Limits of adaptation to a diet low in protein in normal man: urea kinetics

A model to predict the ATP equivalents of macronutrients absorbed from food

Calculating the physiologically available energy of food at the cellular level (ATP), based on known stoichiometric relationships and predicted nutrient uptake from the human digestive tract may be more accurate than using currently available factorial or empirical models for estimating dietary energy. The objective was to develop a model that can be used for describing the ATP costs/yields associated with the total tract uptake of the energy-yielding nutrients for an adult human in a state of weight loss (sub-maintenance energy intakes). A series of predictive equations for determining ATP yields/costs were developed and applied to the uptake of each energy-yielding nutrient, as predicted separately in the upper-digestive tract and the hindgut using a dual in vivo-in vitro digestibility assay. The costs associated with nutrient ingestion, absorption and transport and with the synthesis and excretion of urea produced from amino acid catabolism were calculated. ATP yields (not including costs associated with digestion, absorption and transport) were predicted as 28.9 mol ATP per mol glucose; 4.7-32.4 mol ATP per mol amino acid and 10.1 mol ATP per mol ethanol, while yields for fatty acids ranged from 70.8 mol ATP per mol lauric acid (C12) to 104 mol ATP per mol linolenic acid (C18 : 3). The energetic contribution of hindgut fermentation was predicted to be 101.7 mmol ATP per g organic matter fermented. The model is not proposed as a new system for describing the energy value of foods in the diet generally, but is a means to give a relative ranking of foods in terms of physiologically available energy (ATP) with particular application in the development of specialised weight-loss foods.

Urea nitrogen salvage mechanisms and their relevance to ruminants, non-ruminants and man

Maintaining a correct balance of N is essential for life. In mammals, the major sources of N in the diet are amino acids and peptides derived from ingested proteins. The immediate endproduct of mammalian protein catabolism is ammonia, which is toxic to cells if allowed to accumulate. Therefore, amino acids are broken down in the liver as part of the ornithine-urea cycle, which results in the formation of urea - a highly soluble, biochemically benign molecule. Mammals cannot break down urea, which is traditionally viewed as a simple waste product passed out in the urine. However, urea from the bloodstream can pass into the gastrointestinal tract, where bacteria expressing urease cleave urea into ammonia and carbon dioxide. The bacteria utilise the ammonia as an N source, producing amino acids and peptides necessary for growth. Interestingly, these microbial products can be reabsorbed back into the host mammalian circulation and used for synthetic processes. This entire process is known as 'urea nitrogen salvaging' (UNS). In this review we present evidence supporting a role for this process in mammals - including ruminants, non-ruminants and man. We also explore the possible mechanisms involved in UNS, including the role of specialised urea transporters.

Dietary protein affects urea transport across rat urothelia

Recent evidence suggests that regulated solute transport occurs across mammalian lower urinary tract epithelia (urothelia). To study the effects of dietary protein on net urothelial transport of urea, creatinine, and water, we used an in vivo rat bladder model designed to mimic physiological conditions. We placed groups of rats on 3-wk diets differing only by protein content (40, 18, 6, and 2%) and instilled 0.3 ml of collected urine in the isolated bladder of anesthetized rats. After 1 h dwell, retrieved urine volumes were unchanged, but mean urea nitrogen (UN) and creatinine concentrations fell 17 and 4%, respectively, indicating transurothelial urea and creatinine reabsorption. The fall in UN (but not creatinine) concentration was greatest in high protein (40%) rats, 584 mg/dl, and progressively less in rats receiving lower protein content: 18% diet, 224 mg/dl; 6% diet, 135 mg/dl; and 2% diet, 87 mg/dl. The quantity of urea reabsorbed was directly related to a urine factor, likely the concentration of urea in the instilled urine. In contrast, the percentage of instilled urea reabsorbed was greater in the two dietary groups receiving the lowest protein (26 and 23%) than in those receiving higher protein (11 and 9%), suggesting the possibility that a bladder/urothelial factor, also affected by dietary protein, may have altered bladder permeability. These findings demonstrate significant regulated urea transport across the urothelium, resulting in alteration of urine excreted by the kidneys, and add to the growing evidence that the lower urinary tract may play an unappreciated role in mammalian solute homeostasis.

