Leucine kinetics at graded leucine intakes in young men

A study was carried out with 12 young men to examine the relationships between the intake of leucine and indices of leucine kinetics, using L-[1-13C]leucine as a tracer. Six subjects received L-amino acid diets during 7-day periods supplying leucine in the range of 79 to 20 mg.kg-1.day-1 (Group I) and another six subjects (Group II) received leucine intakes ranging from 20 to 4 mg.kg-1.day-1. Estimations were made of leucine kinetics, at the end of each diet period, when subjects were receiving small isonitrogenous, isocaloric meals during the isotope infusion period. Leucine flux declined with reduced leucine intake and leucine oxidation tended not to change at intakes below 20 mg.kg-1.day-1 (slope not statistically different than zero). Plasma valine increased markedly with further restriction in leucine intake below this level. The daily mass balance of leucine, estimated from the difference between intake and oxidation, became negative at an intake of about 20 mg.kg-1.day-1. These findings are discussed in relation to the published mean and upper range of requirement for leucine in healthy adults, currently taken to be 11 mg.kg-1.day-1 and 14 mg.kg-1.day-1, respectively.     

Lysine kinetics at graded lysine intakes in young men

Healthy young men participated in a study designed to explore the effects of decreasing dietary lysine content on plasma amino acid concentrations and lysine kinetics, studied with L-[1-13C]lysine as tracer. Diets provided adequate energy and the equivalent (N X 6.25) of 0.8 g protein kg-1.day-1 as a synthetic L-amino acid mixture simulating egg protein. Lysine intake was reduced every 7 days. Changes in plasma amino acids suggested that effects characteristic of a dietary lysine inadequacy were prevented by consuming more than 32 mg lysine.kg-1 day-1. Primed, continuous intravenous infusions of L-[1-13C]lysine, at the end of each diet period while subjects were in the fed state, showed that as dietary lysine was reduced, 13C enrichment increased in plasma lysine and decreased in expired CO2. It was estimated that lysine oxidation exceeded, by 4.4 mg kg-1 day-1, the lysine intake of 20 mg kg-1.day-1 indicating that the lysine required for body protein maintenance would probably exceed this latter value. These results are discussed in relation to the physiological requirement in adults for lysine, currently accepted to be met by an intake of 12 mg kg-1.day-1. which is assumed to be the upper range of the lysine requirement for this population group.     

Threonine kinetics at graded threonine intakes in young men

A study was undertaken in eight healthy young men to examine the effects of varying intakes of threonine on plasma free threonine concentrations and threonine kinetics, using a 3 h constant intravenous infusion of L-[1-13C]threonine. Subjects consumed diets based on an L-amino acid mixture, in which the quality of threonine was reduced every 7 days. On the last day of each diet period, determinations of plasma threonine flux and threonine oxidation were carried out while subjects consumed small meals, each supplying 1/12 daily intake, at hourly intervals. Threonine oxidation rates fell with reduced threonine intake, reaching a relatively constant level at intakes of 20 mg.kg-1.day-1 and below. These metabolic data are discussed in relation to the currently established value of 7 mg.kg-1.day-1 as the upper range of the threonine requirement for healthy young adults. It is concluded that actual threonine requirements may be considerably higher for this age group.     

Valine kinetics at graded valine intakes in young men

Twelve young men, six subjects in each group studied in two phases, participated in an experiment to explore the relationships between valine intake, plasma valine concentrations, and valine kinetics, using 1-[13C]valine as a tracer. Below a valine intake of about 20 mg.kg-1.day-1 plasma valine concentrations reached a low and relatively constant level. The rate of valine oxidation fell with the decline in the intake of amino acid. Below valine intakes of 16 mg.kg-1.day-1, the mean daily rate of oxidation was estimated to be generally higher than the intake level, implying a negative valine balance during the 24 h day. These findings indicate that an intake of 10 mg valine kg-1.day-1 would not be adequate to maintain protein nutritional status. Our results are discussed in relation to the currently accepted 1973 FAO/WHO value of 10 mg.kg-1.day-1 as being the upper range of the valine requirement in healthy adult humans.     

