An Acute Reduction in Habitual Protein Intake Attenuates Post Exercise Anabolism and May Bias Oxidation-Derived Protein Requirements in Resistance Trained Men

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.

Keywords: hypertrophy; indicator amino acid oxidation; muscle growth; protein oxidation; protein requirements; protein synthesis; resistance training; stable isotopes.

Copyright © 2020 Tinline-Goodfellow, West, Malowany, Gillen and Moore.

Figures

Figure 1

Study design. (A,B) High and moderate protein intake phase overview. White bars represent non-trial days, with a controlled diet. Black bars represent a metabolic trial day. (C) Metabolic trial day overview. Test drinks provided 1/12th of dietary requirement each, with ingestion of the tracer (L-[1-13C] phenylalanine) beginning with drink five. R.Ex, resistance exercise; VCO2, volume of expired CO2 collected via indirect calorimetry; CHO, Carbohydrate.

Figure 2

Phenylalanine metrics. (A) Phenylalanine rate of appearance (PheRa, Flux) had no effect for time. (B) There was a main effect of condition for fraction of expired 13CO2 (F13CO2). F13CO2 was significantly elevated from H to M1 (d = 2.58), and from H to M3 (d = 1.96). There was a trend for M1 to be elevated compared to M5 (d = 1.75, P = 0.07). (C) Phenylalanine oxidation (PheOx) displayed a main effect of condition and was significantly elevated at M1 compared to H (d = 2.56). There was a trend for M3 to be significantly higher than H (d = 1.66, P = 0.09). There was a trend for M5 to be lower than M1 (d = 1.78, P = 0.08). The effect of condition was determined using a repeated measures one-way ANOVA, with a Holm-Sidak post-hoc for pairwise comparisons. *Significantly different from H (P < 0.05). Data are means ± SE.

Figure 3

Phenylalanine net balance. Whole body phenylalanine net balance is significantly decreased at M1 compared to H (d = −2.57), with a trend for M3 to be less than H (d = −1.68, P = 0.09). There was a trend for M5 to be greater than M1 with a large effect size (d = 1.78, P = 0.08), suggesting adaptation to the moderate protein intake may take 5d. The effect of condition was determined using a repeated measures one-way ANOVA, with a Holm-Sidak post-hoc for pairwise comparisons. *Significantly different from H (P < 0.05). Data are means ± SE.