The mysteries of nitrogen balance

The first part of this review is concerned with the balance between N input and output as urinary urea. I start with some observations on classical biochemical studies of the operation of the urea cycle. According to Krebs, the cycle is instantaneous and automatic, as a result of the irreversibility of the first enzyme, carbamoyl-phosphate synthetase 1 (EC6.3.5.5; CPS-I), and it should be able to handle many times the normal input to the cycle. It is now generally agreed that acetyl glutamate is a necessary co-factor for CPS-1, but not a regulator. There is abundant evidence that changes in dietary protein supply induce coordinated changes in the amounts of all five urea-cycle enzymes. How this coordination is achieved, and why it should be necessary in view of the properties of the cycle mentioned above, is unknown. At the physiological level it is not clear how a change in protein intake is translated into a change of urea cycle activity. It is very unlikely that the signal is an alteration in the plasma concentration either of total amino-N or of any single amino acid. The immediate substrates of the urea cycle are NH3and aspartate, but there have been no measurements of their concentration in the liver in relation to urea production. Measurements of urea kinetics have shown that in many cases urea production exceeds N intake, and it is only through transfer of some of the urea produced to the colon, where it is hydrolysed to NH3, that it is possible to achieve N balance. It is beginning to look as if this process is regulated, possibly through the operation of recently discovered urea transporters in the kidney and colon. The second part of the review deals with the synthesis and breakdown of protein. The evidence on whole-body protein turnover under a variety of conditions strongly suggests that the components of turnover, including amino acid oxidation, are influenced and perhaps regulated by amino acid supply or amino acid concentration, with insulin playing an important but secondary role. Molecular biology has provided a great deal of information about the complex processes of protein synthesis and breakdown, but so far has nothing to say about how they are coordinated so that in the steady state they are equal. A simple hypothesis is proposed to fill this gap, based on the self-evident fact that for two processes to be coordinated they must have some factor in common. This common factor is the amino acid pool, which provides the substrates for synthesis and represents the products of breakdown. The review concludes that although the achievement and maintenance of N balance is a fact of life that we tend to take for granted, there are many features of it that are not understood, principally the control of urea production and excretion to match the intake, and the coordination of protein synthesis and breakdown to maintain a relatively constant lean body mass.

Amino acid availability: aspects of chemical analysis and bioassay methodology

It is important to be able to characterise foods and feedstuffs according to their available amino acid contents. This involves being able to determine amino acids chemically and the conduct of bioassays to determine amino acid digestibility and availability. The chemical analysis of amino acids is not straightforward and meticulousness is required to achieve consistent results. In particular and for accuracy, the effect of hydrolysis time needs to be accounted for. Some amino acids (for example, lysine) can undergo chemical modification during the processing and storage of foods, which interferes with amino acid analysis. Furthermore, the modified amino acids may also interfere with the determination of digestibility. A new approach to the determination of available lysine using a modifiedin vivodigestibility assay is discussed. Research is required into other amino acids susceptible to structural damage. There is recent compelling scientific evidence that bacterial activity in the small intestine of animals and man leads to the synthesis and uptake of dietary essential amino acids. This has implications for the accuracy of the ileal-based amino acid digestibility assay and further research is required to determine the extent of this synthesis, the source of nitrogenous material used for the synthesis and the degree of synthesis net of amino acid catabolism. Although there may be potential shortcomings in digestibility assays based on the determination of amino acids remaining undigested at the terminal ileum, there is abundant evidence in simple-stomached animals and growing evidence in human subjects that faecal-based amino acid digestibility coefficients are misleading. Hindgut microbial metabolism significantly alters the undigested dietary amino acid profile. The ileal amino acid digestibility bioassay is expected to be more accurate than its faecal-based counterpart, but correction of the ileal amino acid flow for amino acids of endogenous origin is necessary. Approaches to correcting for the endogenous component are discussed.

