack of correlation of either carbohydrate intake or breath H2 with stool volume suggests that diarrhea in these patients may be caused by factors other than carbohydrate malabsorption. These data do not support altering nutrition support because of watery diarrhea.

Prediction of daily energy expenditure during a feeding trial using measurements of resting energy expenditure, fat-free mass, or Harris-Benedict equations

BACKGROUND

During feeding trials, it is useful to predict daily energy expenditure (DEE) to estimate energy requirements and to assess subject compliance.

OBJECTIVE

We examined predictors of DEE during a feeding trial conducted in a clinical research center.

DESIGN

During a 28-d period, all food consumed by 26 healthy, nonobese, young adults was provided by the investigators. Energy intake was adjusted to maintain constant body weight. Before and after this period, fat-free mass (FFM) and fat mass were assessed by using dual-energy X-ray absorptiometry, and DEE was estimated from the change (after - before) in body energy (DeltaBE) and in observed energy intake (EI): DEE = EI - DeltaBE. We examined the relation of DEE to pretrial resting energy expenditure (REE), FFM, REE derived from the average of REE and calculated from FFM [REE = (21.2 x FFM) + 415], and an estimate of DEE based on the Harris-Benedict equation (HB estimate) (DEE = 1.6 REE).

RESULTS

DEE correlated (P < 0.001) with FFM (r = 0.78), REE (r = 0.73), average REE (r = 0.82), and the HB estimate (r = 0.81). In a multiple regression model containing all these variables, R(2) was 0.70. The mean (+/-SEM) ratios of DEE to REE, to average REE, and to the HB estimate were 1.86 +/- 0.06, 1.79 +/- 0.04, and 1.02 +/- 0.02, respectively.

CONCLUSIONS

Although a slightly improved prediction of DEE is possible with multiple measurements, each of these measurements suggests that DEE equals 1.60-1.86 x REE. The findings are similar to those of previous studies that describe the relation of REE to DEE measured directly.

Moderate disaccharide malabsorption does not affect weight gain and cecal cell proliferation in piglets

BACKGROUND

Intestinal malabsorption of disaccharides or oligosaccharides occurs normally in preterm infants, the majority of the world's adult population when ingesting dairy products, and in sick patients with abnormal intestinal motility or intestinal damage. Although there may be benefit to colonic fermentation, there also could be mucosal damage from excessive fermentation. Moreover, excess fluid in the colon also may impair fermentation. There are few data that address the effects of moderate amounts of carbohydrate, such as 50% of intake, reaching the colon. We addressed the hypothesis that normal weight gain and stool characteristics would occur during disaccharide malabsorption approximating 50% of intake.

METHODS

Twelve piglets (aged 17 days) were randomized to receive control sow milk replacement formula (CON; lactose 60 g/L) or CON modified so that lactulose replaced 50% of the lactose (LAC). During a 5- to 7-day period, weight gain and fecal characteristics were observed before autopsy for cecal histology and determination of cecal cell proliferation using bromodeoxyuridine.

RESULTS

Neither group developed diarrhea. In LAC and CON, formula intakes (g/kg/d) were similar (279 +/- 30 and 280 +/- 35), as were the ratios of weight gain:formula intake (g/mL): 0.23 +/- 0.04 and 0.22 +/- 0.04. Mean cecal cell proliferation was not diminished in the group receiving lactulose (p > .05).

CONCLUSION

These data suggest that moderate (50%) disaccharide malabsorption per se does not cause diarrhea and does not impair dietary energy use or colonic cell proliferation.

Physical activity in middle school-aged children participating in a school-based recreation program

BACKGROUND

The increasing incidence of obesity in children may be attributed in part to increased sedentary behavior, such as watching television, which leads to less energy expended in physical activity. We have theorized that by middle school, many children lack the physical skills or self-confidence to participate in competitive physical activities. Thus, we hypothesized that if we provided a summer and after-school program featuring noncompetitive, outdoor activities such as gardening and adventure education, we would observe increased physical activity relative to habitual physical activities at home.

