A Deep Dive Into Protein Intake: Research Recommendations

Strength Training 101

Individual and team sport athletes often include resistance based exercises and training protocols in their micro and macrocycles in an attempt to develop specific qualities that transfer to their sport or activity. In performing resistance training, many athletes hope to improve force production capabilities by increasing muscle size. Several months of heavy resistance training cause hypertrophy of the muscle fibers which increases total muscle mass and the possible maximum power output.1 Repetitive bouts of resistance training in combination with protein intake increase net protein balance and promote muscle protein accretion over time.2

In using resistance based exercises within training programs, athletes acutely increase muscle protein breakdown. Muscle protein breakdown is required to dismantle and repair damaged proteins and to help restore muscle function.2The rate of human skeletal muscle protein synthesis remains elevated for up to 24 hours in trained individuals and for 48-72 hours in untrained individuals. However, the restoration and subsequent development of new tissue is dependent upon a positive net balance of protein.

Protein Intake for Strength Training

Recent developments in sport nutrition research have made it possible to determine an optimal intake of protein that allows for maximum muscle protein synthesis. The purpose of this paper is to describe the physiological processes involved in muscle protein synthesis, highlight current recommendations regarding pre, intra and post-workout protein consumption that optimize muscle protein synthesis, and to provide practical recommendations for athletes.

Muscular growth adaptation to resistance exercise is driven primarily by changes in translation, in particular by increases in mRNA activity.3 Stretch, contraction, and damage to muscle fibers during resistance training provide the stimuli for muscle adaptation which involves changes in the expression of different myosin isoforms.1 Muscle fiber undergoes addition of contractile protein mass and increases the fiber size in response to training.4 However, the rate of myofibrillar protein synthesis must exceed the rate of myofibrillar breakdown over a certain period in order to increase muscle hypertrophy.1

Nutrient-gene and nutrient-protein interactions can promote or inhibit the activities of a number of cell signaling pathways and thereby affect training adaptation and subsequent performance capacity.5 In addition, exercise mode, intensity and duration may differentially impact the post-exercise protein meal requirements to optimize the stimulation of muscle protein synthesis.6

The Role of Leucine

Recent studies2, 4 have shown that leucine, an essential amino acid, plays a key role in muscle protein synthesis. While the exact mechanism is yet to be fully determined, Drummond et al.8 noted that leucine may exert its effects through an increase in plasma insulin concentrations and/or through a more pronounced activation of mTORC1 signaling pathway. Moreover, Philips4 added that the mediation of muscle protein synthesis by leucine is through the mechanistic target of mTORC1. 

The multi-protein complex mechanistic Target of Rapamycin Complex 1 (mTORC1) is a key controller of protein synthesis through the control that it exerts on mRNA activity via increasing protein translation initiation.3 Activation of mTORC1 is required for increases in protein synthesis with amino acids and resistance exercise. While the direct activation of mTORC1 is not yet fully understand, a number of signaling proteins and enzymes have been found to play important roles in this process.3 For example, the amino acid sensitive lipid kinase hVPS34 plays a key role in mTORC1 activation, as does the MAPK family member MAP4K3.3 Intracellular amino acid accumulation following resistance exercise significantly actives mTORC1 above resistance exercise alone. The mTORC1 sensing system is incredibly sensitive as just a 7% increase in intracellular leucine leads to 50% maximal activation of mTORC1.3

The recognition of leucine as a key role in muscle protein synthesis has resulted in the leucine trigger hypothesis. A rapid rise (60-90 min post workout) in blood leucine concentrations is thought to be most anabolic for stimulating post-exercise muscle protein synthesis.6 Similarly, Philips4 reported a leucine threshold which is caused by rapid post-prandial leucinemia. This subsequently leads to an increase in intracellular leucine concentration, which then triggers a rise in muscle protein synthesis.