Urea kinetics of a carnivore,Felis silvestris catus

The effect of two levels of dietary protein energy, moderate (20 %; MP) and high (70 %; HP), on urea kinetics in eleven domestic cats was studied. After a 3-week prefeed, a single dose of [15N15N]urea was administered, and urine and faeces collected over the subsequent 5 d. For each 24 h period, total urea and enrichment of [15N15N]- and [15N14N]urea in urine were determined, and a model applied to calculate urea production, entry into the gastrointestinal tract, recycling to urine or faeces and, by difference, retention by the body and potentially available for anabolism. Urea production and excretion increased with dietary protein level (P<0·05). Most of the urea produced was excreted, with only a small proportion entering the gut, and with the pattern of urea disposal not significantly different between the HP and MP diets. Thus, the percentages of urea production available to the gut were 15 % (MP) and 12 % (HP), of which 57 % (MP) and 59 % (HP) was recycled in the ornithine cycle, 40 % (MP and HP) was potentially available for anabolism and the rest lost as faecal N. As a percentage of urea produced the amount potentially available for anabolism was very low at 6·41 % (MP diet) and 4·79 % (HP diet). In absolute terms urea entering the gut, being recycled in the ornithine cycle and potentially available for anabolism was significantly higher on the HP diet (P<0·05). These results show that cats operate urea turnover, but at a lower rate, and with less nutritional sensitivity than has been reported for other species.

Relationship between birth weight and urea kinetics in children

OBJECTIVE

To explore the effect of birth weight on urea kinetics in young healthy children.

DESIGN

Observational study.

SETTING

Tertiary center for treatment of malnutrition.

SUBJECTS

A total of 17 male children, 6-24 months old, who had recovered from malnutrition.

INTERVENTIONS

Urea kinetics were measured using stable isotope methodology with [(15)N(15)N]-urea over 36 h.

RESULTS

Birth weight was negatively related to urea hydrolysis after controlling for the intake of protein (adjusted R (2 ) = 0.91, P = 0.001) and separately for energy intake (adjusted R (2) = 0.95, P = 0.001), age (adjusted R (2) = 0.90, P = 0.001) and rate of weight gain (adjusted R (2) = 0.91, P = 0.001). There was a tendency for higher urea production in the children with lower birth weight after controlling for nitrogen intake (adjusted R (2) = 0.93, P = 0.099), and separately for age (adjusted R (2) = 0.94, P = 0.06) and rate of weight gain (adjusted (R (2) = 0.92, P = 0.096). Urea excretion was not significantly related to birth weight.

CONCLUSIONS

The salvaging of urea nitrogen following urea hydrolysis contributed significantly more to the nitrogen economy in children with lower birth weight compared to those with higher birth weight. This may be as a result of reductive adaptation in the children with lower birth weight as a consequence of inappropriate prenatal nutrition and growth.

Expression of the peptide transporter hPepT1 in human colon: a potential route for colonic protein nitrogen and drug absorption

Substrates of the proton-coupled peptide transporter, hPepT1, include dietary di- and tripeptides plus therapeutically important drugs such as the beta-lactam antibiotics and angiotensin-converting enzyme inhibitors. Expression and function of hPepT1 in the small bowel is well established. We have compared levels of hPepT1 mRNA expression in regions of human gut by RT-PCR methods and examined the expression of hPepT1 in normal human colon using an anti-hPepT1 antipeptide antibody. hPepT1 mRNA was expressed in the large intestine, although at lower levels than in the small intestine. Quantitatively, expression in ileum was 4.6-fold greater than in sigmoid colon. Immunoreactive hPepT1 was detected in human colon at lower levels than in ileum. The pattern of expression differed between the two tissues: whilst expression in the ileum was localised to the apical enterocyte membrane along the length of the crypt-villus axis, expression in the colonocyte was detected at the apical membrane towards the luminal surface but predominantly at the basal membrane towards the base of the crypt. We conclude that distal regions of the bowel express hPepT1, which may provide a mechanism for colonic protein-nitrogen absorption and for absorption of therapeutically important peptidomimetic drugs.

