Leucine kinetics in reference to the effect of the feeding mode as three discrete meals

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.

Mild-to-moderate chronic cholestatic liver disease increases leucine oxidation in children

Malnutrition is prevalent in children with chronic cholestatic liver disease. Using the noninvasive indicator amino acid oxidation (IAAO) technique, we recently determined that mild-to-moderate chronic cholestatic (MCC) liver disease increases the need for branched-chain amino acids (BCAA) in children. To examine the underlying mechanisms responsible for this increased need for BCAA in liver disease, we measured L-[1-(13C)]-leucine oxidation in the postabsorptive and fed states in 10 children with MCC liver disease (8.8 +/- 3.5 y) and in 11 healthy children (9.4 +/- 2.2 y). The oxidation of L-[1-(13C)]-leucine to 13CO2 [F13CO2 in micromol/(kg.h)] was determined after a primed, continuous oral administration of the tracer. Total BCAA in diet was provided at 300 mg/(kg.d) to ensure that leucine oxidation was measured when leucine intake was in excess of requirements. In the postabsorptive state, the rate of release of 13CO2 from 13C-leucine oxidation (F13CO2) and whole-body leucine oxidation were significantly higher in children with MCC liver disease (P < 0.05). However, F13CO2 and whole-body leucine oxidation did not differ in the fed state. We conclude that the increased need for dietary BCAA in MCC liver disease is mediated in part by increased leucine oxidation in the postabsorptive state.

Effect of cystine on the methionine requirement of healthy Indian men determined by using the 24-h indicator amino acid balance approach

BACKGROUND

The 1985 FAO/WHO/UNU requirement for methionine in healthy adults consuming a cystine-free diet is 13 mg.kg(-1).d(-1). It is unclear whether this daily requirement is influenced by dietary cystine.

OBJECTIVE

We assessed the effect of 2 intakes of cystine (5 and 12 mg.kg(-1).d(-1)) on methionine requirements in well-nourished Indian men by using 7 test methionine intakes (3, 6, 9, 13, 18, 21 and 24 mg.kg(-1).d(-1)) and the 24-h indicator amino acid oxidation (24-h IAAO) and balance (24-h IAAB) methods. We combined these data with those from an experiment with zero cystine intake and in which the exact same method was used.

DESIGN

Two studies were performed in which a diet containing either 5 or 12 mg cystine.kg(-1).d(-1) was fed to 21 well-nourished Indian men over three 7-d periods. The 24-h IAAO and 24-h IAAB values were measured on day 7 with the use of a 24-h intravenous [13C]leucine tracer infusion. The breakpoints in the relation between these values and methionine intake in each study were assessed by two-phase linear regression.

RESULTS

Breakpoints in the response curve were obtained at methionine intakes of 20 (95% Fiellers CI: 17, 26) and 10 (95% Fiellers CI: 8, 16) mg.kg(-1).d(-1) with cystine intakes of 5 and 12 mg.kg(-1).d(-1) intakes, respectively, which suggested a sparing effect of cystine. Although the 5- and 12-mg cystine breakpoints differed from one another, they did not differ significantly from that estimated previously with 0 mg cystine.

CONCLUSION

Cystine may spare the methionine requirement in healthy men, although the amount of sparing is difficult to quantify.

The rate of protein digestion affects protein gain differently during aging in humans

In young men ingesting protein meals, slowly digested proteins (caseins: CAS) induce a higher protein gain than those that are rapidly digested (whey proteins: WP). Our aim was to assess whether or not this is true in elderly men receiving mixed meals. The effects of meals containing either CAS or two different amounts of WP (WP-iN: isonitrogenous with CAS, or WP-iL: providing the same amount of leucine as CAS) on protein metabolism (assessed by combining oral and intravenous leucine tracers) were compared in nine healthy, elderly (mean +/- S.E.M. age 72 +/- 1 years) and six young men (24 +/- 1 years). In both age groups, WP-iL and WP-iN were digested faster than CAS (P < 0.001, ANOVA). Proteolysis was inhibited similarly whatever the meal and age groups (P = NS). Protein synthesis was higher with WP-iN than with CAS or WP-iL (P < 0.01), irrespective of age (P = NS). An age-related effect (P < 0.05) was found with postprandial leucine balance. Leucine balance was higher with CAS than with WP-iL (P < 0.01) in young men, but not in elderly subjects (P = NS). In isonitrogenous conditions, leucine balance was higher with WP-iN than with CAS (P < 0.001) in both age groups, but the magnitude of the differences was higher in the elderly men (P = 0.05). In conclusion, during aging, protein gain was greater with WP (rapidly digested protein), and lower with CAS (slowly digested protein). This suggests that a 'fast' protein might be more beneficial than a 'slow' one to limit protein losses during aging.