METHODS

To test this hypothesis, 2 experiments were conducted. In the first, 4 children aged 12 years were evaluated while they participated in a summer recreation program for 2 hours and again while they watched a videotape. They wore a uniaxial accelerometer to assess physical movement, and we used a bicarbonate labeled with 13C tracer technique to assess energy expenditure. In a second experiment, we evaluated 8 children (aged 10-12 years) twice using uniaxial accelerometry only, once while they attended the after-school program for 2 hours and then during a similar period at home.

RESULTS

The first study showed that the estimated energy expenditure (kilocalories. kilograms-1. hours-1) was 60% increased during the program (mean +/- SD) (2.6 +/- 0.5) compared with watching a videotape (1.6 +/- 0.3) (P =.02). Physical movement (accelerations per minute) also was significantly increased (3959 +/- 896 vs 513 +/- 182) (P =.004). In the second experiment, movement was 95% increased during the program (4578 +/- 1004) compared with the behavior at home (2345 +/- 746) (P =.005).

CONCLUSION

These results show that an organized, noncompetitive, leisure-time program can increase physical activity in children.

Quantitation of colonic luminal synthesis of butyric acid in piglets

OBJECTIVES

Butyric acid, synthesized via bacterial fermentation in colonic lumen, may play an important role in the nutrition of the colonic mucosa. Since disaccharides, especially lactose, are the principal dietary carbohydrates during infancy, it is important to determine if their fermentation is associated with butyric acid synthesis. The objective of this paper is to describe a newly developed stable isotope method for quantifying butyric acid synthesis in the colonic lumen and to demonstrate its application during cecal infusions of lactose and lactulose in piglets.

METHODS

Nine piglets aged 21 to 30 days were studied during acute anesthesia. The C enrichment of butyric acid was monitored in the portal vein before and during a 120 minutes cecal infusion of [1- C]-butyric acid and either unlabeled lactose (N = 4) or lactulose (N = 5).

RESULTS

The luminal synthesis of BA (micromol x kg x min ) (Mean +/- S.D.) was respectively 1.5 +/- 0.9 and 1.2 +/- 0.6 during lactulose and lactose infusion.

CONCLUSIONS

This study provides new quantitative data on in vivo butyric acid production in the mammalian colon.

Complete biotinidase deficiency presenting as reversible progressive ataxia and sensorineural deafness

Most symptomatic patients with biotinidase deficiency have both neurologic and cutaneous symptoms and typical organic aciduria. We encountered a previously healthy girl with complete biotinidase deficiency presenting initially at age 17 months with episodic ataxia that became severe progressive ataxia in 2 months, but without skin rash or typical organic aciduria, which resolved completely with biotin treatment. Additionally, moderate sensorineural deafness also improved to the normal range. Even without typical cutaneous findings or organic aciduria, biotinidase deficiency should be considered among the differential diagnosis in any child presenting with either episodic or progressive ataxia or sensorineural deafness as prompt diagnosis and treatment with biotin may induce an excellent recovery.

Evaluation of methods to assess physical activity in free-living conditions

PURPOSE

The purpose of this study was to compare different methods of measuring physical activity (PA) in women by the doubly labeled water method (DLW).

METHODS

Thirteen subjects participated in a 7-d protocol during which total daily energy expenditure (TDEE) was measured with DLW. Body composition, basal metabolic rate (BMR), and peak oxygen consumption were also measured. Physical activity-related energy expenditure (PAEE) was then calculated by subtracting measured BMR and the estimated thermic effect of food from TDEE. Simultaneously, over the 7 d, PA was assessed via a 7-d Physical Activity Recall questionnaire (PAR), and subjects wore secured at the waist, a Tritrac-R3D (Madison, WI), a Computer Science Application Inc. activity monitor (CSA; Shalimar, FL), and a Yamax Digi Walker-500 (Tokyo, Japan). Pearson-product moment correlations were calculated to determine the relationships among the different methods for estimating PAEE. Paired t-tests with appropriate adjustments were used to compare the different methods with DLW-PAEE.

