Combined ingestion of protein and carbohydrate improves protein balance during ultra-endurance exercise

Abstract

The aims of this study were to compare different tracer methods to assess whole body protein turnover during 6 h of prolonged endurance exercise when carbohydrate was ingested throughout the exercise period and to investigate whether addition of protein can improve protein balance. Eight endurance-trained athletes were studied on two different occasions at rest (4 h), during 6 h of exercise at 50% of maximal O₂uptake (in sequential order: 2.5 h of cycling, 1 h of running, and 2.5 h of cycling), and during subsequent recovery (4 h). Subjects ingested carbohydrate (CHO trial; 0.7 g CHO·kg^−1·h⁻¹) or carbohydrate/protein beverages (CHO + PRO trial; 0.7 g CHO·kg⁻¹·h⁻¹and 0.25 g PRO·kg⁻¹·h⁻¹) at 30-min intervals during the entire study. Whole body protein metabolism was determined by infusion of l-[1-¹³C]leucine, l-[²H₅]phenylalanine, and [¹⁵N₂]urea tracers with sampling of blood and expired breath. Leucine oxidation increased from rest to exercise [27 ± 2.5 vs. 74 ± 8.8 (CHO) and 85 ± 9.5 vs. 200 ± 16.3 mg protein·kg⁻¹·h⁻¹(CHO + PRO), P < 0.05], whereas phenylalanine oxidation and urea production did not increase with exercise. Whole body protein balance during exercise with carbohydrate ingestion was negative (−74 ± 8.8, −17 ± 1.1, and −72 ± 5.7 mg protein·kg⁻¹·h⁻¹) when l-[1-¹³C]leucine, l-[²H₅]phenylalanine, and [¹⁵N₂]urea, respectively, were used as tracers. Addition of protein to the carbohydrate drinks resulted in a positive or less-negative protein balance (−32 ± 16.3, 165 ± 4.6, and 151 ± 13.4 mg protein·kg⁻¹·h⁻¹) when l-[1-¹³C]leucine, l-[²H₅]phenylalanine, and [¹⁵N₂]urea, respectively, were used as tracers. We conclude that, even during 6 h of exhaustive exercise in trained athletes using carbohydrate supplements, net protein oxidation does not increase compared with the resting state and/or postexercise recovery. Combined ingestion of protein and carbohydrate improves net protein balance at rest as well as during exercise and postexercise recovery.