Exercise-Induced Dehydration Limits the Effectiveness of Post-Exercise Whey Protein Supplementation in Healthy Young Men

Background/Objectives: Post-exercise protein ingestion is widely used to promote recovery from exercise, but whether exercise-induced dehydration limits the subsequent protein supplementation response remains unclear. This study examined the effects of different hydration strategies during exercise on post-exercise whey protein supplementation. Methods: Thirteen healthy young men completed three partially crossover trials designed to produce approximately 2% (DEH: a dehydration trial without fluid replacement), 1% (EUH_50%: a 50% fluid-replacement trial), and 0% (EUH: a euhydration trial with 100% fluid replacement) body mass loss. Within 5 min after each exercise, participants ingested 45 g of whey protein dissolved in 450 mL of water. Serum amino acid concentrations were measured before exercise, immediately after exercise, and at 60 and 120 min after ingestion. Results: The protocol produced distinct exercise hydration status, with body mass loss of 2.20 ± 0.19% (DEH), 1.18 ± 0.30% (EUH_50%), and 0.27 ± 0.35% (EUH), respectively. Post-exercise whey protein ingestion increased serum amino acid concentrations in all trials. However, at 60 min after ingestion, the DEH trial showed lower total amino acid, branched-chain amino acid, leucine, isoleucine, and glutamine concentrations than both EUH_50% and EUH (all p < 0.05). The 0–120 min percentage incremental area under the curve for total amino acids, branched-chain amino acids, isoleucine, and glutamine was also lower in DEH. Gastrointestinal symptoms remained low across all conditions, with only 1.0% of individual item scores exceeding 4. Conclusions: These findings indicate that exercise-induced hypohydration of approximately 2.2% body mass attenuates the early circulating amino acid response to post-exercise whey protein ingestion, suggesting that inadequate fluid intake during exercise may compromise the efficacy of whey protein supplementation for post-exercise recovery.