Excretion of tryptophan-niacin metabolites by young men: effects of tryptophan, leucine, and vitamin B6 intakes

Three human metabolic studies, each 35 days in length, were performed to investigate the relationship between tryptophan intake and the proportion of dietary tryptophan converted to niacin and the effect of supplements of L-leucine and vitamin B6 on this conversion. Nine college men consumed a basal diet that provided 8 mg of niacin, 1 mg of vitamin B6, and either 245, 548, or 845 mg of tryptophan from proteins per day. During each 35-day study, for one 15-day period basal diet alone was consumed, for another 15-day period basal diet plus 10 g of L-leucine per day was consumed, and for the last 5-day period, 20 mg of vitamin B6 per day was added to the diets of both groups. N1-methylnicotinamide, N1-methyl-2-pyridone-5-carboxamide, and quinolinic acid were measured in 24-hr urine samples. There were no significant or consistent effects of L-leucine or vitamin B6 supplements on the excretin of any of the metabolites measured. The proportion of tryptophan converted to niacin tended to increase as tryptophan consumption increased; however, this change was small and was probably not significant over the range of tryptophan intakes studied. The average conversion ration of tryptophan to niacin was approximately 72:1 in these subjects.     

Regulatory role of dietary leucine on plasma branched-chain amino acid levels in young men.

Four healthy young men were studied to explore the effects of dietary leucine intake on plasma levels of leucine,isoleucine and valine. Leucine-, valine-, leucine-valine-, and leucine-isoleucine-valine-free amino acid mixtures were studied during four-day experimental diet periods. The effects of giving amino acid mixtures with high-carbohydrate, low-fat or low-carbohydrate, high-fat diets were also studied. Deficient leucine intake increased plasma levels of valine and isoleucine, but a valine-free diet did not affect plasma levels of the other branched-chain amino acids. Leucine influence was evident during the postprandial and fasted phases. Valine and isoleucine levels were less markedly reduced when diets devoid of these amino acids were also leucine-free, as compared with a diet providing adequate leucine. Changes in the major dietary energy source failed to influence the qualitative effects of dietary leucine adequacy on the branched-chain amino acid levels in plasma. The results suggest that leucine facilitates both tissue uptake of branched-chain amino acids and their intracellular metabolism.     

Effects of meal consumption on whole body leucine and alanine kinetics in young adult men

The effects of meal consumption on plasma leucine and alanine kinetics were studied using a simultaneous, primed, continuous infusion of L-[1-13C]leucine and L-[3,3,3-2H3]alanine in four healthy, young, adult male subjects. The study included an evaluation of the effect of sampling site on plasma amino acid kinetics, with blood being drawn simultaneously from an antecubital and dorsal heated hand vein. In comparison with the postabsorptive state, the ingestion of small hourly meals resulted in a 35% increase in plasma leucine flux and a 77% increase in leucine oxidation. Calculated entry of leucine into the plasma compartment from endogenous sources decreased by 65%. Plasma alanine flux more than doubled, indicating a significant enhancement in de novo alanine synthesis. 13C enrichment of leucine in venous and arterialized plasma did not differ significantly, but alanine flux calculated from isotopic measurement in venous plasma was substantially greater than that based on analysis of arterialized blood plasma.     

Cysteine kinetics and oxidation at different intakes of methionine and cystine in young adults

