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

Obligatory urinary and fecal nitrogen losses and urinary urea nitrogen and creatinine excretion levels were determined in 11 young women (four of whom used oral contraceptives), eight older men, and five young men. Values for obligatory nitrogen losses in the young women who did not use oral contraceptives were similar to those observed previously in young women and lower than those reported for young men. Our values for older men were lower than previously reported values for obligatory losses observed in young and elderly men, but similar to previously reported values for young women and to values for our women who did not use oral contraceptives. The four oral contraceptive users had significantly higher levels of total urinary and fecal obligatory nitrogen losses than the nonusers in our study. When values from our study and values from other recent studies are considered, variations in obligatory nitrogen losses appear to be larger among individuals within an age-sex group than the observed differences between groups. When the intakes of protein recommended as safe by the Food aculated from obligatory nitrogen losses per se, the recommended levels appear to be only marginally adequate.     

Long-term validation of 1 g of protein per kilogram body weight from a predominantly vegetable mixed diet to meet the requirements of young adult males

Eight healthy young men participated in a 90-d metabolic balance study designed to assess the adequacy of 1 g of protein (N X 6.25) per kilogram body weight per day provided by a predominantly vegetable diet. The mean gross energy intake of the subjects was 49 kcal/(kg X d). The individuals received the experimental diet for eleven 8-d experimental periods. Three isoenergetic, isonitrogenous meals were served at 0800, 1200 and 1900. Anthropometric measurements and biochemical determinations in blood were made at the beginning of the study and afterwards at regular 15-d intervals. Body weight was measured daily. The mean overall weight change for the group was -0.8 kg. The mean daily urinary nitrogen output decreased from an initial value of 134.6 mg/(kg X d) to a final figure of 115.1 mg/(kg X d) (P less than 0.01), while the daily fecal N decreased from 31.5 to 26.3 mg/(kg X d) (P less than 0.05). Mean cumulative nitrogen balances were slightly positive for seven out of the eight individuals with an overall mean nitrogen balance value of +6.7 mg N/(kg X d). Biochemical and hematological values showed no significant changes throughout the study, except for a decrease in serum cholesterol. It is concluded that the "Chilean" mixed diet fed at the level of 1 g of protein per kilogram body weight per day with sufficient energy meets the long-term protein-energy needs of healthy young men, thus supporting the 1984 FAO/WHO/UNU safe recommended daily protein allowance of 0.75 g of reference protein, with a correction for the lower digestibility of vegetable sources.     

Change of urinary nitrogen excretion in response to low-protein diets in adults.

Data from nitrogen balance studies in young men and elderly women were analyzed for the pattern of urinary nitrogen excretion after the subjects had changed from an adequate to an essentially protein-free diet. The single exponential equation: y = P1e-p2t+P3, provided an adequate fit of the data, suggesting that a single-pool model describes the adaptive decline in urinary nitrogen excretion under these dietary conditions. Comparisons of the estimated parameters showed that the fractional rate constant, P2, was the same for both the young men and elderly women, but that the other coefficients differed between the two groups depending upon whether they were expressed per day, per kg of body weight, per unit creatinine excreted, or per kg of body cell mass. The length of time between the change in diet and the attainment of a new equilibrium in urinary nitrogen excretion (ts, or time to stability) is defined as ts = 1/p2 loge(p1/s) where s is the standard error of the fit. The average time to stability was the same in both groups (4.5 days), and 95% of the subjects reached the new equilibrium in nitrogen output within 8 days.     

Utilization and requirement of dietary protein taking into account the dermal and miscellaneous nitrogen losses in Japanese women

Utilization and requirement of mixed protein in the conventional Japanese diet and the obligatory integumental and miscellaneous nitrogen losses were measured in female subjects. Twelve female students aged 18 to 24 years were given conventional low-protein diets at N intake levels of 50, 70, and 90 mg/kg/day for 10 days. Constituents of the diets corresponded to those of average intake of the Japanese in recent years (1982). N balances were estimated and regression equations between N intake (X) and N balance (Y) were calculated by the multiple level-constant variation method. The equation was Y = 0.426X - 40.0 (n = 36, r = 0.615, p less than 0.01) and the intersection of the regression line with zero nitrogen balance was 93.9 +/- 14.3 mg N/kg/day. The mean digestibility was 92.2 +/- 4.7%. In another experimental group, nitrogen losses due to hair, nails, and menstruation in ten Japanese women were 0.624 +/- 0.172, 0.020 +/- 0.005, and 1.76 +/- 0.68 mg/kg/day, respectively. In a third experimental group, mean of the integumental N losses was 2.8 mg/kg/day in both the subjects given a low-protein diet (19 women) and an ordinary-protein diet (4 women). It increased to 12.7 mg N/kg/day when subjects exercised in summer (4 subjects). The protein requirement in the conventional Japanese diet estimated as the sum of the mean requirement of dietary protein and obligatory dermal and miscellaneous nitrogen losses in Japanese women was 99.1 mg N/kg/day or 0.62 g protein/kg/day. The net protein utilization (NPU) of the proteins at the N intake level for N equilibrium was estimated as 48.     

