Obligatory urinary and fecal nitrogen losses in young women, older men, and young men and the factorial estimation of adult human protein requirements

Effect of five hours of mixed exercise on urinary nitrogen excretion in healthy moderate-to-well-trained young adults

Introduction

Carbohydrates and fats are the primary energy substrates during exercise, but proteins can also contribute. When proteins are degraded in the body, the amino groups are mainly converted to urea and excreted. Therefore, nitrogen excretion has been used as a marker of protein degradation, but a clear conclusion has yet to be reached on the effect of exercise on nitrogen excretion. Thus, we tested whether exercise increases nitrogen excretion.

Methods

Fifteen young, healthy, moderate-to-well-trained participants (4 females, 11 males, VO2max 54.4 ± 1.7 mL·kg-1·min-1; mean ± SEM) participated in a randomized, balanced cross-over design investigation consisting of 1 day with 5 h of exercise (exercise day, EX) and 1 day with no exercise (control day, CON). The participants recorded their dietary intake the day before from 16:00 and throughout the intervention day. They then repeated these dietary intakes on the second trial day. A standardized lunch was provided on both days. In addition, participants were allowed to consume almost protein-free snacks in EX to ensure the same energy balance during both trial days. Urine was collected throughout the whole testing period, and urinary 3-methylhistidine (3-MH) excretion was measured to examine muscular catabolism. The sweat rate was calculated during the exercise period.

Results and discussion

The urinary nitrogen and 3-MH excretions did not differ significantly between EX and CON (p = 0.764 and p = 0.953). The sweat rate was 2.55 ± 0.25 L in EX and 0.14 ± 0.15 L in CON (p < 0.001), and by estimating sweat nitrogen excretion, total nitrogen excretion was shown to differ with exercise. Our results showed that 5 hours of mixed exercise did not significantly impact urinary nitrogen and 3-MH excretions in healthy moderate-to-well-trained young adults.

Protein Catabolism and the Dysregulation of Energy Intake-Related Hormones May Play a Major Role in the Worsening of Malnutrition in Hospitalized Cirrhotic Patients

Malnutrition in cirrhotic patients is extremely common and has a multifactorial aetiology, whose constitutive elements have not been completely elucidated yet. Protein depletion is particularly important and an imbalance of hormones regulating hunger and satiety may be an important additive factor. The diagnosis and treatment of malnutrition are extremely important since malnutrition is associated with higher complication rates and mortality. Our observational study aimed to study protein status and energy intake-related hormone levels in a cohort of hospitalized cirrhotic patients. We enrolled 50 hospitalized and clinically stable cirrhotic patients and assessed their nutritional status with anthropometric measurements and nitrogen balance. In a subgroup of 16 patients and 10 healthy controls, circulating ghrelin and leptin levels were studied. We observed that 60% of our patients were malnourished on the basis of the mid-arm muscle circumference values; the recorded daily protein intake was tendentially insufficient (mean protein intake of 0.7 ± 0.5 g protein/kg vs. recommended intake of 1.2–1.5 g of protein/kg/die). Cirrhotic patients had lower circulating levels of both ghrelin and leptin compared to healthy controls. In conclusion, hospitalized cirrhotic patients face a catabolic state and an imbalance in hormones regulating food intake and satiety, and these elements may play a major role in the genesis and/or the worsening of malnutrition.

Macronutrients in Parenteral Nutrition: Amino Acids

The right amount and quality of amino acids (AAs) supplied to patients on parenteral nutrition (PN) reduces muscle mass loss, may preserve or even increase it, with significant clinical benefits. Several industrial PN mixtures are available so that nutrition specialists can choose the product closest to the patient's needs. In selected cases, there is the possibility of personalizing compounded mixtures in a hospital pharmacy that completely meets the individual nutritional needs of PN patients. This narrative review deals with the AA solutions used in PN mixtures. The physiology, the methods to calculate the AA needs, and the AA and energy requirements suggested by scientific guidelines for each patient type are also reported.

Human Amino Acid Requirements: A Re-Evaluation

The most recent internationally stated estimates of the amino acid requirements in adult humans are those given in the 1985 report of the Joint FAO/WHO/UNU Expert Consultation on Energy and Protein Requirements. In this review we present, in brief a number of scientific concerns and problems that lead us to conclude that these current recommendations for amino acid requirements are not valid and that the appropriate values are likely to be considerably higher. Following a short review of the C-labelled amino acid tracer studies carried out at the Massachusetts of Technology (MIT) and designed to reassess the requirements for specific indispensable amino acids, we focus particular attention on the lysine requirement in adults. When various criteria and methods are used to estimate this requirement, it appears that a cohesive body of data indicates the mean requirement value for lysine in healthy adults to be about 30 mg/kg/day or 50 mg/g protein. Although this value contrasts with the FAO/WHO/UNU value of 12 mg/kg/day or 16 ma/g protein, this new, tentative requirement value is consistent with findings from studies carried out earlier at MIT on the nutritional quality of wheat proteins. We propose that it would be prudent to apply the MIT amino acid requirement pattern (see Food and Nutrition Bulletin 1990;12:298–300), rather than the 1985 FAO/WHO/UNU adult amino acid requirement pattern, in the design and implementation of sound nutrition policies and programmes that include considerations of the amount and quality of the protein component of national and regional diets.

