Low-Carbohydrate Training Increases Protein Requirements of Endurance Athletes

Introduction Training with low-carbohydrate (CHO) availability enhances markers of aerobic adaptation and has become popular to periodize throughout an endurance-training program. However, exercise-induced amino acid oxidation is increased with low muscle glycogen, which may limit substrate availability for postexercise protein synthesis. We aimed to determine the impact of training with low-CHO availability on estimates of dietary protein requirements. Methods Eight endurance-trained males (27 +/- 4 yr, 75 +/- 10 kg, 67 +/- 10 mL center dot kg body mass(-1)center dot min(-1)) completed two trials matched for energy and macronutrient composition but with differing CHO periodization. In the low-CHO availability trial (LOW), participants consumed 7.8 g CHO-1 center dot kg(-1) before evening high-intensity interval training (10 x 5 min at 10-km race pace, 1 min rest) and subsequently withheld CHO postexercise (0.2 g center dot kg(-1)). In the high-CHO availability trial (HIGH), participants consumed 3 g CHO center dot kg(-1) during the day before high-intensity interval training, and consumed 5 g CHO center dot kg(-1) that evening to promote muscle glycogen resynthesis. A 10-km run (similar to 80% HRmax) was performed the following morning, fasted (LOW) or 1 h after consuming 1.2 g CHO center dot kg(-1) (HIGH). Whole-body phenylalanine flux and oxidation were determined over 8 h of recovery via oral [C-13]phenylalanine ingestion, according to standard indicator amino acid oxidation methodology, while consuming sufficient energy, 7.8 g CHO center dot kg(-1)center dot d(-1), and suboptimal protein (0.93 g center dot kg(-1)center dot d(-1)). Results Fat oxidation (indirect calorimetry) during the 10-km run was higher in LOW compared with HIGH (0.99 +/- 0.35 g center dot min(-1) vs 0.60 +/- 0.26 g center dot min(-1), P < 0.05). phenylalanine flux during recovery was not different between trials (P > 0.05) whereas phenylalanine oxidation (reciprocal of protein synthesis) was higher in LOW compared with HIGH (8.8 +/- 2.7 mu mol center dot kg(-1)center dot h(-1) vs 7.9 +/- 2.4 mu mol center dot kg(-1)center dot h(-1), P < 0.05), suggesting a greater amino acid requirement to support rates of whole-body protein synthesis. Conclusions Our findings suggest that performing endurance exercise with low-CHO availability increases protein requirements of endurance athletes.