ENDOGENOUS NITROGEN REQUIREMENT

Endogenous urinary nitrogen excretion of normal young men fed a "protein free" diet for seven to ten days was 36.6 mg. per kilogram body weight per day; fecal excretion was 9.9 mg. N per kilogram of body weight.     

Nutrient intake and nitrogen metabolism in cancer patients during oncological chemotherapy

Aggressive oncological chemotherapy often impairs the nutritional status of tumor patients. To evaluate the pathogenetic mechanisms, food intake in 13 cancer patients was investigated in correlation with nitrogen losses, N balances, muscle wasting, and weight course, during cytostatic therapy. Median daily N and energy intakes were reduced only in patients with weight loss [0.55 g protein, 16.5 kcal/kg ideal body wt (IBW)]. Patients with constant weight had the same intake as control subjects (1.27 g protein, 37.2 kcal IBW). N balances and creatinine height index (CHI) correlated with daily nutrient intake. Fecal N excretions did not correlate with urinary losses; there was no excess of fecal N loss because of cytostatic treatment. The impairment of cancer patients' nutritional status seems to depend primarily on the decrease of spontaneous oral intake as a consequence of the side effects of tumor therapy. Changes in CHI, compared before and after chemotherapy, indicated muscle wasting of weight-losing patients.     

Human protein requirements: obligatory urinary and faecal nitrogen losses and the factorial estimation of protein needs of Nigerian male adults

1. The present study was designed to use the factorial approach to estimate protein requirements of Nigerian male adults by measuring obligatory nitrogen losses via urine, faeces and sweat when N intake was very low and energy intake adequate. 2. Eight adult men from Osegere village near Ibadan and seven medical students from the University of Ibadan, who volunteered to participate as subjects in the study, were given a low-protein diet (based on staple foods habitually consumed by subjects) for 10 d. Mean daily total protein intake was 4.68 g while that of energy was 0.2 MJ/kg body-weight. After an initial 5 d adaptation period, 24 h urine and faces were collected in marked containers for five consecutive days for N determination. N losses from the skin were also determined in the village adults. 3. Mean daily urinary, faecal and sweat N losses (mg N/kg body-weight) were 45.88 (SD4.84), 21.79 (SD4.19), and 7.46 (SD1.71) from the village adults. The corresponding urinary and faecal N losses from the university students were 43.45 (SD2.28) and 18.32 (SD4.66) (sweat N loss not measured). Thus the total daily obligatory N losses (per kg body-weight) from the village adults and university students were 75.13 and 69.23 mg N respectively (assuming a sweat N loss of 7.46 mg for the university students). After adjusting for requirement and making a 30% allowance for individual variability, the safe level of protein intake was calculated to be 0.78 and 0.73 g protein/kg body-weight for the village men and university students respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nitrate and nitrogen balances in men

Nitrate is a nitrogen-containing compound that is not detected by the traditional Kjeldahl method of nitrogen analysis. Nitrate balance studies were done in order to determine if nitrate production in the human body contributes to the irrationally positive nitrogen balances that have been reported in healthy adults. Seven healthy young men, confined to a metabolic unit, received five diets for 9 days each: a mixed foods diet, a fiber-free egg formula diet, and egg formula with California small white beans, lima beans, and wheat bran added. Nitrate-nitrogen intake with the mixed foods diet was 75 mg/day and nonnitrate nitrogen intake was 19 g/day; crude Kjeldahl nitrogen balance (intake--(urinary + fecal)) was 0.64 +/- 1.23 g/day. Nitrate-nitrogen excretion exceeded intake by 0.10 +/- 0.05 g/day. With the four other diets, nitrate-nitrogen intake was almost nil (about 2 mg/day) and nonnitrate nitrogen intake was 95 mg/kg body weight; crude Kjeldahl nitrogen balances ranged from -0.63 +/- 0.73 to 0.02 +/- 0.45 g N/day. With these four diets, feces contained about 80 mg nitrate-nitrogen/day and urine contained about 8 mg. Saliva obtained before lunch had about 1 ppm nitrate-nitrogen with the formula diets and 5 ppm with the mixed foods diet. Net synthesis of nitrate is quite variable but appears to be of the order of 100 mg nitrate-nitrogen/day. Although the excess nitrate excretion increased total nitrogen excretion by less than 5%, it could account for as much as 10 to 20% of unexplained positive nitrogen balances previously reported in well-controlled studies.     