Sex differences in amino acids lost via sweating could lead to differential susceptibilities to disturbances in nitrogen balance and collagen turnover

Fluid collected during sweating is enriched with amino acids derived from the skin's natural moisturising factors and has been termed "faux" sweat. Little is known about sex differences in sweat amino acid composition or whether faux sweat amino acid losses affect nitrogen balance. Faux sweat collected by healthy adults (n = 47) after exercise, and at rest by chronic fatigue patients, was analysed for amino acid composition. Healthy females had higher total amino acid concentrations in sweat (10.5 ± 1.2 mM) compared with healthy males (6.9 ± 0.9 mM). Females had higher levels of 13 amino acids in sweat including serine, alanine and glycine. Higher hydroxyproline and proline levels suggested greater collagen turnover in females. Modelling indicated that with conservative levels of exercise, amino acid losses in females via faux sweat were triple than those predicted for urine, whereas in males they were double. It was concluded that females were more susceptible to key amino acid loss during exercise and/or hot conditions. Females reporting chronic fatigue had higher levels of methionine in faux sweat than healthy females. Males reporting chronic fatigue had higher levels of numerous amino acids in faux sweat compared to healthy males. Higher amino acid loss in faux sweat associated with chronic fatigue could contribute to a hypometabolic state. Depending on activity levels, climatic conditions and gender, amino acid losses in sweat and skin leachate could influence daily protein turnover where periods of continuously high turnover could lead to a negative net nitrogen balance.

Meta-analysis of nitrogen balance studies for estimating protein requirements in healthy adults

BACKGROUND

The most recent international dietary protein recommendations for healthy adults are those developed and proposed by the 1985 FAO/WHO/UNU Joint Expert Consultation.

OBJECTIVE

The objective was to analyze available nitrogen balance data to establish new recommendations for the protein required by healthy adults.

DESIGN

Data were gathered from published nitrogen balance studies that had as their primary objective either the estimation of basal or maintenance requirements or the testing of the adequacy of specific nitrogen intakes in healthy adults. These data were synthesized to characterize the distribution of individual protein requirements; the effects of climate of the study site, adult age, sex, and dietary protein source on individual requirements; and the midpoint of and the variability between the protein requirements of healthy persons.

RESULTS

Data for 235 individual subjects, each studied at >or= 3 test protein intakes, were gathered from 19 studies. The median estimated average requirement (EAR) of nitrogen from these data was 105 mg N x kg(-1) x d(-1). Individual requirements were found to fit a log-normal distribution. The median EAR was estimated as the median of this distribution, 105 mg N x kg(-1) x d(-1), whereas the 97.5th percentile (the recommended dietary allowance; RDA) was estimated from the distribution of the log of the requirement (after correction of the total observed variability to remove within-individual variability) as 132 mg N x kg(-1) x d(-1). No significant differences between the climate of the study site, adult age class, sex, or source of dietary protein were observed, although there was an indication that women might have a lower requirement than do men.

CONCLUSION

This meta-analysis provides new recommendations for dietary reference values, ie, an EAR (median) and RDA (97.5th percentile) for healthy adults of 105 and 132 mg N x kg(-1) x d(-1) (0.65 and 0.83 g good-quality protein x kg(-1) x d(-1)), respectively.

Contrasting plasma free amino acid patterns in elite athletes: association with fatigue and infection

AIM

There is little information on the plasma free amino acid patterns of elite athletes against which fatigue and nutrition can be considered. Therefore the aim was to include analysis of this pattern in the medical screening of elite athletes during both especially intense and light training periods.

METHODS

Plasma amino acid analysis was undertaken in three situations. (1) A medical screening service was offered to elite athletes during an intense training period before the 1992 Olympics. Screening included a blood haematological/biochemical profile and a microbial screen in athletes who presented with infection. The athletes were divided into three groups who differed in training fatigue and were considered separately. Group A (21 track and field athletes) had no lasting fatigue; group B (12 judo competitors) reported heavy fatigue at night but recovered overnight to continue training; group C (18 track and field athletes, one rower) had chronic fatigue and had been unable to train normally for at least several weeks. (2) Athletes from each group were further screened during a post-Olympic light training period. (3) Athletes who still had low amino acid levels during the light training period were reanalysed after three weeks of additional protein intake.