RESULTS

There was no significant difference between PAEE determined from PAR and DLW. The differences between the two methods ranged from -633 to 280 kcal.d(-1). Compared with DLW, PAEE determined from CSA, Tritrac, and Yamax was significantly underestimated by 59% (-495 kcal.d(-1)), 35% (-320 kcal.d(-1)) and 59% (-497 kcal.d(-1)), respectively. VO2peak explained 43% of the variation in DLW-PAEE.

CONCLUSION

Although the group average for PAR-PAEE agreed with DLW-PAEE, there were differences in the methods among the subjects. PAEE determined by Tritrac, CSA, and Yamax significantly underestimate free-living PAEE in women.

Butyric acid is synthesized by piglets

We hypothesized that there is no synthesis of butyric acid within organs or tissues not drained by the portal vein (PV). Two experiments were performed. In six piglets, the colonic vasculature was clamped (n = 4) or the entire colon resected while [1-13C]-butyric acid (99% enriched) was infused into a jejunal vein for 120 min; 13C enrichment of butyric acid was measured in the PV and carotid artery (ART) during the last 30 min of the infusion. In a second experiment, butyric acid tracer and unlabeled disaccharide were infused into the cecum for 120 min, and blood again was sampled from the PV and ART. For the four piglets studied during ligation of the colonic vasculature, the mean (+/- SD) ratio of the butyric acid enrichment in the ART to that in the PV (ART/PV) was 0.80+/-0.05 (ART vs. PV, P = 0.002) and for all six piglets in expt. 1, the ART/PV ratio was 0.74+/-0.1 (ART vs. PV, P = 0.001). The enrichment of butyric acid in the PV averaged 96.0% for the six studies, implying that splanchnic tissues other than the colon did not produce a substantial amount of butyric acid. For the second experiment, the ART/PV ratio was 0.80+/-0.15 (ART vs. PV, P = 0.03). These studies provide the first evidence for endogenous synthesis of butyric acid by piglets.

Fractional oxidation of chylomicron-derived oleate is greater than that of palmitate in healthy adults fed frequent small meals

Differences in oxidation of individual dietary fatty acids could contribute to the effect of dietary fat composition on risk factors for non-insulin-dependent diabetes mellitus and cardiovascular disease. Using a novel stable isotope technique, we compared fractional oxidation of chylomicron-derived oleate and palmitate in 10 healthy adults in a crossover study. 1-(13)C-labeled oleate or palmitate was emulsified into a eucaloric formula diet administered each 20 min for 7 h to produce a plateau in excretion of (13)C label in breath CO(2). Unlabeled oleate and palmitate each provided 16% of dietary energy, and other fatty acids provided 8% of energy. Total dietary fat was 40% of energy, carbohydrate was 46%, and protein was 14%. Diet without tracer was fed for 2 h before beginning tracer administration to establish a baseline fed state. Relative oxidation of oleate versus palmitate was defined as fractional oxidation of oleate divided by fractional oxidation of palmitate. Relative oxidation averaged 1.21 (99.5% confidence interval = 1.03;-1.39), indicating that fractional oxidation of oleate was significantly greater than that of palmitate.

Lactulose feeding in piglets: a model for persistent diarrhea and colitis induced by severe sugar malabsorption

We sought to determine, in a piglet model, whether severe sugar malabsorption causes colonic injury or inflammation. Twenty-four piglets were randomized to receive either control formula (CON) or CON supplemented with lactulose (LAC) (N = 12 each group). After seven days, inflammation, apoptosis, and crypt cell proliferation were assessed in the proximal colon (cecum). Lactulose feeding caused persistent diarrhea. In both groups, breath H2 concentration was low, suggesting no increased fermentation in the LAC group. Weight gain/volume formula intake was identical in the CON and LAC groups (0.09+/-0.13 and 0.09+/-0.11 g/ml) respectively. Injury to the colon did not occur, but inflammation of the colon (scale 0-5) was greater in LAC (score of 1.5+/-1.38) than in CON (0.42+/-0.79; P<0.05). Cell proliferation at the basal 40% of the crypt was 92% increased in CON (labeling index 22.8+/-9.9 vs. 11.9+/-2.8; P<0.05). We conclude that persistent feeding during severe sugar malabsorption permits weight gain but may cause colitis.