Basal urinary zinc/creatinine ratio as an indicator of dietary zinc intake in healthy adult women

OBJECTIVE: To study, in healthy women, the correlation between the basal urinary zinc/creatinine ratio and dietary zinc intake. SUBJECTS: A group of 36 healthy female University students was evaluated. Mean age and body weight were, respectively, 25.6+/-3.3 years and 54.4+/-7.0 kg. METHODS: Basal urine was collected; Zn was determined by AAS and Creatinine (Creat) by the Jaffe method. A nutritional survey of seven days was recorded. Mean daily dietary intake of energy (DE) and zinc (DZn) were calculated according to the INCAP and English or German Food Composition Tables, respectively. RESULTS: Mean dietary daily intake were as follows (x +/- SD): Energy (kcal): 1606+/-570; zinc (mg): 9.1+/-3.8; basal urine Zn/Creat ratio: 0.41+/-0.24. Individual values of the Zn/Creat ratio correlated with dietary Zn (r=0.481, p=0.0339); data grouped according to ranges of dietary Zn fit the following equation: Zn/Creat=0.160+/-0.034 DZn (mg/day); (r=0.870, p=0.00497). CONCLUSIONS: These results showed that the basal urinary Zn/Creat ratio could be a useful indicator of dietary Zn intake in healthy adult women.     

Effect of season and exercise on dermal nitrogen losses and their relation to urinary nitrogen excretion

The present study was designed to estimate dermal nitrogen losses in summer and winter under the conditions of minimal daily activities, on a diet of standard Japanese protein intake level and to determine whether the increased dermal nitrogen losses induced by hot climate or exercise were compensated for by the decrease in urinary nitrogen excretion. Six healthy male university students served as the subjects. The daily dermal nitrogen losses (mean +/- SD) were 0.22 +/- 0.07 g or 3.10 +/- 0.58 mg/kg in winter and 0.44 +/- 0.19 g or 6.35 +/- 2.46 mg/kg in summer, showing significantly higher dermal nitrogen losses in summer than in winter. On the contrary, urinary nitrogen excretion tended to be larger in winter than in summer. Thus, renal compensation seemed to exist for the seasonal changes in dermal nitrogen losses. In the summer experiment, the subjects took light exercise besides the minimal daily activities for a 2-day exercise period. The pooled mean of daily dermal nitrogen losses during the exercise period was significantly larger than that during the sedentary period, while the urinary nitrogen excretion was almost the same in the two periods. No compensatory reduction in the urinary nitrogen excretion during the exercise period was observed under the conditions of the present study.     

The effects of acute clostridium difficile diarrhea on fecal nitrogen content in adult hospitalized patients

Clostridium difficile diarrhea (CDD) is a frequent cause of hospital-associated infectious diarrhea and a common reason for nutritional consultation. Limited data are available on fecal nitrogen losses during infectious diarrhea in adults. Eleven patients with acute CDD were studied for stool volume and fecal nitrogen. The mean stool output was 630 +/- 110 g/day and fecal nitrogen 2.5 +/- 0.3 g per 24 hour period (mean +/- SEM). Fecal nitrogen loss was increased from the predicted 12.0 mg/kg BW/day to 38.0 +/- 5.9 mg/kg BW/day. Fecal nitrogen loss in CDD can be an important source of nitrogen loss and should be measured in nitrogen balance studies in such patients.     