Type and timing of protein feeding to optimize anabolism

PURPOSE OF REVIEW

The delivery rate of amino acids to an organism significantly affects protein anabolism. The rate can be controlled by the type and the timing of feeding. Our aim was to bring new insights to the way they may act.

RECENT FINDINGS

During young and adult ages, when food supply is liberal, subjects can adapt to various modes of protein feeding. However, during food restriction, protein anabolism is favored when the delivery of amino acids is evenly distributed over the day, either with frequent meals, or through the use of slowly absorbed proteins like casein. In contrast, during aging, quickly absorbed protein sources become more efficient. During recovery after exercise, the timing of protein feeding after the end of exercise may or may not influence its anabolic effect, depending on the subject's age and the type of exercise.

SUMMARY

The synchronization of variations in anabolic capability with amino acid supply partly explains the effects of the type and timing of protein feeding. This effect is modulated by the amount of amino acids required to increase whole-body proteins and by the signaling properties of some amino acids to stimulate protein synthesis. Indeed, the anabolic effect of amino acids is determined by their interaction with other anabolic factors (other nutrients or physiological factors, whose efficiency is mainly related to their effect on protein degradation). It is clear that benefits can be obtained from adapted protein feeding patterns.

Approaches to quantifying protein metabolism in response to nutrient ingestion

The investigation of protein metabolism under various nutritional and physiological conditions has been made possible by the use of indirect, principally tracer-based methods. Most studies were conducted at the whole-body level, mainly using steady-state isotopic techniques and equations based on simple two-pool models, in which amino acids are either free or protein bound. Because whole-body methods disregard regional contributions to protein metabolism, some regional approaches have tried to distinguish the distribution of protein kinetics in the different tissues. The organ-balance tracer technique, involving the arteriovenous catheterization of regions or organs with concomitant isotopic tracer infusion, distinguishes between amino acid uptake and release in the net amino acid balance and measures protein synthesis and degradation under steady-state conditions. Last, the importance has become clear of the difference in dietary and endogenous amino acids recycled from proteolysis for anabolic and catabolic pathways. In humans, the dual tracer technique, which consists of the simultaneous oral/enteral administration and intravenous infusion of different tracers of the same amino acid, allows an estimate of the splanchnic uptake of amino acids administered. Furthermore, the whole-body retention of labeled dietary nitrogen after the ingestion of a single protein meal has enabled a clearer understanding of the metabolic fate of dietary amino acids. Based on such data, a newly developed compartmental model provides a simulation of the regional distribution and metabolism of ingested nitrogen in the fed state by determining its dynamic fate through free and protein-bound amino acids in both the splanchnic and peripheral areas in humans.

Threonine requirement of healthy adults, derived with a 24-h indicator amino acid balance technique

BACKGROUND

Because we question the validity of the 1985 FAO/ WHO/UNU upper requirement for threonine of 7 mg x kg(-1) x d(-1), we proposed a tentative mean requirement of 15 mg x kg(-1) x d(-1).

OBJECTIVE

Our goal was to assess threonine adequacy at 3 test intakes and the consequences of a 6-d compared with a 13-d dietary adaptation phase.

DESIGN

We used a 24-h indicator amino acid balance technique ([1-(13)C]leucine as indicator) to assess the threonine requirement. Fifteen healthy adults were randomly assigned to receive 7, 15, or 46 mg threonine x kg(-1) x d(-1) and were studied after 6 and 13 d of adaptation to the experimental diets. Diets were based on an L-amino acid mixture in which the threonine content was varied. At 1700 on days 6 and 13, a 24-h intravenous [(13)C]leucine tracer infusion protocol was begun to assess leucine oxidation and daily leucine balances.

RESULTS

There was no detectable effect of duration of dietary adaptation in leucine oxidation or balance, but the 24-h leucine oxidation and balances differed significantly between the 7-mg intake and each of the 2 higher intakes (P < 0.05). The latter were not significantly different. The 24-h leucine oxidation rate decreased across threonine intakes (P < 0.01 for main effect of diet, independent of infusion day). Leucine oxidation was highly correlated (r = 0.80) between the 2 dietary adaptation phases across all test intakes.

CONCLUSION

The 1985 FAO/WHO/UNU threonine recommendation is inadequate, and 15 mg x kg(-1) x d(-1) is sufficient to achieve mean indicator (leucine) amino acid balance.

Daily requirement for and splanchnic uptake of leucine in healthy adult Indians

BACKGROUND

The 1985 FAO/WHO/UNU requirement for leucine is too low according to tracer-derived estimates of leucine oxidation and balance in adults from developed regions.

OBJECTIVE

The leucine requirement in populations in developing countries was assessed with use of the 24-h tracer balance method and on the basis of nitrogen balances.