BACKGROUND: We previously studied methionine kinetics and oxidation with the tracer L-[1-(13)C, methyl-(2)H(3)]methionine. OBJECTIVES: We sought to explore methionine-cysteine interrelations in adults by using L-[1-(13)C]cysteine under different dietary conditions. DESIGN: In experiment 1, 12 adults consumed a protein-free diet for 6 d. On day 7, methionine (n = 6) or cysteine (n = 6) oxidation rates were measured during an 8-h continuous infusion of L-[1-(13)C, methyl-(2)H(3)]methionine or L-[1-(13)C]cysteine, respectively. In experiment 2, 6 young men consumed 3 diets for 6 d each before a tracer study on day 7 with L-[1-(13)C]cysteine. The amounts (in mg*kg(-)(1)*d(-)(1)) of methionine and cysteine, respectively, were: high-methionine (HM) diet, 13 and 0; low-methionine (LM) diet, 6.5 and 0; and methionine-plus-cystine (MC) diet, 6.5 and 5.6. Cysteine flux and oxidation rates were determined and sulfur amino acid (SAA, methionine plus cysteine) balances were estimated. RESULTS: In experiment 1, rates of methionine and cysteine oxidation were similar to losses predicted from obligatory nitrogen losses. In experiment 2, SAA balance was less negative when subjects consumed the HM diet than the LM and MC diets (interaction, P = 0.034), largely because of a difference in fed-state balance (HM compared with LM, P < 0.01; HM compared with MC, P < 0.05). There was no evidence of a sparing effect of dietary cystine on the methionine requirement. CONCLUSION: These studies support use of [1-(13)C]cysteine for studying whole-body SAA oxidation and conclusions that maintenance of SAA balance is best achieved by supplying methionine at approximately the FAO/WHO/UNU recommendations for total SAA intake (13 mg*kg(-)(1)*d(-)(1)).     

Plasma proline kinetics and concentrations in young men in response to dietary proline deprivation

This study examined plasma proline concentration flux, oxidation, and endogenous biosynthesis in five healthy young men given three isocaloric, isonitrogenous diets for 1 wk [a complete egg-pattern amino acid diet (diet 1), an amino acid mixture devoid of proline (diet 2), and a diet composed solely of indispensable amino acids (diet 3)]. At the end of each dietary period, a 360-min postabsorptive, primed, continuous stable-isotope-tracer infusion of L-[1-13C]proline and L-[methyl-2H3]leucine was performed in all subjects. Plasma proline concentrations declined by 22% on diet 2 (p less than 0.02) and by 29% on diet 3 (p less than 0.01). No statistically significant (p greater than 0.2) changes were observed for proline oxidation, endogenous biosynthesis, or flux. The data suggest that the absence of proline in the human diet does not trigger changes in proline dynamics during the postabsorptive state. The metabolic significance of the reduction of plasma proline concentrations requires elucidation.     

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.     

Excess energy and nitrogen balance at protein intakes above the requirement level in young men

Effects of excess energy on nitrogen balance at above the safe level of protein intake were studied in six young men. They were given test diets with a fixed protein intake of 1.2 g.kg-1.d-1 but with three successive energy (E) levels: at maintenance (1.0 E), 15% above maintenance (1.15 E) and 30% above maintenance (1.3 E), both in ascending (1.0 E, 1.15 E, 1.3 E) and descending (1.3 E, 1.15 E, 1.0 E) sequences. Duration of each dietary period was 10 d. N balance increased from 7.2 to 23.8 to 33.3 mg N.kg-1.d-1 in the ascending series and decreased from 27.8 to 17.6 to 4.8 mg N.kg-1.d-1 in the descending series. The changes in N balance per 100 kcal change in energy intake ranged between 144 and 243 mg with smaller changes at higher energy levels. Biological value and net protein utilization of the dietary protein were positively correlated with energy intake in both the ascending series and the descending series (p less than 0.001).     

Resistance training reduces fasted- and fed-state leucine turnover and increases dietary nitrogen retention in previously untrained young men