Studies of nitrogen balance in male highlanders in Papua New Guinea

Nitrogen metabolism in Papua New Guinea highlanders was studied by examining their nitrogen balance when they ate their usual diet (experiment 1) or an experimental diet (experiment 2). Studies were made on 39 male highlanders between October and December in 1980 and 1982. In experiment 1, the average protein and energy intakes (mean +/- SD) for three consecutive days of usual diets were 116.2 +/- 31.2 mg N/kg body weight and 46.9 +/- 8.9 kcal/kg body weight in 1980 and 143.5 +/- 54.8 mg N/kg body weight and 47.3 +/- 10.5 kcal/kg body weight in 1982, and thus the nitrogen balances were 27.7 +/- 28.3 and 18.4 +/- 24.2 mg N/kg body weight, respectively. Most dietary protein and energy was derived from sweet potatoes and other vegetable foods, but between 1980 and 1982 the diet changed slightly with increased consumption of imported foods such as rice and canned fish. In the second experiment, when 10 highlanders were given adequate protein diet (about 200 mg N/kg body weight and about 44 kcal total energy/kg body weight for 13 d, much of the dietary nitrogen was retained in their bodies (45.1 +/- 19.3 mg N/kg body weight). When another group of men were fed a sweet-potato diet (81.1 mg N/kg body weight for 8 d followed by a low protein sweet-potato diet containing only 42 mg N/kg body weight with 50 kcal/kg body weight for 14 d, nitrogen true digestibility and net protein utilization increased and their nitrogen balance showed no significant change (-11.0 +/- 5.6 mg N/kg body weight).(ABSTRACT TRUNCATED AT 250 WORDS)     

Nitrogen balance in obese patients receiving a very low calorie liquid formula diet.

Eleven obese patients were placed on a liquid formula diet containing 320 kcal (1.34 MJ), 31 g protein, 44 g oligosaccharides, 1.5 g fat, vitamins, and essential minerals for a period of 4 weeks under metabolic ward conditions. The diet was well tolerated, and mean weight loss was 2.49 kg/week during the 4-week period. Nitrogen excretion diminished in all patients during the period of treatment, but nitrogen balance remained slightly negative in most patients, mean daily deficit being 1.3 g N/day at the end of the study. The rate at which nitrogen was loss declined in a biphasic fashion, a slower second phase after an initial rapid period of adjustment to the diet. During the study a mean of 4.1 +/- 1.4% SD of calculated total body nitrogen was lost. Potassium excretion studies did not reveal significant potassium losses. The amount of nitrogen loss was correlated with the urinary creatinine excretion, suggesting that nitrogen loss during reduced dietary intake of protein is largely dependent on the size of the lean body mass.     

Obligatory nitrogen losses in pregnant rats with special reference to estimation of net protein utilization.

For estimation of net protein utilization of dietary proteins during pregnancy, obligatory nitrogen losses were measured in protein-deficient rats in which pregnancy was maintained by administration of ovarian steroids. On shift from normal to protein-free diet, urinary nitrogen, expressed as mg/day or mg/100 g BW per day, decreased initially rapidly and then gradually during the first two weeks in both pregnant and nonpregnant rats. However, urinary endogenous nitrogen increased during the final week of pregnancy, whereas it continued to decrease in nonpregnant controls. The endogenous urinary nitrogen excretions during early-mid and late pregnancies were significantly higher in pregnant rats (666 mg/15 days and 234 mg/6 days, respectively) than in nonpregnant animals (585 mg/15 days and 153 mg/6 days, respectively), indicating pregnancy-induced protein hypercatabolism. The metabolic fecal nitrogen excretions in pregnant and nonpregnant rats were comparable. In pregnant rats, a protein-free diet resulted in decrease of basal energy expenditure, from 24 kcal/day on day 1 to about 15 kcal/day on days 16, 19 and 22 of pregnancy. Thus, the ratio of endogenous urinary nitrogen to basal energy expenditure increased in late pregnancy, indicating that "the law of a constant relationship of minimal nitrogen and energy output" is not applicable to the pregnant animals. We discuss which values for obligatory nitrogen loss should be used for estimating the net utilization efficiency of dietary proteins in pregnancy.     

Effect of methionine supplementation of a soy protein isolate on short-term nitrogen balance in young women

The effect of L-methionine supplementation on the utilization of a soy protein isolate (SPI) was evaluated by short-term nitrogen balance studies in young women. Thirteen female students were given SPI in an initial period and SPI supplemented with 1% methionine in a second period immediately after menstruation as the sole source of protein. After one day on protein-free diet, each subject received conventional low-protein diet for three days, and then low protein, semisynthetic diet containing 0.5 g/kg/day (seven subjects) or 0.3 g/kg/day (six subjects) of SPI or SPI supplemented with methionine for seven days. The energy intake was approximately of a maintenance level of 36.5 +/- 3.8 kcal/kg/day. The mean N balances of the subjects at an intake level of 0.5 g/kg/day in the SPI period and methionine supplemented period were -6.2 +/- 12.6 mg N/kg and -9.8 +/- 9.8 mg N/kg, respectively, while their N balances at an intake level of 0.3 g/kg/day were -17.8 +/- 7.2 mg N/kg in the SPI period and -15.5 +/- 3.0 mg N/kg in the methionine supplemented period. There was no significant difference between the values in the SPI and methionine supplemented periods at both levels of protein intake. Blood analyses were carried out before and after the SPI period and after the period of methionine supplementation. The urinary creatinine and urea excretions during these periods were not markedly affected.     

