EVIDENCE-INFORMED RECOMMENDATIONS ON PROTEIN AND ITS ROLE IN SUPPORTING EXERCISE ADAPTATION

The health benefits of exercise are universally recognized and supported by evidence.¹ Exercise provides a set of signals to skeletal muscles to adapt. The stress of exercise, be it energetic or structural stress, leads to muscle adaptations that try to lessen the stress of the subsequent exercise sessions.¹

Aerobic (endurance) exercise is typified by activities that can be repeated for long(er) durations with sustained effort (biking, walking, running, swimming), and promotes a myriad of muscle adaptations. All of which improve what is known as the oxidative capacity of the muscle tissue due to the proliferation of a mitochondrial network within the muscle.¹ This local tissue adaptation is accompanied by improved heart and blood vessel function as fitness improves.¹ The outward manifestation of fitness is captured by VO_{2 peak}, the peak (highest) amount of oxygen the body can consume when we provide maximum effort.

Resistance exercise is a form of exercise that requires the repeated generation of muscular resistance against an external load. The external load could be our body weight (bw) or an external weight (load). Regardless, the practice of resistance training increases strength and, in certain circumstances, can increase muscle size. An increase in muscle size in adults (beyond the period of growth) is referred to as hypertrophy.²

A common question is, what diet best supports muscle adaptation? When discussing this question with athletes, I clarify that the 4Rs are important: Rehydrate, Refuel, and Repair to Replay.

1. Rehydration is the most easily understood concept for sports performance, and the need to do this is almost implicit.³
2. Refueling is best accomplished by a balance of carbohydrates, favoring carbohydrates to fuel higher-intensity work,³ and lipid, which, beyond our small requirement for necessary lipids,⁴ is the almost exclusive role of these macronutrients.
3. Repairing uses dietary protein as the macronutrient building block substrate for remodeling and restoring tissues to refine the phenotype of tissues like skeletal muscle.⁵
4. Replaying happens when the first 3 Rs are acted on, and athletes recover and return to play (or practice).

How much protein do athletes need? The recommended dietary allowance (RDA) defines protein needs and is set at 0.8g protein/kg of bw (actual scale weight)/day or 0.37g protein/lb of bw/day. The RDA defines a minimum protein intake and not an optimal protein intake. The acceptable macronutrient distribution range for protein is 10-30% of total energy intake. Numerous studies and meta-analyses have shown that protein intake greater than the RDA promotes superior muscular adaptations for endurance exercise,^6,7 resistance exercise,^8,9 and mixed arduous (concurrent) training.^10,11 An important consideration is that the repair and remodeling of muscle (and other tissues) takes place over days after exercise, so protein supports this repair and remodeling long after the exercise is over.

Some recommendations focus heavily on the importance of timing of protein consumption with respect to exercise.^12,13 Insofar as protein timing is concerned; however, meta-analyses show no specific advantage to timing protein intake to be in close temporal proximity to exercise to gain an anabolic advantage.¹⁴ By far, the most important variable for protein-related benefits is the total daily protein intake. If there is any advantage of protein timing on exercise-induced adaptations, the effect is small mainly because the effect of exercise in sensitizing muscle to hyperaminoacidemia persists for at least 24h.¹⁵

The dietary protein intake recommended in the most comprehensive meta-analyses for promoting resistance exercise-induced muscle adaptation is 1.6g protein/kg of bw/day (0.73g protein/lb of bw/day).^8,9 Similar intakes appear beneficial to promote endurance-based or concurrent exercise adaptations.^6,7,10,11

Since promoting optimal adaptations to exercise requires greater protein intake than the RDA, an important question is whether the source of the “extra” dietary protein matters. However, the axiom that dietary protein from plant-based sources is inherently inferior to animal-derived sources appears to be without merit when considering enhancing exercise adaptations. A randomized controlled trial showed that vegan novice resistance trainees gained similar strength and lean mass as an omnivorous group.¹⁶ It should be noted that both groups were supplemented with protein, from isolated soy in the vegan group, or whey in the omnivorous group, to an intake of 1.6g protein/kg of bw/day.¹⁶

Importantly, internet-offered arguments that soy affects male reproductive hormones are without substance.¹⁷ Perhaps more surprising is why the concept that male reproductive hormone levels somehow affect hypertrophy is even relevant, given that women gain the same relative amount of muscle as men when undertaking resistance training.¹⁸ A substantive body of evidence shows that testosterone, in particular, is not a primary driver of exercise-induced hypertrophy.¹⁹

Conversely, a relevant question is how big of an influence added protein (even when optimized) has on the adaptive responses to exercise. The short answer is that the effect is small. It is rare that individual studies, usually employing sample sizes of 8-25 people per group (with versus without protein), show differences between groups.^7-11,20 Pooled effects, with analyses of hundreds or thousands of subjects, show a small but statistically significant effect on outcomes such as strength gain, lean mass (note: not muscle mass) accretion, and increases in peak aerobic capacity.^7-11,20 The pooled estimates indicate benefits, but individual studies do not, which highlights how small the protein effect is.

Protein quality is measured using the protein digestibility corrected amino acid score (PDCAAS) and, more recently, the digestible indispensable amino acid score (DIAAS).²¹  While the approaches used to calculate scores share some commonalities, the important differences are around the use of ileal–DIAAS–as opposed to fecal digestibility–PDCAAS. Also, PDCAAS values are truncated at 1.0 with the assumption that essential amino acids (EAA) consumed above the intakes needed for tissue growth and maintenance do not provide any additional benefit. In contrast, EAA are recognized as individual nutrients in DIAAS, and when their quantities exceed that of the reference protein, proteins can have scores greater than 1.²² The practical utility of the DIAAS score and what it means for human nutrition has been challenged, especially in the context of a plant-based diet.^23-25 No meta-analysis has identified protein source (plant versus animal) as a variable that affected the outcome, whether in response to aerobic or resistance exercise.^7-11 Meta-analyses specifically focusing on soy-based protein supplements have likewise found no difference between soy-derived versus non-soy-derived protein supplements in promoting hypertrophy and strength gains.²⁰

There is minimal evidence showing that plant-based diets are inferior to animal-based diets (or the inverse) in promoting exercise-induced adaptations, especially when protein intakes are 1.6g protein/kg of bw/day (0.73g protein/lb of bw/day).^7-11,20 Soy protein is a high-quality plant protein that does not affect male reproductive hormone levels.¹⁷ In sum, plant-based diets support exercise-mediated adaptations, and soy-derived protein is one of the highest-quality plant proteins and supports aerobic and resistance exercise adaptations well.

REFERENCES

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