We aimed to determine the impact of intense resistance training, designed to increase lean body mass (LBM), on both fasted and fed whole body protein kinetics in untrained young men. Twelve healthy males (22 +/- 2 y of age; BMI, 24.3 +/- 2.4 kg/m(2)) participated in a 12-wk (5-d/wk) resistance training program. Before and after training, a primed constant infusion of [1-(13)C]leucine was used to measure whole body leucine turnover, protein breakdown, and nonoxidative leucine disposal in the fasted and fed states. Participants were studied during 5-d controlled diet periods that provided a moderate protein intake [1.4 g/(kg body wt . d)]. We estimated protein turnover and nitrogen balance. Training increased LBM (61.6 +/- 6.9 vs. 64.8 +/- 6.7 kg, P < 0.05). After training, whole body leucine turnover was reduced (P < 0.01) in both fasted (167 +/- 18 vs. 152 +/- 17) and fed (197 +/- 23 vs. 178 +/- 21) states [all values micromol/(kg LBM . h)]. Training-induced decreases (P < 0.01) in protein breakdown occurred in the fasted (165 +/- 18 vs. 144 +/- 17) and fed (111 +/- 23 vs. 93 +/- 20) states. Following training, nonoxidative leucine disposal was similarly reduced (P < 0.01) in the fasted (144 +/- 18 vs. 126 +/- 18) and fed (151 +/- 20 vs. 133 +/- 19) states. Nitrogen balance was more positive after training (13.7 +/- 8.1 vs. 33.4 +/- 12.5 g/(kg LBM . d), P < 0.01) indicating an increased retention of dietary nitrogen. Intense resistance training alters whole body protein kinetics in novice weightlifters regardless of feeding status. The increase in nitrogen balance after training demonstrates a more efficient utilization of dietary nitrogen, suggesting that protein requirements for novice weightlifters are not elevated.     

Branched-chain amino acid interactions with reference to amino acid requirements in adult men: leucine metabolism at different valine and isoleucine intakes

Recent estimates of the leucine requirement of adult men based on 13C-tracer studies are substantially higher than those proposed by FAO/WHO/UNU (1985). To explore whether leucine oxidation and requirements are affected by the dietary amount of valine. 11 healthy young adult men received, in random order, for 5 d, one of four L-amino acid diets providing 40 or 15 mg leucine.kg-1.d-1 together with variable amounts mg.kg-1.d-1 of valine and isoleucine in the following combinations (Val:Ile): 80:62 and 20:62 (six subjects; phase 1); 20:62 and 20:20 (five subjects, phase 2). On the morning of day 6, a continuous intravenous infusion of L-[1-13C]leucine was given for 7-8 h; the subject was in the fasting state for the initial 2.5 or 3 h and in the fed state for the remainder of the time. Also, [2H3]leucine was added to the diet. Leucine oxidation was similar for all diet groups in the fasted state. During the fed state, leucine oxidation was not affected by the Val:Ile pattern. Thus, changes in the pattern of branched-chain amino acid intake within a physiological range do not affect isotopically derived estimates of the leucine requirement.     

Zinc utilization in young men fed adequate and low zinc intakes

Zinc absorption and balances were measured in six young men who were confined while participating in a 75-d metabolic study. A diet of conventional food providing either 16.5 or 5.5 mg Zn/d was fed. Apparent absorption of zinc was calculated from the zinc balance data and from the absorption of a zinc stable isotope added to the diet. The apparent zinc absorption calculated from the isotope data was about 25% when 16.5 mg zinc was fed; it increased to about 53 and 49% after 13 and 42 d, respectively, when 5.5 mg zinc was fed. Total zinc absorption fell from 4.1 mg/d to 2.7-2.9 mg/d during the low zinc period, however. This zinc absorption was sufficient to maintain crude zinc balances in five of the six subjects. Neither serum nor urinary zinc levels changed significantly during the period of feeding low levels of zinc. It appears that zinc absorption readily responds to changes in dietary zinc and that an intake of 5.5 mg Zn/d for about 8 wk does not cause serum or urinary zinc level to fall.     