Post-Workout Nutrition

According to Burd et al.6 & Yeo et al7.  post-exercise muscle protein synthesis rates in a dose response curve reach their breaking points at 0.25g protein per kg per meal in healthy young men. Stokes2 reported a similar finding, adding that 0.24g protein per kg body mass maximally stimulated rates of muscle protein synthesis. 

Some post-workout recommendations include the addition of carbohydrate post resistance training.7 Burd et al.6 noted that non-protein components may influence how dietary amino acids are used for protein synthesis by aiding in protein translation. The addition of carbohydrate may be warranted as carbohydrate can affect AMPK by decreasing its activation which reduces AMPK inhibition of the mTOR pathway. Drummond et al.8 added that insulin and insulin growth factor pathways play an important role in muscle hypertrophy through the Akt-mTOR signaling cascade. However, Stokes et al.2 cautioned against the addition of carbohydrate post training, adding that carbohydrate co-ingestion with protein does not enhance the anabolic effect of protein and does not contribute to hypertrophic potential following resistance exercise. 

Volek et al.9 compared the ingestion of whey protein (high leucine content) supplementation versus soy-protein supplementation (low leucine content) post resistance training to determine if the amount of leucine changes rates of muscle protein synthesis, and subsequently lean body mass. In this study the whey protein supplement was shown to significantly enhance gains in lean body mass over those seen in a soy protein-supplemented group by 83%. In agreement with Philips4 and Drummond et al.8 Volek et al.9 noted that the leucine content of a protein is the strongest determinant of the capacity of a protein to affect muscle protein synthesis. In addition, Stokes et al.2 reported that lower quality proteins that lack or are low in one or more essential amino acids fail to stimulate muscle protein synthesis to the same degree as higher quality sources. 

Athletes should consume high quality, rapidly digested, leucine rich proteins immediately after exercise. The ingestion of protein or amino acids immediately after exercise can promote muscle protein synthesis and reduce protein breakdown such that a net gain of tissue protein occurs, thereby stimulating muscle protein gain and the promotion of hypertrophy.1

Common post-workout nutritional strategies include the consumption of chocolate milk and whey-protein based ready to drink fluids such as Muscle Milk. Chocolate milk has become an affordable recovery beverage for many athletes, and provides fluid and sodium to aid in post-workout recovery. In a recent study, chocolate milk had a more positive effect on strength development in adolescents compared to carbohydrate drink only over the course of a 4 day/week, five week resistance training program.10 Perhaps this is not surprising as chocolate milk contains protein and over 700mg of leucine per serving. However, chocolate milk may be more beneficial for endurance training athletes versus strength training athletes due to its high carbohydrate content.

Interestingly, vendors from sport nutrition companies often tout the value of enhanced protein drinks, whereby the premise of higher protein contents equal superior recovery and greater muscle building compared to products with lower protein or carbohydrate contents. Muscle Milk, a ready to drink protein supplement, is often found in strength and conditioning facilities across the country. This company offers three different ready to drink protein drinks, a regular RTD shake (25g protein), Muscle Milk pro (40g protein) and a collegiate series shake (18g protein).11

In using an example of a 180-lb (81.8kg) athlete, based upon the recommendations within the research2, 6, 7) regarding amount of protein required for muscle protein synthesis, this athlete should consume 20.4g to 24.5g of protein immediately post workout. If muscle hypertrophy is the primarily goal for this athlete, and if a food-first approach is not feasible, the athlete would be best served by consuming the regular shake (25g protein) and not the collegiate or pro shake. This finding is surprising to me given my previous, falsely held assumption that higher protein content must equal greater muscle hypertrophy.  

How Much Protein Do You Really Need?