Splanchnic uptake of leucine in healthy children and in children with cystic fibrosis

Interpretation of tracer studies of amino acid kinetics in the fed state is dependent on knowledge of splanchnic uptake of diet-derived amino acids. We studied five healthy control children and five children with cystic fibrosis (CF). After an overnight fast, the children ingested, hourly, a formula diet for 11 h. 5,5,5-[2H3]Leucine was added to the feedings during the last 6 h, and an i.v. infusion of 1-[13C]leucine was administered during the last 2 h of the formula feeding. The mean rate of splanchnic uptake of leucine was similar in the CF and control group, 23.8 +/- 24.0 and 21.5 +/- 21.2 mumol.kg-1.h-1, respectively. Fractional splanchnic uptake of leucine was not significantly different in the patients with CF (0.16 +/- 0.112 mean +/- SD) compared with the control children (0.244 +/- 0.256(-1)). The rate of whole body protein breakdown was not significantly different between the groups (CF versus control) with (159 +/- 18 versus 135 +/- 28 mumol.kg-1.h-1) or without (135 +/- 14 versus 114 +/- 20 mumol.kg-1.h-1) correction for splanchnic leucine uptake. However, for the 10 cases combined, protein breakdown corrected for splanchnic leucine uptake (147 +/- 26 mumol.kg-1.h-1) was 18% greater than uncorrected protein breakdown (124 +/- 20 mumol.kg-1.h-1) (p = 0.009). The data suggest that companion studies of splanchnic uptake might enhance the interpretation of leucine kinetics in the fed state.

Differential impact of obesity on glucose metabolism in black and white American adolescents

BACKGROUND

The authors have previously demonstrated abnormalities in glucose and insulin metabolism in nondiabetic black American (BA) adults versus white American (WA) adults. Whether similar glucoregulatory alterations extend to BA adolescents remain unknown. In addition, obesity, a known risk factor for insulin resistance and hyperinsulinemia, occurs in a greater proportion of BA adults and children when compared to WA. The objective of the present study was to examine the differential effects of obesity on glucose homeostasis in BA and WA adolescents.

METHODS

We examined glucose homeostasis in BA and WA adolescents using oral glucose tolerance test (OGTT), intravenous glucose tolerance test (IVGTT), and [6,6-2H2]-glucose infusion. The study consisted of four age-, sex-, and pubertal stage-matched groups: 15 lean BA, 29 lean WA, 7 obese BA, and 9 obese WA.

RESULTS

Both obese groups had significantly increased insulin and C-peptide area under the curve (AUC) during OGTT and IVGTT when compared to their same-race lean counterparts. During OGTT, obese BA demonstrated greater insulin and C-peptide when compared to obese WA. During IVGTT, first- and second-phase insulin were significantly greater in obese BA versus obese WA.

CONCLUSION

In summary, BA adolescents demonstrated insulin resistance which is markedly exaggerated in the face of obesity when compared to WA adolescents, implying a differential impact for obesity on glucose homeostasis that is unique to the obese BA adolescent group. In conclusion, there is a need for early aggressive weight management in obese BA adolescents.

Effects of lactose intake on lactose digestion and colonic fermentation in preterm infants

The purpose of this study was to determine whether doubling the lactose concentration in formula for preterm infants lowers the fraction of lactose digested and/or increases the fraction of lactose fermented. Six preterm infants, 31 to 36 weeks' postconceptional age, were fed a standard preterm formula (carbohydrate is 50% lactose and 50% glucose polymer)(SC) and/or the same volume of formula modified to contain lactose as the sole carbohydrate (LAC). Relative lactose digestion during the LAC formula feeding compared with SC formula feeding was measured by using a stable isotope approach for quantifying the fractional contribution of formula lactose to plasma glucose enrichment. Relative lactose digestion was 0.98 +/- 0.17 (range, 0.70 to 1.19). Fractional fermentation of lactose was estimated from breath H2 excretion (0.52 +/- 0.34 during LAC feeding and 0.23 +/- 0.22 during SC feeding, P = .11). The rate of breath H2 excretion was much higher with LAC (1.34 +/- 0.98 mL/h) than with SC (0.27 +/- 0.29, P = .029). In conclusion, doubling the lactose concentration had only modest effects on fractional lactose digestion. Increased breath H2 excretion with LAC may relate to fermentation of nonlactose sugar or to ill-defined changes in colonic physiology or motility, which could enhance colonic fermentation of malabsorbed sugar by H2-producing bacteria.