Vampire bat, shrew, and bear: comparative physiology and chronic renal failure

In the typical mammal, energy flux, protein metabolism, and renal excretory processes constitute a set of closely linked and quantitatively matched functions. However, this matching has limits, and these limits become apparent when animals adapt to unusual circumstances. The vampire bat and shrew have an extremely high protein intake, and the glomerular filtration rate (GFR) is not commensurate with the large urea load to be excreted. The vampire bat is chronically azotemic (blood urea concentration 27-57 mmol/l); yet there is no information as to how this animal has adjusted to such an azotemic internal environment. A high protein intake should also lead to chronic glomerular hyperfiltration; yet neither animal appears to develop progressive renal failure. The American black bear, on the other hand, has adapted to a prolonged period without intake or urine output. Despite continued amino acid catabolism with urea production, this mammal is able to completely salvage and reutilize urea nitrogen for protein synthesis, although the signals that initiate this metabolic adaptation are not known. The vampire bat, shrew, and bear are natural models adapted to circumstances analogous to chronic renal failure. Unraveling these adaptations could lead to new interventions for the prevention/treatment of chronic renal failure.

Endogenous glycine and tyrosine production is maintained in adults consuming a marginal-protein diet

BACKGROUND

The adequacy of indispensable amino acid supplies has received much attention in studies of protein requirements, but the availability of nitrogen for synthesis and maintenance of the supply of dispensable amino acids has been overlooked.

OBJECTIVE

We aimed to determine whether nitrogen balance and the endogenous supply of the dispensable amino acids glycine and tyrosine can be maintained with a marginal protein intake.

DESIGN

Phenylalanine, glycine, and tyrosine kinetics were measured in young adults (6 men, 6 women) on 4 occasions during a reduction in habitual protein intake (1.13 g x kg(-1) x d(-1)) to a marginal intake (0.75 g x kg(-1) x d(-1)) by using a multiple stable-isotope-infusion protocol.

RESULTS

During the 10-d period of marginal protein intake, nitrogen excretion fell initially, then remained constant such that nitrogen balance was negative for the first 2 d and then positive or zero thereafter. Whole-body protein degradation and synthesis predicted from phenylalanine kinetics declined significantly (P < 0.05) over the period of marginal protein intake. Despite the reduction in the amount of glycine and tyrosine derived from whole-body proteolysis, the fluxes of glycine and tyrosine were maintained.

CONCLUSIONS

The results show that adaptation to a marginal intake of dietary protein consisted of an overall reduction in whole-body protein turnover, net protein catabolism, and the rate of nitrogen excretion. The conserved nitrogen was sufficient to maintain the endogenous synthesis and hence the supply of glycine and tyrosine.

Substrate-induced regulation of the human colonic monocarboxylate transporter, MCT1

Butyrate is the principal source of energy for colonic epithelial cells, and has profound effects on their proliferation, differentiation and apoptosis. Transport of butyrate across the colonocyte luminal membrane is mediated by the monocarboxylate transporter 1 (MCT1). We have examined the regulation of expression of human colonic MCT1 by butyrate, in cultured colonic epithelial cells (AA/C1). Treatment with sodium butyrate (NaBut) resulted in a concentration- and time-dependent upregulation of both MCT1 mRNA and protein. At 2 mM butyrate, the magnitude of induction of mRNA (5.7-fold) entirely accounted for the 5.2-fold increase in protein abundance, and was mediated by both activation of transcription and enhanced mRNA stability. The other monocarboxylates found naturally in the colon, acetate and propionate, had no effect. The properties of butyrate uptake by AA/C1 cells were characteristic of MCT1. Induction of the MCT1 protein resulted in a corresponding increase in the maximal rate of butyrate transport. The V(max) for uptake of [U-(14)C]butyrate was increased 5-fold following pre-incubation with 2 mM NaBut, with no significant change in the apparent K(m). In conclusion, this study is the first to show substrate-induced regulation of human colonic MCT1. The basis of this regulation is a butyrate-induced increase in MCT1 mRNA abundance, resulting from the dual control of MCT1 gene transcription and stability of the MCT1 transcript. We suggest that butyrate-induced increases in the expression and resulting activity of MCT1 serve as a mechanism to maximise intracellular availability of butyrate, to act both as a source of energy and to influence processes maintaining cellular homeostasis in the colonic epithelium.