RESULTS

(1) The pre-Olympics amino acid patterns were as follows. Group A had a normal amino acid pattern (glutamine 554 (25.2) micromol/l, histidine 79 (6.1) micromol/l, total amino acids 2839 (92.1) micromol/l); all results are means (SEM). By comparison, both groups B and C had decreased plasma glutamine (average 33%; p<0.001) with, especially in group B, decreased histidine, glucogenic, ketogenic, and branched chain amino acids (p<0.05 to p<0.001). None in group A, one in group B, but ten athletes in group C presented with infection: all 11 athletes had plasma glutamine levels of less than 450 micromol/l. No intergroup differences in haematological or other blood biochemical parameters, apart from a lower plasma creatine kinase activity in group C than in group B (p<0.05) and a low neutrophil to lymphocyte ratio in the athletes with viral infections (1.2 (0.17)), were found. (2) During post-Olympic light training, group A showed no significant amino acid changes. In contrast, group B recovered normal amino acid levels (glutamine 528 (41.4) micromol/l, histidine 76 (5.3) micromol/l, and total amino acids 2772 (165) micromol/l) (p<0.05 to p<0.001) to give a pattern comparable with that of group A, whereas, in group C, valine and threonine had increased (p<0.05), but glutamine (441 (24.5) micromol/l) and histidine (58 (5.3) micromol/l) remained low. Thus none in group A, two in group B, but ten (53%) in group C still had plasma glutamine levels below 450 micromol/l, including eight of the 11 athletes who had presented with infection. (3) With the additional protein intake, virtually all persisting low glutamine levels increased to above 500 micromol/l. Plasma glutamine rose to 592 (35.1) micromol/l and histidine to 86 (6.0) micromol/l. Total amino acids increased to 2761 (128) micromol/l (p<0.05 to p<0.001) and the amino acid pattern normalised. Six of the ten athletes on this protein intake returned to increased training within the three weeks.

CONCLUSION

Analysis of these results provided contrasting plasma amino acid patterns: (a) a normal pattern in those without lasting fatigue; (b) marked but temporary changes in those with acute fatigue; (c) a persistent decrease in plasma amino acids, mainly glutamine, in those with chronic fatigue and infection, for which an inadequate protein intake appeared to be a factor.

Relationship between tissue mobilization and storage in the rat

The amount of energy mobilized or stored as protein, expressed as a proportion of the total energy stored or mobilized (defined as the P ratio, Payne & Dugdale, 1977), was investigated in the young male (rapid growth) and adult female (slow growth) rat. Energy mobilization was induced by a 3 d fast and the changes in body content of fat and lean tissues were used to estimate the fasting P ratio (Pfast). Tissue storage was subsequently effected by 17 d of refeeding and the corresponding ratio (P(refed)) was calculated from the amounts of lean and fat tissue regained. The same experimental protocol was used for measuring Pfast and P(refed) in weanling (30 d) and adult (130 d) rats. Weight-matched individuals were assigned to three groups. All animals in group 1 were killed at the beginning of the experiment. Animals in group 2 were fasted for 3 d and then killed. Group 3 animals were first fasted for 3 d, then had free access to a stock diet for a further 17 d before being killed. During fasting, a close linear relationship was found between weight change and body energy changes (r 0.985, and r 0.92, P < 0.001 for weanlings and adult rats respectively) and between carcass N loss and urinary N loss (r 0.933, P < 0.001). These relationships were used to estimate the body energy and N content of each animal at the end of the initial fasting period. Hence, both Pfast and P(refed) values could be calculated for all individuals at both ages. When Pfast and P(refed) were calculated for adult rats, the mean values were similar (0.138 (SE 0.002) and 0.130 (SE 0.006) respectively). Individually, the Pfast and P(refed) values were correlated (r 0.54, P = 0.058). In weanling rats, the mean P(refed) value was about 2.5 times the Pfast (0.421 (SE 0.0113) v. 0.156 (SE 0.004)). Despite the differences in Pfast and p(refed), the individual values were again correlated (r 0.668, P < 0.05). These results support the suggestion made by Payne & Dugdale (1977), that particular P. ratio values are characteristic of individuals and describe the extent to which protein is mobilized or stored during fasting or refeeding. These observations are discussed in relations to the predictions made by the Payne-Dugdale model.