Nitrogen balance of healthy Dutch women before and during pregnancy

BACKGROUND: Experimental studies including longitudinal nitrogen balance studies could provide insight into protein metabolism in pregnancy. OBJECTIVE: Our aim was to determine the development of nitrogen balance during pregnancy compared with nitrogen balance before pregnancy in women consuming imposed constant diets. We also tracked changes in muscle mass and lean body mass by measuring urinary 3-methylhistidine (3-MeH) and urinary creatinine. DESIGN: Nitrogen balance was determined over 8 d in 12 healthy Dutch women before pregnancy and at weeks 12, 23, and 34 of gestation. Complete daily diets were supplied during each balance period so that each subject's energy, protein, and macronutrient intakes were similar in amount and composition in all 4 balance periods. RESULTS: Throughout pregnancy there was no significant change in loss of nitrogen in feces and therefore no change in protein digestibility. The amount of nitrogen excreted in urine in late pregnancy (11.0 +/- 1.4 g/d) was significantly (P < 0.01) less than in early pregnancy (12.6 +/- 1.3 g/d). Nitrogen retention increased toward term, even though energy balance became progressively negative. The difference between the first (-0.4 +/- 1.7 g N/d) and third (1.2 +/- 1.6 g N/d) trimester was significant (P < 0.05). No differences were found in either 3-MeH or creatinine excretion between trimesters. CONCLUSIONS: These urinary nitrogen excretion and nitrogen retention data show that when the dietary supply remains constant, nitrogen balance increases toward the end of pregnancy, suggesting a more efficient use of dietary protein later in pregnancy. Urinary 3-MeH and creatinine excretion indicated no change in protein metabolism.     

Muscularity and adiposity in addition to net acid excretion as predictors of 24-h urinary pH in young adults and elderly

OBJECTIVE: In patients with nephrolithiasis, an inverse relationship between 24-h urinary pH (24h-UpH) and body weight has been reported. Whether body composition indices and 24h-UpH are similarly associated in healthy subjects needs investigation. DESIGN: Cross-sectional, retrospective analysis. SETTING: Dortmund, Germany and Gothenburg, Sweden. SUBJECTS: Healthy young adults (18-23 years; n=117) and elderly (55-75 years; n=85) having a mean body mass index (BMI) of 22.80+/-3.4 and 25.3+/-3.9 kg/m2, respectively. METHODS: Anthropometric data, 24h-UpH, and 24-h urinary excretion rates of net acid (NAE), creatinine, and urea were determined. After adjusting for urea (reflecting protein intake), renal creatinine output was used as a biochemical marker for muscularity. The BMI served as a marker of adiposity. RESULTS: NAE, body weight, and BMI were significantly (P<0.05) higher, and height and creatinine significantly lower in the elderly, whereas body-surface area (BSA) was not different. Step-wise multiple regression analysis using BSA-corrected urinary variables revealed NAE as the primary predictor of 24h-UpH (with R2 values of 0.64 and 0.68 in young adults and elderly, respectively, P<0.0001), followed by urea (P<0.0001), creatinine (P<0.05), and BMI (P<0.05 for the young adults and P=0.12 for the elderly). These associations were negative for NAE and BMI, and positive for urea and creatinine. CONCLUSIONS: Muscularity (i.e. creatinine adjusted for urea) and particularly in the group of young adults, adiposity (i.e. BMI) proved to be modest, but significant predictors of 24h-UpH. Future research should focus on more obese subjects in whom insulin resistance and particular kidney functions should also be examined to further substantiate the role of obesity in low-urine pH-associated conditions, for example, nephrolithiasis.     

An analysis of temporal patterns in urinary nitrogen excretion of young adults receiving constant diets at two nitrogen intakes for 8 to 11 weeks.