DESIGN

Twenty healthy Indian men were studied during their consumption for 6 d of 2 L-amino acid diets that supplied either 14 and 30 (n = 10) or 22 and 40 (n = 10) mg leucine x kg(-1) x d(-1) in random order. At 1800 on day 7, a 24-h constant intravenous [13C]leucine tracer-infusion protocol was conducted to determine leucine oxidation and daily leucine balance. During the intake of 40 mg leucine/d, [2H3]leucine was given orally to assess the splanchnic uptake of leucine.

RESULTS

Mean 24-h leucine oxidation rates were 29.8, 30.6, 33.6, and 39.3 mg x kg(-1) x d(-1) at leucine intakes of 14, 22, 30, and 40 mg x kg(-1) x d(-1), respectively; daily leucine balances were -16.5, -9.0, -3.3, and 0.5 mg x kg(-1) x d(-1), respectively. Mixed-models linear regression of balance against leucine intake resulted in a zero balance at a leucine intake of 37.3 mg x kg(-1) x d(-1). Nitrogen balances were -12.7, -17.9, -3.9, and 1.0 mg x kg(-1) x d(-1) at leucine intakes of 14, 22, 30, and 40 mg x kg(-1) x d(-1). Regression of nitrogen balance against intake resulted in a zero balance at a leucine intake of 37.6 mg x kg(-1) x d(-1). The first-pass splanchnic uptake of leucine was 45.7% and 33.9% in the fasted and fed periods, respectively.

CONCLUSION

A tentative mean leucine requirement of 40 mg x kg(-1) x d(-1) is proposed for healthy Indian adults, as it is for Western subjects.

Lysine requirements of healthy adult Indian subjects, measured by an indicator amino acid balance technique

BACKGROUND

In an earlier study, using a modification of the indicator amino acid oxidation approach, we concluded that the 1985 FAO/WHO/UNU-proposed lysine requirement of 12 mg x kg(-1) x d(-1) is likely inadequate to maintain body amino acid homeostasis in apparently healthy south Asian subjects and that our proposed requirement of 30 mg x kg(-1) x d(-1) is more appropriate.

OBJECTIVE

We assessed the lysine requirement in a similar population by using 4 test lysine intakes (12, 20, 28, and 36 mg x kg(-1) x d(-1)) with an indicator amino acid balance approach.

DESIGN

Sixteen healthy male Indians were studied during each of 2 randomly assigned 8-d L-amino acid diets that supplied either 12 and 28 or 20 and 36 mg lysine. At 1800 on day 8, a 24-h intravenous [(13)C]leucine tracer-infusion protocol was conducted to assess leucine oxidation and daily leucine balance at each lysine intake.

RESULTS

Mean 24-h leucine oxidation rates decreased significantly (P = 0.005) across different lysine intakes and were 104.1, 97.8, 87.3, and 87.3 mg x kg(-1) x d(-1) at intakes of 12, 20, 28, and 36 mg x kg(-1) x d(-1), respectively; mean 24-h leucine balances were 3.3, 9.1, 19.7, and 20.7 mg x kg(-1) x d(-1), respectively (P = 0.015, mixed-model analysis of variance). Oxidation and balances differed significantly between the lower and higher lysine intakes but were not significantly different between the 12- and 20-mg and 28- and 36-mg test intakes. Two-phase regression analysis indicated a mean breakpoint at 29 mg lysine x kg(-1) x d(-1) in the relation between lysine intake and leucine oxidation or balance.

CONCLUSION

We propose a mean lysine requirement of 30 mg x kg(-1) x d(-1) for healthy Indian adults, which is the same amount we proposed previously for Western populations.

The digestion rate of protein is an independent regulating factor of postprandial protein retention

To evaluate the importance of protein digestion rate on protein deposition, we characterized leucine kinetics after ingestion of "protein" meals of identical amino acid composition and nitrogen contents but of different digestion rates. Four groups of five or six young men received an L-[1-13C]leucine infusion and one of the following 30-g protein meals: a single meal of slowly digested casein (CAS), a single meal of free amino acid mimicking casein composition (AA), a single meal of rapidly digested whey proteins (WP), or repeated meals of whey proteins (RPT-WP) mimicking slow digestion rate. Comparisons were made between "fast" (AA, WP) and "slow" (CAS, RPT-WP) meals of identical amino acid composition (AA vs. CAS, and WP vs. RPT-WP). The fast meals induced a strong, rapid, and transient increase of aminoacidemia, leucine flux, and oxidation. After slow meals, these parameters increased moderately but durably. Postprandial leucine balance over 7 h was higher after the slow than after the fast meals (CAS: 38 +/- 13 vs. AA: -12 +/- 11, P < 0.01; RPT-WP: 87 +/- 25 vs. WP: 6 +/- 19 micromol/kg, P < 0.05). Protein digestion rate is an independent factor modulating postprandial protein deposition.