Rates of urea production and hydrolysis and leucine oxidation change linearly over widely varying protein intakes in healthy adults

The quantitative relationships between nitrogen (N) intake, urea production, excretion and amino acid oxidation are currently a matter of debate. Some investigators have proposed that urea production is essentially constant over a wide range of N intakes and that urea hydrolysis is regulated according to the N needs of the organism. We have assessed this proposal by compiling results from four separate experiments in healthy young adults (n = 34) carried out in our laboratories and all at the end of the respective diet periods using an identical 24-h continuous intravenous infusion of [(15)N, (15)N]urea and L-[1-(13)C]leucine. The N intakes were: expt. 1; protein-free diet for 5 d; expt. 2; N at 44 mg N. kg(-1). d(-1) from a balanced L-amino acid mixture for 13 d; expt. 3; N at 161 mg. kg(-1). d(-1) from egg protein for 6 d; expt. 4 -one group received 157 mg. kg(-1). d(-1) and the other 392 mg. kg(-1). d(-1) from milk-protein-based diets for 6 d. Urea production and excretion were linearly correlated with N intake (r = 0.98 and 0.94, respectively; P < 0.01). Urea hydrolysis increased linearly with N intake (r = 0.7; P < 0.05), with considerable variation in the rate among individuals, especially at the N intake of approximately 160 mg N. kg(-1)d(-1). These findings are consistent with the generally accepted view that a control of body N balance is via a regulation of urea production. They do not support the concept that urea hydrolysis is the more important site in the control of body N loss.

Protein metabolism in the extremely low-birth weight infant

Although extensive data are available on the impact of nutrient and protein administration on growth, plasma amino acids, and nitrogen balance in the newborn and growing infants, relatively few studies have carefully examined the dynamic aspects of protein metabolism in vivo and particularly in the micropremie or ELBW infant. These studies show that the very preterm infants, either because of immaturity or because of the intercurrent illness, have high rates of protein turnover and protein breakdown. This high rate of proteolysis is not as responsive to nutrient administration. Intervention strategies aimed at promoting nitrogen accretion, such as insulin, human growth hormone, or glutamine, have not thus far resulted in enhanced protein accretion and growth. This may be, in part, due to limitations in delivery of adequate calorie and nitrogen.

Incorporation of urea and ammonia nitrogen into ileal and fecal microbial proteins and plasma free amino acids in normal men and ileostomates

BACKGROUND

The importance of urea nitrogen reutilization in the amino acid economy of the host remains to be clarified.

OBJECTIVE

The objective was to explore the transfer of (15)N from orally administered [(15)N(2)]urea or (15)NH(4)Cl to plasma free and intestinal microbial amino acids.

DESIGN

Six men received an L-amino acid diet (167 mg N*kg(-)(1)*d(-)(1); 186 kJ*kg(-)(1)*d(-)(1)) for 11 d each on 2 different occasions. For the last 6 d they ingested [(15)N(2)]urea or, in random order, (15)NH(4)Cl (3.45 mg (15)N*kg(-)(1)*d(-)(1)). On day 10, a 24-h tracer protocol (12 h fasted/12 h fed) was conducted with subjects receiving the (15)N tracer hourly. In a similar experiment, (15)NH(4)Cl (3.9 mg (15)N*kg(-)(1)*d(-)(1)) was given to 7 ileostomates. (15)N Enrichments of urinary urea and plasma free and fecal or ileal microbial protein amino acids were analyzed.

RESULTS

(15)N Retention was significantly higher with (15)NH(4)Cl (47.7%; P < 0.01) than with [(15)N(2)]urea (29.6%). Plasma dispensable amino acids after the (15)NH(4)Cl tracer were enriched up to 20 times (0. 2-0.6 (15)N atom% excess) that achieved with [(15)N(2)]urea. The (15)N-labeling pattern of plasma, ileal, and fecal microbial amino acids (0.05-0.45 (15)N atom% excess) was similar. Appearance of microbial threonine in plasma was similar for normal subjects (0.14) and ileostomates (0.17).

CONCLUSION

The fate of (15)N from urea and NH(4)Cl differs in terms of endogenous amino acid metabolism, but is similar in relation to microbial protein metabolism. Microbial threonine of normal and ileostomy subjects appears in the blood plasma but the net contribution to the body threonine economy cannot be estimated reliably from the present data.

