Abstract
Protein recommendations for resistance-trained athletes are generally lower than their habitual intakes. Excess protein consumption increases the capacity to oxidize amino acids, which can attenuate post-exercise anabolism and may impact protein requirements determined by stable isotope techniques predicated on amino acid tracer oxidation. We aimed to determine the impact of an acute (5d) reduction in dietary protein intake on post-exercise anabolism in high habitual consumers using the indicator amino acid oxidation (IAAO) technique. Resistance trained men [n = 5; 25 ± 7 y; 73.0 ± 5.7 kg; 9.9 ± 2.9% body fat; 2.69 ± 0.38 g·kg−1·d−1 habitual protein intake) consumed a high (H; 2.2 g·kg−1·d−1) and moderate (M; 1.2 g·kg−1·d−1) protein diet while training every other day. During the High protein phase, participants consumed a 2d controlled diet prior to determining whole body phenylalanine turnover, net balance (NB), and 13CO2 excretion (F13CO2) after exercise via oral [13C]phenylalanine. During the Moderate phase, participants consumed 2.2 g protein·kg−1·d−1 for 2d prior to consuming 1.2 g protein·kg−1·d−1 for 5d. Phenylalanine metabolism was measured on days 1, 3, and 5 (M1, M3, and M5, respectively) of the moderate intake. F13CO2, the primary outcome for IAAO, was ~72 and ~55% greater on the 1st day (M1, P < 0.05) and the third day of the moderate protein diet (M3, P = 0.07), respectively, compared to the High protein trial. Compared to the High protein trial, NB was ~25% lower on the 1st day (M1, P < 0.01) and 15% lower on the third day of the moderate protein diet (M3, P = 0.09). High habitual protein consumption may bias protein requirements determined by traditional IAAO methods that use only a 2d pre-trial controlled diet. Post-exercise whole body anabolism is attenuated following a reduction in protein intake in resistance trained men and may require ~3–5d to adapt. This trial is registered at clinicaltrials.gov as NCT03845569.
Highlights
Dietary protein ingestion acts synergistically with resistance exercise to elevate rates of muscle protein synthesis (MPS) [1, 2]and enhance whole-body net protein balance [3], which over time supports training adaptations such as muscle hypertrophy.The importance of dietary protein for the recovery from and adaptation to resistance exercise is reflected in the generally accepted greater daily recommendations in athletic populations relative to their weight-stable, non-training counterparts [4]
We demonstrate that reducing dietary protein intake in habitually high protein consumers from a standardized 2.2 g·kg−1·d−1 to a moderate 1.2 g·kg−1·d−1 resulted in a reduction in whole-body net protein balance during recovery from resistance exercise for up to 3 days, greater durations (i.e., ≥5 days) may be needed for full adaptation
This reduced net balance was not related to any accommodation in whole body protein breakdown (i.e., PheRa) but rather was the result of an elevated excretion of our indicator amino acid (i.e., F13CO2), suggesting that metabolic adaptation in high habitual protein consumers requires greater durations than the traditional 2 days adaptation period employed in indicator amino acid oxidation technique (IAAO) studies [11, 12, 20, 30]
Summary
Dietary protein ingestion acts synergistically with resistance exercise to elevate rates of muscle protein synthesis (MPS) [1, 2]and enhance whole-body net protein balance [3], which over time supports training adaptations such as muscle hypertrophy.The importance of dietary protein for the recovery from and adaptation to resistance exercise is reflected in the generally accepted greater daily recommendations in athletic populations relative to their weight-stable, non-training counterparts [4]. The safe protein intake (i.e., +2 SD of the estimated average requirement; EAR) by nitrogen balance has been suggested to be greater in novice weight lifters as compared to trained body builders (i.e., ∼1.7 vs 1.2 g·kg−1·d−1) [7, 8]. This may be related in part to an increased requirement to support muscle damage repair and growth that is typically greatest during the initiation of a resistance training program [9, 10]. Recent estimates of the protein requirements to maximize whole body protein synthesis on a non-training day by the indicator amino acid oxidation technique (IAAO)
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