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There is general agreement that individuals engaged in strength and endurance exercise training require more dietary protein than the generally healthy population. As noted by Paddon-Jones, the RDA (0.8 g/kg bodyweight) “was never designed to provide prescriptive guidance for populations with extraordinary demands, be they clinical or athletic.” Just how much dietary protein is needed by exercisers is a matter of some debate, and (not surprisingly) will depend upon the type and intensity of the exercise. But in general, recommendations range from 1.2 to 2.0 g/kg, although a recent meta-analysis on protein supplementation involving resistance exercise trainees reported an upper 95% confidence interval of 2.2 g/kg/day.
While these recommendations are considerably higher than the RDA, they are compatible with guidelines from the Institute of Medicine to consume a diet that ranges in protein content of between 10 and 35% of calories. The upper end of that range represents a daily protein intake of 175 g for a person consuming 2,000 calories per day, which for a 160-pound person is about 2.4 g/kg body weight. Furthermore, according to NHANES data, the average American male consumes about 99 g of protein per day (about 1.4 g/kg body weight for a 160 pound person).6 Thus, it is not difficult to consume the amount of protein needed by exercisers-- especially when considering that caloric needs will increase as a result of exercise.
Over the years concerns have arisen about the potentially harmful effects of high-protein diets. One concern frequently mentioned is the adverse effect of protein intake on renal function. However, a very recently published meta-analysis of the clinical data found that high protein intakes do not adversely influence kidney function or glomerular filtration rate in healthy adults.7
In addition to total protein intake, questions have arisen about the impact of the timing of protein ingestion on performance, especially gains in strength and lean body mass in response to resistance exercise training. The anabolic window refers to the time period after resistance exercise during which dietary protein maximally stimulates muscle protein synthesis. A commonly held belief is that the anabolic window lasts for about 2 hours. However, more recent research refutes this belief and downplays the importance of timing.5,8 Enhanced protein synthesis following exercise may last for as long as 24 hours.
Another factor potentially influencing muscle mass is dietary protein distribution; that is, the distribution of protein intake throughout the day. Americans tend to consume most of their protein at dinner and via after-dinner snacking.9 However, evidence suggests that for optimal benefit, protein intake should be evenly distributed throughout the day. To this point, Schoenfeld and Aragon,10 propose that to maximize anabolism, one should consume protein at a target intake of 0.4 g/kg/meal across a minimum of 4 meals in order to reach a minimum of 1.6 g/kg/day. Using the upper daily intake of 2.2 g/kg/day reported in the literature and spreading intake over the same 4 meals would necessitate a maximum of 0.55 g/kg/meal. These recommendations mimic those made earlier by Paddon-Jones and Rasmussen11 to consume 25-30 g protein per meal.
Finally, there is the question of the influence of protein type on gains in muscle mass and strength. Because of its high leucine concentration and perhaps also because it is so quickly digested, whey protein is generally considered to be the optimal protein source for building muscle.12 Certainly, acute studies that measure muscle protein synthesis over a 4-hour period support this conclusion.13,14
However, a recent systematic review concluded that over a period of many weeks, protein source plays a minor (if any) role in determining gains in lean body tissue and strength in response to resistance exercise training.5 This point was recently confirmed by a meta-analysis that included 9 clinical trials that compared the effect of soy protein supplementation with whey protein supplementation and supplementation with other animal proteins on gains in strength and lean body mass in response to resistance exercise training programs lasting between 6 and 36 weeks.15 This analysis found no difference in gains between soy protein and whey and soy protein and the other animal proteins. Despite this finding, more work needs to be done to determine whether these findings apply to extremely highly-trained individuals. Still, overall, the evidence indicates that protein amount is more important than protein source.
Finally, there is speculative evidence to suggest that a blend of proteins may have subtle advantages over any single protein. For example, data from a 12-week trial by Reidy et al.16 found that a blend of casein (50%), whey (25%), and soy protein (25%) enhanced overall whole body lean mass in young men undergoing resistance exercise training more than whey protein alone. Also, Mobley et al.17 recently found that soy protein preferentially increased the size of type I (slow twitch) muscle fibers whereas whey protein supplementation preferentially increased the size of type II fibers (fast twitch).