Differences in fat, carbohydrate, and protein metabolism between lean and obese subjects undergoing total starvation

Despite extensive experimental studies on total starvation, many of the findings relating to protein, fat (plus ketone body), and carbohydrate metabolism remain confusing, although they become more consistent when considered in relation to the degree of initial obesity. During prolonged starvation, protein loss and percent energy derived from protein oxidation are 2- to 3-fold less in the obese than in the lean; percent urine N excreted as urea is 2-fold less in the obese; and the contribution of protein to net glucose production is only about half in the obese compared to lean subjects. During short-term starvation (first few days) the following differences are reported: hyperketonaemia is typically 2-fold greater in lean subjects, but associated with a 2-fold lower uptake of ketone bodies by forearm muscle; glucose tolerance becomes impaired more in lean subjects; and both protein turnover and leucine oxidation increase in the lean, but may show no significant change in the obese. It is no longer acceptable to describe the metabolic response to starvation as a single typical response. The differences between lean and obese subjects have important physiological implications, some of which are of obvious relevance to survival.

Dietary supplementation with L-methionine impairs the utilization of urea-nitrogen and increases 5-L-oxoprolinuria in normal women consuming a low protein diet

Urea kinetics were measured in normal women after 5 d consuming a low protein diet [LP, 67 mg N/(kg.d), 0.42 g protein/(kg.d)]. To determine whether the availability of methionine limits the utilization of nonessential nitrogen from low protein diets, the study was repeated on four further occasions with the addition of dietary supplements of L-methionine, 9 mg N/(kg.d) (LP-M); urea, 52 mg N/(kg.d) (LP-U); urea and methionine (LP-UM); or urea, 26 mg N/(kg.d), and glycine, 26 mg N/(kg.d), (LP-UG). Urea kinetics were derived after prime and intermittent oral doses of [15N15N]urea from the measurements of enrichment by isotope ratio mass spectrometry in urea isolated from urine. Nitrogen balance was significantly improved when the women consumed LP-U and LP-UG, but not LP-M or LP-UM. The urinary excretion of 5-L-oxoproline was measured as a marker of glycine availability and was significantly lower when women consumed LP-U and LP-UG compared with either LP or LP-M and LP-UM. There was a significant correlation between urinary 5-L-oxoproline and urinary sulfate excretion (r = 0.68, P = 0.00003). The availability of methionine was not limiting for nitrogen metabolism when women consumed these diets, whereas the response to supplementation with urea alone or urea with glycine showed that the availability of nonessential nitrogen was limiting. Glycine is consumed in the detoxification of excess methionine, and supplementation with methionine appeared to place a competitive demand on the availability of glycine for other metabolic processes.

Urea and protein metabolism in burned children: effect of dietary protein intake

The response of urea metabolic kinetics, the rate of whole-body protein breakdown, and muscle and skin protein synthesis rates to dietary protein intake (1.15 to 2.92 g/kg/d) was assessed in children with 20% to 40% total body surface area burn injury using a primed continuous infusion of 15N2-urea and L-13C6-phenylalanine. Plasma urea concentration, production, and excretion rates increased with dietary protein intake without evidence of approaching maximum plateau values. There was no consistent evidence of urea recycling in these subjects (urea production = excretion) at any level of protein intake. The rate of appearance (Ra) of phenylalanine (an index of whole-body protein breakdown) and rate of muscle protein synthesis were independent of dietary protein, whereas there was a significant increase in skin protein synthesis with higher protein intake. We conclude that there seems to be little benefit of high protein intake on whole-body protein breakdown and muscle protein synthesis rates in these burn patients, although high-protein diets may enhance wound healing.