Protein requirement of young adult Nigerian females on habitual Nigerian diet at the usual level of energy intake

A short-term N balance study was conducted in twelve healthy female adults aged 21-32 years to determine their protein requirement. Four dietary protein levels (0.3, 0.4, 0.5 and 0.6 g protein/kg per d) were used. Energy intake of the subjects was kept constant at 0.18 MJ/kg per d. All subjects maintained their normal activity throughout the study period. N excretion was determined from the measurements of N in a total collection of urine, faeces, sweat and menstrual fluid for each dietary period. N balance during the four protein levels were -15.15 (SD 5.95), -5.53 (SD 6.71), +6.15 (SD 4.76) and +12.05 (SD 8.63) mg N/kg per d for 0.3, 0.4, 0.5 and 0.6 g protein/kg per d respectively. The calculated average N requirements from regression analysis was 76.0 (SD 3.37) mg N/kg per d (0.48 g protein/kg per d). The estimate of allowance for individual variation to cover the 97.5% population was 95 mg N/kg per d (0.6 g protein/kg per d). The net protein utilization (NPU) of the diet was 0.55. When compared with a similar study with men, there was a significant difference in the protein requirement between sexes. Thus, the unjustifiable sex difference in the protein allowance recommended by the Food and Agriculture Organization/World Health Organization/United Nations University (1985) Expert Consultation group must be reviewed.

Plasma leucine kinetics and urinary nitrogen excretion in intensively treated diabetes mellitus

It is well known that inadequate insulin therapy stimulates body protein loss in insulin-dependent diabetes mellitus (IDDM). It is less well known, however, that accelerated body protein loss (as indicated by increased leucine oxidation) occurs in IDDM even during conventional glycemic control. It is not known whether intensified insulin therapy fully normalizes protein oxidation or, more importantly, whether such therapy is sufficient to allow the adaptive decrease of protein oxidation that normally occurs when protein intake is restricted below the customary surfeit level. We used two measures of protein oxidation [daily urinary nitrogen (N) excretion over several days of intensive insulin therapy and plasma [1-13C]leucine oxidation during short-term strict euglycemia] to assess the response of 7 men with IDDM and 12 normal men after adaptation first to a control diet providing maintenance energy and conventional (surfeit) protein then to an isoenergetic protein-free diet. After adaptation to the protein-free diet and during short-term strict euglycemia achieved using intravenous insulin, leucine turnover and oxidation decreased equivalently in normal and diabetic subjects. However, daily urinary obligatory N excretion, which indicated the effect of the low-protein diet and intensive subcutaneous insulin therapy over several days, was increased by 18% in the diabetic group (P less than 0.05). Even mildly elevated average blood glucose values well within the guidelines for intensive therapy were strongly correlated with high rates of urinary N excretion (r = 0.97, P = 0.0002). Thus insulin therapy of IDDM that imposes strict euglycemia is compatible with a normal ability to diminish body protein oxidation in response to protein restriction.(ABSTRACT TRUNCATED AT 250 WORDS)

The protein requirement of adult marmosets: nitrogen balances and net protein utilization of milk proteins, soy protein, and amino acid mixtures

Nitrogen balance studies were conducted in adult male marmosets (Callithrix jacchus) using purified and semipurified diets with protein levels between 0% and 7%. Daily nitrogen loss in a state of prolonged protein free nutrition was 131 +/- 16 mg/kg body weight0.75. Zero nitrogen balance resulted from mean daily intake of 261 mg nitrogen/kg0.75 when high quality protein sources were used. Very low protein intake or the lack of arginine and histidine in an amino acid mixture induced coprophagy. It is concluded that the protein requirement of adult marmosets is very similar to the protein requirement of adult humans (based on metabolic body weight). About 6-7% high quality protein, based on dry matter, suffice to avoid a negative nitrogen balance in all individuals.

Protein digestibility of the same protein preparations by human and rat assays and by in vitro enzymic digestion methods

The apparent and true digestibilities of the same preparations of six proteins (spray dried whole egg, cottage cheese, canned tuna, peanut flour, soy isolate, and wheat gluten) were estimated in four to five men and in rats and compared to estimates of digestibility from three different in vitro enzymic digestion procedures. For all six proteins, the correlation coefficient was 0.46 between true digestibility in humans and in rats; with values for tuna excluded, r = 0.96. With all six proteins, none of the in vitro values was significantly correlated with values from humans or rats. However, with either the three animal proteins alone or the three plant proteins alone, correlations were high (r greater than 0.90) between one or more of the in vitro estimates and the observed true or apparent human and rat digestibilities. The differences in the relationship between enzymic digestion estimates and the human digestibility estimates for plant or animal proteins suggest that for accurate prediction of protein digestibility in humans by these enzymic methods, different equations would have to be used for plant and animal proteins. For protein sources containing both plant and animal protein, use of the in vitro enzymic procedures would give only an approximate estimate of digestibility in humans.