There appears to be a ceiling in the amount of daily protein required for muscle protein synthesis. Beyond a daily protein intake of 1.6g/kg/day, Stokes2 reported that any additional protein supplementation failed to augment resistance exercise-induced muscle hypertrophy. Schoenfeld and Aragon12 recommend a simple practical approach to protein consumption per day; consume protein at a target intake of 0.4g/kg/meal across a minimum of four meals in order to reach a minimum of 1.6g/kg/day. They add that such a tactic would apply what is currently known to maximize acute anabolic responses as well as chronic anabolic adaptations. While many practical recommendations regarding protein intake to optimize muscle protein synthesis exist, sports dietitians should consider the typical eating pattern and travel or training schedules of an athlete when developing meal plans.6 

There are several limitations from the research studies highlighted above. Published studies have only been undertaken for a maximum of 10 weeks.7Many studies detailing the mechanism of action of AMPK and mTORC pathways are from rodent studies and not based upon human subjects.7 Drummond8 noted few data, if any, are available on mTORC1 signaling in human skeletal muscle in response to an anabolic stimulus. Additionally, within sport nutrition research subjects are often well trained at best, often male, and almost always sub-elite.13 

Continued research is needed to confirm theories (leucine threshold) and recommendations (0.24-0.25g protein/kg/meal). I think Tipton14 summarizes this thought, as he wrote “it may be beneficial to clarify whether successful athletes have already refined optimal nutrient-training protocols that enhance endurance performance and can better inform future scientific enquiry.”

References:

  1. Jeukendrup, A. & Gleeson, M. Sports Nutrition. Metabolic responses to exercise. 2019. 3rd Ed. 
  2. Stokes, T., Hector, A.J., Morton, R.W., McGlory, C. & Phillips, S.M. Recent perspectives regarding the role of dietary protein for the promotion of muscle hypertrophy with resistance exercise training. Nutrients. 2018; 10(180)
  3. Close, G.L., Hamilton, D.L., Philp, A., Burke. L.M. & Morton, J.P. New strategies in sport nutrition to increase exercise performance. Free Radical Biology and Medicine. 2016; 98, 144-158
  4. Phillips, S.M. The impact of protein quality on the promotion of resistance exercise-induced changes in muscle mass. Nutrition & Metabolism. 2016; 13(64) 1-9
  5. Barr, K. Nutrition and the molecular response to strength training. Sports Science Exchange. 2014; 27(123) 1-4
  6. Burd, N.A., Beals, J.W., Martinez, I.G., Salvador, A.F. & Skinner, S.K. Food-first approach to enhance the regulation of post-exercise skeletal muscle protein synthesis and remodeling. Sports Medicine. 2019; 49(1) 559-568
  7. Yeo, W.K., C.D. Paton, A.P. Garnham, L.M. Burke, A.L. Carey, and J.A. Hawley (2008). Skeletal muscle adaptation and performance responses to once a day versus twice every second day endurance training regimens. J. Appl. Physiol. 2008. 105:1462-1470
  8. Drummond, M.J., Dreyer, H.C., Fry, C.S., Glynn, E.L. & Rasmussen, B.B. Nutritional and contractile regulation of human skeletal muscle protein synthesis and mTORC1 signaling. J Appl Physiol. 2009; 106, 1374-1384
  9. Volek et al. Whey protein supplementation during resistance training augments lean body mass. J Am Coll Nutr. 2013; 32(2), 122-135
  10. Born, K.A., Dooley, E.E., Cheshire, P.A., McGill, L.E., Cosgrove, J.M., Ivy, J.L. & Bartholomew, J.M. Chocolate milk versus carbohydrate supplements in adolescent athletes: a field based study. J Int Soc Spt Nutr. 2019; 16(6) 
  11. Muscle Milk. Website. https://shop.musclemilk.com/ Accessed February 22nd, 2020. 
  12. Schoenfeld, B. & Aragon, A. How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. J Intl Soc Spt Nutr. 2018; 15,10
  13. Burke, L.M. & Hawley, J.A. Swifter, higher, stronger: What’s on the menu? Science. 2018. 781-787
  14. Tipton, K.D. Factors that influence the amount of protein necessary to maximize the anabolic response of muscle following resistance exercise. Sports Science Exchange. 2017; 28(172) 1-6