Urea kinetics in healthy women during normal pregnancy

Urea kinetics were measured in normal women aged 22-34 years at weeks 16, 24 and 32 on either their habitual protein intake (HABIT) or a controlled intake of 60 g protein/d (CONTROL), using primed-intermittent oral doses of [15N15N]urea and measurement of plateau enrichment in urinary urea over 18 h (ID) or a single oral dose of [15N15N]urea and measurement of enrichment of urea in urine over the following 48 h (SD). The intake of protein during HABIT-ID (80 g/d) was greater than that on HABIT-SD (71 g/d); urea production as a percentage of intake was significantly greater at week 16 for HABIT-ID than HABIT-SD, whereas urea hydrolysis at week 16 was greater for HABIT-SD than HABIT-ID and urea excretion at week 32 was greater for HABIT-ID than HABIT-SD. The combined results for HABIT-ID and HABIT-SD showed a significant reduction in urea production at week 32 compared with week 24. Urea excretion decreased significantly from week 16 to week 24 with no further decrease to week 32 and urea hydrolysis was significantly greater at week 24 than either week 16 or week 32. Compared with HABIT, on CONTROL there was a decrease in urea production at week 16, and urea excretion was significantly reduced at week 16. For all time periods urea production was closely related to the sum of intake plus hydrolysis. Hydrolysis was greatest at week 24 and closely related to urea production. There was a significant inverse linear relationship overall for hydrolysis as a proportion of production and excretion as a proportion of intake. The results show that on HABIT N is more effectively conserved in mid-pregnancy through an increase in urea hydrolysis and salvage, and during late pregnancy through a reduction in urea formation. Lowering protein intake at any stage of pregnancy increased the hydrolysis and salvage of urea. The staging of these changes was later than that in pregnancy in Jamaica.

Urea kinetics in healthy young women: minimal effect of stage of menstrual cycle, contraceptive pill and protein intake

Urea kinetics were measured using prime/intermittent oral doses of [15N15N]urea, on five separate protocols in thirteen normal young women. Each woman underwent either two or three study protocols. Measurements were made at day 12 and day 22 of the menstrual cycle, whilst consuming their habitual protein intake in seven women not taking the contraceptive pill and in six women taking the contraceptive pill. In three women taking the pill, and three not taking the pill, urea kinetics were measured whilst taking a diet in which the intake was restricted to 55 g protein/d. There was no difference in the rate of urea production, urea excretion or urea hydrolysis between the women taking the pill and those not taking the pill at day 22. In the women not taking the pill there was no difference in any measure between day 12 and day 22. In the women taking the pill there was a significant difference in the disposal of urea N to excretion or hydrolysis on day 12 compared with day 22, with a relative decrease in excretion and enhancement of hydrolysis at day 12 compared with day 22. On the restricted diet, an intake of 55 g protein/d represented 77% of the habitual intake and urea production, excretion and hydrolysis were reduced to about 84% of the rate found on the habitual intake. In paired studies the reduction in urea production was statistically significant, and there was a statistically significant linear relationship between urea production and either intake or the sum of intake plus hydrolysis. The within-individual variability for urea production was about 10%, for excretion 15% and for hydrolysis 44%. The between-individual variability for intake was about 17% on the habitual intake. The variability for production, excretion and hydrolysis (14, 13, 36%) was less in the women not taking the contraceptive pill than in those taking the pill 23, 32, 42% respectively). The variability was reduced on the controlled low intake of 55 g protein compared with the habitual intake. These results confirm the wide variability in aspects of urea kinetics between individuals. In women this variability is not, to any large extent, accounted for by changes associated with the menstrual cycle.

Protein quality and urea kinetics in prepubertal Chilean schoolboys

Urea kinetics were measured non-invasively in 12 Chilean schoolboys aged 8-10 years who were receiving one of two diets, either predominantly animal protein or predominantly vegetable protein. Both the diets provided an equivalent level of gross protein, 1.2 g/kg/day. The study diets were given for 10 days to enable adaptation to take place. On the eighth day a single oral dose of 15N15N-urea, 100 mg, was given and the amount of label excreted as 15N15-urea in urine over the subsequent 48 hours was measured. There was little difference in any aspect of urea kinetics between the two diets with urea production (animal, 173 +/- 50 mgN/kg/day; vegetable 179 +/- 53 mgN/kg/day), urea excretion (animal, 86 +/- 19 mgN/kg/day; vegetable, 105 +/- 13 mgN/kg/day), urea nitrogen hydrolysis (animal, 87 +/- 49 mgN/kg/day; vegetable, 74 +/- 42 mgN/kg/day), and the salvaged urea-nitrogen derived from hydrolysis which returned to urea formation (animal, 12 +/- 5 mgN/kg/day; vegetable, 17 +/- 9 mgN/kg/day) all being similar. A very high proportion of the salvage nitrogen derived from urea hydrolysis was maintained within the metabolic pool, about 80%, which was equivalent to 0.4 g protein/kg/day. This is the first time urea kinetics have been measured in children of this age and shows that 57% of the ura produced is excreted in urine on average with about 43% of the urea-nitrogen being salvaged for further metabolic interaction. It is concluded that the vegetable based protein diet taken habitually by Chilean children is metabolically equivalent in terms of urea kinetics to a diet based upon animal protein at this level of intake, but that high rates of salvage of urea nitrogen are found on both diets.