Leigh syndrome, cytochrome C oxidase deficiency and hypsarrhythmia with infantile spasms

A rare patient with infantile spasms, hypsarrhythmia, cytochrome c oxidase deficiency and Leigh syndrome is reported. Although rare, infantile spasms and Leigh syndrome may occur simultaneously. Leigh syndrome should be included in the differential diagnosis of infantile spasms.

Measuring energy costs of leisure activity in adolescents using a CO2 breath test

To determine whether a 13C-bicarbonate, isotope dilution technique could be used to estimate relative changes in energy expenditure of leisure activities of short duration, we studied eight adolescents who performed the following activities: watching television (120 min); playing a stringed instrument (60 min plus 60 min of sitting); and walking plus rest during two approximately isocaloric sessions (slow walk at 40% of peak VO2 for 43 min plus 77 min of sitting; fast walk at 73% of peak VO2 for 22 min plus 98 min of sitting). The rate of appearance of CO2 (RaCO2) was determined from the ratio of the oral dose of 13C-bicarbonate and the isotopic enrichment of breath CO2. The net rates of excretion of CO2 (VCO2) and oxygen consumption were measured. VCO2 and RaCO2, were correlated (r = 0.93; P < 0.05). To adjust for the systematic difference in CO2 production between methods, determinations were expressed as a fraction of that during television viewing. For RaCO2, the ratios for instrument playing, walking at 40% peak VCO2, and walking at 73% peak VO2 were respectively 133 +/- 20%, 186 +/- 38%, and 206 +/- 34%; for VCO2, the respective ratios were 129 +/- 19, 210 +/- 50, and 232 +/- 39 (P > 0.05 for methods and interaction, two-way ANOVA). RaCO2 may be a useful method for detecting relative differences in energy expenditure associated with leisure activities of brief duration.

Technical note: pig model for studying nutrient assimilation by the intestine and colon

We have developed a system for chronically catheterizing 10- to 25-d-old pigs that permits stable isotope tracer studies of intestinal or colonic assimilation of nutrients. This model also can be used to ensure constant enteral feeding or to assess the rate of entry into the terminal ileum of carbohydrates, fats, and amino acids. A plastic cannula with a luminal flange can be surgically placed in the stomach for tracer studies of sugar digestion or for controlled infusion of any formula diet. A similar cannula can be placed in the cecum for infusion of tracer and(or) substrates for studies of fermentation. The cannula has been machined so that a washer and nut can be threaded onto it, allowing the entire apparatus to be fixed to the abdominal wall. The distal end protruding above the skin was tapered to fit standard i.v. extension tubing. A carotid arterial catheter was used to sample substrates for isotopic enrichment measurements.

Measurement of the rate of entry of intact colon-derived lactose into the circulation: a model for assessing gut uptake of molecules not endogenously synthesized

BACKGROUND

Results of in vitro studies have documented colonic absorption of lactose in the newborn. A stable isotope model was developed for assessing the entry rate of intact lactose into the portal circulation in newborn piglets.

METHODS

In experiment 1, unlabeled and [D-1-(13C)]-lactose were infused into two separate mesenteric veins, and in experiment 2, labeled lactose was infused into a mesenteric vein and unlabeled lactose was infused into the colon. The 13C-enrichment of plasma lactose was assessed by high performance liquid chromatography gas chromatography combined with mass spectrometry.

RESULTS

The isotopic estimate of the mesenteric venous infusion rate of lactose was 91% of the theoretical. In the second experiment 13% of the unlabeled lactose infused into the colon reached the portal circulation.