The temporal pattern of daily urinary nitrogen (UN) excretion was investigated in 21 young adult men and women who participated in two metabolic studies involving constant diets and lasting 8 to 11 weeks. For one group (16 subjects), nitrogen (N) intake was about 280 mg N/kg per day (mixed animal and plant protein sources) for 8 weeks; for the second group (five subjects), N intake was about 90 mg N/kg per day (egg protein) for 11 weeks. Two mathematical models were examined to determine how well they described the variations in UN. The first modeled only random variability while the second added a component of serial correlation (correlation between successive daily observations). After correcting the UN data for linear trend, to minimize effects of possible alterations in body composition, only two of the 21 subjects showed significant serial correlation of daily UN. Moreover, it is shown that any serial correlation undetectable in these data would not be of practical importance in estimating UN levels in short-term dietary experiments. It is concluded that the interpretation of metabolic N balance measurements involving constant N intakes over relatively short dietary periods in not complicated by cyclic, time-dependent variations in UN.     

Protein and amino acid requirements in the elderly

Estimates of protein and amino acid requirements in this paper are proposed for healthy elderly people. The estimate of protein requirement was based on nitrogen (N) balance, as well as functional indicators such as immune function or muscle strength. Data suggest that the protein requirement for nitrogen equilibrium in the elderly, is greater than 0.8 gm/kg body weight/day. There do not appear to be any adverse consequences with protein intakes that are about 1 gm/kg body weight/day. The tentative recommendation in this paper is higher than the current mean recommended intake of protein (FAO/WHO/UNU, 1985). However, because of methodological difficulties, the data does not allow for a confident prediction of what the exact level of protein intake should be. Further studies are needed to come to a firm conclusion on the exact protein requirement. Indispensable amino acid requirements based on nitrogen balance data, in the elderly, are fragmentary and conflicting. These requirements can alternatively be based on obligatory nitrogen loss, for which data is available in the elderly. The overlap of the obligatory nitrogen loss between the young and the elderly, suggest that the amino acid requirement based on this technique is similar in young and elderly individuals. Tracer based techniques measuring amino acid balance at different amino acid intakes, also support the view that there are no differences in the amino acid requirements between young and elderly people. In general, these amino acid requirement studies have been performed in healthy USA subjects, and data is needed to know if these estimates can be extended to populations from other, less-developed countries.     

Effects of a beta-agonist (clenbuterol) on growth, carcass composition, protein and energy metabolism of veal calves

Twenty-two British Friesian bull calves were used in a comparative slaughter experiment to determine the effects of a beta-agonist (clenbuterol) on body composition and energy retention. Four calves were slaughtered at 18 d of age and constituted the initial slaughter group. Of the remaining calves, eight (group A, controls) were given milk replacer only, and ten calves (groups B and C, five calves per group) were given milk replacer plus clenbuterol (0.1 and 1.0 mg clenbuterol/kg milk replacer equivalent to approximately 2 and 20 micrograms/kg body-weight respectively over the 105 +/- 3 d of the experimental period). Calves were slaughtered over the weight range 146-177 kg. Clenbuterol had no significant effect on dry matter (DM) intake, daily live-weight gain or feed conversion ratio. DM digestibility of the milk replacer was not affected by treatment. Nitrogen balance was measured on three separate occasions starting when the calves weighed approximately 60, 110 and 130 kg. N retention was increased over the experimental period in clenbuterol-treated calves, although the effect only achieved significance in calves weighing approximately 110 kg live weight (P less than 0.05). Clenbuterol (20 micrograms/kg body-weight) increased estimated mean daily N retention in the carcass of the calves from 22 to 25 g whilst N retention in the non-carcass components decreased from to 8 g/d. Effects of clenbuterol on N retention occurred mainly in skeletal muscle. Fat in both carcass and non-carcass components was reduced by treatment with clenbuterol. The total energy content of live-weight gain was reduced from 1077 to 897 MJ in clenbuterol-treated calves and mean daily heat production was estimated to increase from 23.1 in controls to 25.9 MJ/d in calves in group C. In calves of mean live weight during balance of 120 and 136 kg, clenbuterol significantly increased daily urinary creatinine excretion and in 120 kg calves N tau-methylhistidine was significantly decreased (P less than 0.05). Based on estimates of muscle mass from urinary creatinine and protein degradation from N tau-methylhistidine excretion, the fractional breakdown rate of muscle protein in clenbuterol-treated calves was only 0.66 of that in the controls when the calves weighed 120 kg.