Quantification of incorporation of [15N]ammonia into plasma amino acids and urea

The incorporation of 15N into individual plasma amino acids and urea was quantified in five human subjects who received 15NH4Cl either orally or intravenously for 6 h. After oral tracer administration, the highest enrichment was achieved by arginine, followed by urea and glutamine; distribution of 15N within glutamine was 55% amide and 45% amino N. Glutamine achieved the highest enrichment after the intravenous administration of tracer, with a distribution of 92% amide and 8% amino N. The relative distribution pattern of 15N incorporation was quantified from the rate at which 15N initially appeared in each plasma component. Amino acids (especially arginine, glutamine, and glutamate) accounted for greater than one-half (54%) of the orally administered tracer that was initially recovered in plasma components, compared with 46% initial appearance for urea; for the intravenous tracer, amino acids accounted for 78% of initial appearance of tracer compared with 22% for urea. Our results highlight the involvement of the splanchnic bed in the utilization of orally administered ammonia (preferential incorporation of oral tracer into arginine, urea, glutamate, and the amino N of glutamine) in contrast to the preferential incorporation of systemically administered ammonia into the amide N of glutamine and alanine.

Urea kinetics: effect of severely restricted dietary intakes on urea hydrolysis

Urea kinetics (urea-N production-P, excretion-E, hydrolysis-H, recycling-R and retention-S) were measured in 7 healthy adults consuming a standard diet compared with 4 fasted for 24 and/or 96 h, using primed/intermittent doses of [(15)N (15)N]-urea and mass spectrometry. Standard values were P = 196, E = 132, H = 65, R = 13 and S = 51, mgN/kg/day. After 24 h fasting all urea kinetics were reduced, and P and H were significantly reduced compared with the standard diet (p < 0.01 and < 0.05 respectively). After 96 h fasting, urea kinetics returned to standard values (P = 187, E = 136, H = 51, R =13 and S = 38, mgN/kg/day), although nitrogen intake was significantly lower (p < 0.001). Relative urea excretion (E/P) was 67%, standard diet, and 75% after fasting. Consequently H/P was slightly reduced from 33 to 25%. S/P was 26%, standard diet, 15% after 24 h and 20% after 96 h fasting, suggesting increased urea-N retention with prolonged fasting. These results imply a slight temporary shift towards increased nitrogen excretion at 24 h and subsequent return to the kinetics of the fed state after 96 h. Urea-N retention increases with prolonged fasting.

Urea kinetics in neonates receiving total parenteral nutrition

Urea kinetics were measured on 10 occasions in eight neonates who had not received an oral intake from birth and were maintained on total parenteral nutrition. After a prime/intermittent oral dose of 15N15N-urea over 14 hours urine was collected every three to four hours, urea isolated, and kinetics determined from the plateau level of enrichment in urea, measured by isotope ratio mass spectrometry. The total parenteral nutrition provided 393 kJ (94 kcal)/kg/day and 360 mg nitrogen/kg/day. Urea production was mean (SD) 84 (44) mg nitrogen/kg/day, or 50% of intake. Urinary excretion of urea, 39 (16) mg nitrogen/kg/day, was 40% of production. Therefore 54% of urea production was salvaged through the lower bowel, 45 (35) mg nitrogen/kg/day. It is concluded that even in infants who have never had a regular dietary intake the microflora of the lower bowel is sufficiently developed to salvage urea nitrogen for further metabolic interaction, however it is not clear whether the rate of salvage is adequate to satisfy the metabolic demand.

Urea salvage in a neonate with cloacal exstrophy

Urea kinetics were measured in a child with congenital absence of the colon on days 15, 19, and 23 of age. Urea salvage was 5% of urea production in the first study, increasing to 79% by the third. This provides evidence that the colonic microflora play a more active part in urea salvage than the mucosa and that the establishment of an active lower ileal microflora takes over some of the metabolic functions of the intact colon.