CONCLUSIONS

The current study provides the first, direct, in vivo confirmation of colon absorption of intact lactose. The tracer model could be used to evaluate intestinal or colonic absorption of other organic compounds not endogenously synthesized, including vitamins or drugs.

Stable isotope model for assessing production of short chain fatty acids from colon-derived sugar: application in pigs

Sugar reaching the colon because of intestinal maldigestion or malabsorption may be fermented to acetate and other short-chain fatty acids, resulting in stimulation of colonic water absorption and cell proliferation. To explore this phenomenon in more detail, we have developed a stable isotope model for estimating the fraction of colon-derived glucose or lactose that is fermented to acetate, propionate and butyrate. In an initial application of the model, [d3]-acetate and either [1-(13)C]-glucose or [D-1-(13)C]-lactose were infused into the cecum or colon of piglets, and plateau plasma acetate enrichment was monitored in the carotid artery. In acutely anesthetized piglets, the fractions of glucose and lactose fermented to acetate were 17.0 and 20.0%, respectively. In a chronically catheterized piglet, fermentation was higher (34.2%). When conducted in chronically catheterized animals or via a colostomy or ileostomy in infants, this model may be used to determine how age, previous surgery or antibiotic therapy affects the efficiency of colonic assimilation of carbohydrate.

Estimation of CO2 production in enterally fed preterm infants using an isotope dilution stable tracer technique

BACKGROUND

Estimates of the rate of CO2 production may be useful in preterm infants, but assessment of the rate of respiratory excretion of CO2 (VCO2) may not always be practical in infants requiring constant care. We hypothesized that the rate of dilution of 13CO2 (RaCO2) would be a valid index of CO2 production in preterm infants.

METHODS

Twelve studies of RaCO2 and VCO2 were performed in six enterally fed preterm infants. RaCO2 was measured using a 2-hour, primed, constant, orogastric infusion of NaH13CO3 with formula and an assessment of the plateau 13C enrichment of expired CO2. VCO2 was measured over two 10-minute intervals during the infusion using a flow-through system. Energy expenditure was estimated from these data and the food quotient.

RESULTS

Mean (+/- SD) rate of CO2 production using RaCO2 (348 +/- 32 mumol/kg/min) was 114% of that estimated using VCO2 (304 +/- 51 mumol/kg/min). The ratio of VCO2/RaCO2 is equal to the fractional recovery of tracer CO2 in the expired air during the course of the tracer infusion. In studies of short duration, this ratio is generally less than 100% because of isotope exchange. For five pairs of studies performed on consecutive days, each individual value of RaCO2 on day 2 was multiplied by the mean of the individual ratios of VCO2/RaCO2 on day 1 (0.78); corrected RaCO2 was 306 +/- 19 mumol/kg/min compared with 307 +/- 59 mumol/kg/min for VCO2.

CONCLUSIONS

Thus, RaCO2, particularly when corrected for isotope recovery, may be a useful index of group mean CO2 production and energy expenditure in preterm infants.

In vivo lactose digestion in preterm infants

In vitro studies of intestinal lactase activity and breath-hydrogen studies have suggested that the capacity for lactose digestion in preterm infants is less than the usual intake. To explore this question using an in vivo approach, we determined the fraction of dietary lactose hydrolyzed to glucose (and galactose) in 14 preterm infants with a gestational age of 26-31 wk at the time of birth but a postconceptional age of 31-37 wk at the time of study. The percentage of lactose digested was estimated after 6-h, primed, constant gastric infusions of [1-(13)C]glucose and D-[-1-(13)C]lactose on alternate days. A coefficient of lactose fermentation was derived from the rates of pulmonary excretion of hydrogen and carbon dioxide. Mean (+/- SD) lactose digestion was 79 +/- 26%. There was a significant inverse rank (r = -0.799, P < 0.01) and linear (r = -0.587, P < 0.05) correlation between this variable and postconceptional age. The percentage of lactose fermented averaged 35 +/- 27%.