Differences in reported winter and summer dietary intakes in young adults in Spain

Representative data of usual food consumption are necessary in order to establish accurate conclusions on the relationship between diet and disease. In most studies data are collected at one particular time of the year. The aim of this study was to determine the influence of season of the year on dietary intake estimates. Three hundred and three people aged 19-40 years from northwest Spain recorded their intake twice (in winter and summer) using the seven non-consecutive-day dietary record method. A total of 247 foods (arranged into 16 major food groups) and 27 nutrients were determined. Most fruits and vegetables, as well as some milk products showed significant seasonal differences. Daily total food consumption amount (g) was higher in winter than summer in males. Higher average intakes of energy (men) and a large number of nutrients (both sexes) were observed in winter. This seasonal influence on dietary intake estimates implies significant differences in fractions of Spanish recommendations covered (lower in summer) for energy (men) and most nutrients (both sexes). As expressed in amount/MJ (densities), most nutrients showed higher values in winter in women. Data in our study suggest that the substantial variations of nutrient intakes throughout the course of the year mainly depend on food (energy) amount variations (quantitative diet changes) in males, while changes in densities of nutrients (qualitative diet changes) are mainly responsible for dietary variations in females. Results in the present study suggest that considering the season of the year it will enhance the results of dietary surveys and ensure the accuracy of studies that link diet with disease.     

Differences in reported winter and summer dietary intakes in young adults in Spain

Representative data of usual food consumption are necessary in order to establish accurate conclusions on the relationship between diet and disease. In most studies data are collected at one particular time of the year. The aim of this study was to determine the influence of season of the year on dietary intake estimates. Three hundred and three people aged 19–40 years from northwest Spain recorded their intake twice (in winter and summer) using the seven non-consecutive-day dietary record method. A total of 247 foods (arranged into 16 major food groups) and 27 nutrients were determined. Most fruits and vegetables, as well as some milk products showed significant seasonal differences. Daily total food consumption amount (g) was higher in winter than summer in males. Higher average intakes of energy (men) and a large number of nutrients (both sexes) were observed in winter. This seasonal influence on dietary intake estimates implies significant differences in fractions of Spanish recommendations covered (lower in summer) for energy (men) and most nutrients (both sexes). As expressed in amount/MJ (densities), most nutrients showed higher values in winter in women. Data in our study suggest that the substantial variations of nutrient intakes throughout the course of the year mainly depend on food (energy) amount variations (quantitative diet changes) in males, while changes in densities of nutrients (qualitative diet changes) are mainly responsible for dietary variations in females. Results in the present study suggest that considering the season of the year it will enhance the results of dietary surveys and ensure the accuracy of studies that link diet with disease.     

Copper status and urinary and salivary copper in young men at three levels of dietary copper

Eleven young men were confined to a metabolic research unit for 90 d to determine the effect of the amount of dietary copper on copper nutriture. The study was divided into three metabolic periods (MP): 1) with an adequate-copper diet (1.68 mg/d) for 24 d, 2) with a low-copper diet (0.79 mg/d) for 42 d, and 3) with a high-copper diet (7.53 mg/d) for 24 d. Three indices of copper status, urinary copper, and salivary copper were determined at intervals throughout the study. Neither copper status, urinary copper, nor salivary copper differed among MPs. Sweat collections from three subjects suggested that losses of copper through sweat were very low and would not contribute significantly to copper balance. These results suggest that an amount of dietary copper slightly less than 0.8 mg/d is adequate to maintain copper status for greater than or equal to 42 d in normal, healthy men and that neither urinary nor salivary copper is affected by the amount of Cu in the diet.     

Longitudinal study of dietary intakes and plasma lipids in healthy elderly men and women.

Dietary intakes and blood lipid concentrations were assessed in elderly men (n = 65) and women (n = 92) from 1980 through 1989. Mean age in 1980 was 70 y (range 60-84 y). Health and socioeconomic status was above that found in the general US elderly population. Cross-sectional and longitudinal changes in variables were determined by using least-squares linear regression. No significant cross-sectional differences in energy (kJ/kg), protein (g/kg), total fat, and carbohydrate intake with age was noted. Significant longitudinal decreases in total fat, saturated and polyunsaturated fatty acids, and cholesterol intakes were noted in both men and women. Significant decreases in total, HDL, and LDL plasma cholesterol concentrations were noted over time in both men and women. Ratios of both total and LDL cholesterol to HDL cholesterol increased over time. The decrease in total fat and cholesterol intakes were significantly correlated with the decrease in total plasma cholesterol.