Purpose: This study aimed to determine the effect of postexercise protein-leucine coingestion with CHO-lipid on subsequent high-intensity endurance performance and to investigate candidate mechanisms using stable isotope methods and metabolomics. Methods: In this double-blind, randomized, crossover study, 12 male cyclists ingested a leucine/protein/CHO/fat supplement (LEUPRO 7.5/20/89/22 g/h, respectively) or isocaloric CHO/fat control (119/22 g/h) 1-3 h after exercise during a 6-d training block (intense intervals, recovery, repeated-sprint performance rides). Daily protein intake was clamped at 1.9 g/kg·d (LEUPRO) and 1.5 g/kg·d (control). Stable isotope infusions (1-13C-leucine and 6,6-2H2-glucose), mass spectrometry-based metabolomics, and nitrogen balance methods were used to determine the effects of LEUPRO on whole-body branched-chain amino acid (BCAA) and glucose metabolism and protein turnover. Results: After exercise, LEUPRO increased BCAA levels in plasma (2.6-fold; 90% confidence limits = ×/÷1.1) and urine (2.8-fold; ×/÷1.2) and increased products of BCAA metabolism plasma acylcarnitine C5 (3.0-fold; ×/÷0.9) and urinary leucine (3.6-fold; ×/÷1.3) and ß-aminoisobutyrate (3.4-fold; ×/÷1.4), indicating that ingesting ~10 g leucine per hour during recovery exceeds the capacity to metabolize BCAA. Furthermore, LEUPRO increased leucine oxidation (5.6-fold; ×/÷1.1) and nonoxidative disposal (4.8-fold; ×/÷1.1) and left leucine balance positive relative to control. With the exception of day 1 (LEUPRO = 17 ± 20 mg N/kg, control = -90 ± 44 mg N/kg), subsequent (days 2-5) nitrogen balance was positive for both conditions (LEUPRO = 130 ± 110 mg N·kg-1, control = 111 ± 86 mg N·kg-1). Compared with control feeding, LEUPRO lowered the serum creatine kinase concentration by 21%-25% (90% confidence limits = ±14%), but the effect on sprint power was trivial (day 4 = 0.4% ± 1.0%, day 6 = -0.3% ± 1.0%). Conclusions: Postexercise protein-leucine supplementation saturates BCAA metabolism and attenuates tissue damage, but effects on subsequent intense endurance performance may be inconsequential under conditions of positive daily nitrogen balance.
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