In conclusion, exercise increases dietary protein requirements. With respect to gains in muscle mass and strength, data suggest protein type plays a much lesser role than protein amount. Although speculative, blends of proteins may have advantages over single proteins. For maximal benefit, dietary protein should be evenly distributed throughout the day.
1. Paddon-Jones D. Protein recommendations for bodybuilders: In this case, more may indeed be better. J Nutr. 2017;147(5):723-4.
2. Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics. 2016;116(3):501-28.
3. äger R, Kerksick CM, Campbell BI, et al. International Society of Sports Nutrition Position Stand: protein and exercise. J Internal Soc Sports Nutr. 2017;14(20):1-25.
4. American Dietetic A, Dietitians of C, American College of Sports M, et al. American College of Sports Medicine position stand. Nutrition and athletic performance. Med Sci Sports Exerc. 2009;41(3):709-31.
5. Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2017;52376-84.
6. Pasiakos SM, Agarwal S, Lieberman HR, et al. Sources and amounts of animal, dairy, and plant protein intake of US adults in 2007-2010. Nutrients. 2015;7(8):7058-69.
7. Devries MC, Sithamparapillai A, Brimble KS, et al. Changes in kidney function do not differ between healthy adults consuming higher- compared with lower- or normal-protein diets: A systematic review and meta-Analysis. J Nutr. 2018;148(11):1760-75.
8. Schoenfeld BJ, Aragon AA, Krieger JW. The effect of protein timing on muscle strength and hypertrophy: a meta-analysis. Journal of the International Society of Sports Nutrition. 2013;10(1):53.
9. Berner LA, Becker G, Wise M, et al. Characterization of dietary protein among older adults in the United States: Amount, animal sources, and meal patterns. Journal of the Academy of Nutrition and Dietetics. 2013;113(6):809-15.
10. Schoenfeld BJ, Aragon AA. How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. Journal of the International Society of Sports Nutrition. 2018;1510.
11. Paddon-Jones D, Rasmussen BB. Dietary protein recommendations and the prevention of sarcopenia. Current opinion in clinical nutrition and metabolic care. 2009;12(1):86-90.
12. Devries MC, Phillips SM. Supplemental protein in support of muscle mass and health: advantage whey. J Food Sci. 2015;80 Suppl 1A8-A15.
13. Gran P, Larsen AE, Bonham M, et al. Muscle p70S6K phosphorylation in response to soy and dairy rich meals in middle aged men with metabolic syndrome: a randomised crossover trial. Nutr Metab (Lond). 2014;11(1):46.
14. Mitchell CJ, Della Gatta PA, Petersen AC, et al. Soy protein ingestion results in less prolonged p70S6 kinase phosphorylation compared to whey protein after resistance exercise in older men. Journal of the International Society of Sports Nutrition. 2015;126.
15. Messina M, Lynch H, Dickinson JM, et al. No difference between the effects of supplementing with soy protein versus animal protein on gains in muscle mass and strength in response to resistance exercise. International journal of sport nutrition and exercise metabolism. 2018;28(6):674-85.
16. Reidy PT, Borack MS, Markofski MM, et al. Protein supplementation has minimal effects on muscle adaptations during resistance exercise training in young men: A double-blind randomized clinical trial. J Nutr. 2016;146(9):1660-9.
17. Mobley CB, Haun CT, Roberson PA, et al. Effects of whey, soy or leucine supplementation with 12 weeks of resistance training on strength, body composition, and skeletal muscle and adipose tissue histological attributes in college-aged males. Nutrients. 2017;9(9).
Mark Messina, PhD, MS, is the executive director of the Soy Nutrition Institute, president of Nutrition Matters, Inc., a nutrition consulting company, and an adjunct professor in the School of Public Health at Loma Linda University. He has published more than 100 peer-reviewed articles on soyfoods and soybean components. Messina is the editorial chairperson for The Soy Connection newsletter and has been since its inception.
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