Pretty much every modern fitness enthusiast knows that protein is important for building muscle. The protein we consume breaks down into amino acids, and those compounds trigger and support muscle protein synthesis. But how do carbohydrates play into all this?
We already know that consuming carbohydrates will cause blood insulin levels to increase. What is less known is that insulin, like amino acids, is an important player in both muscle protein synthesis (MPS) and muscle protein breakdown (MPB).
Yes, that’s right. Contrary to popular belief, insulin is a wonderful hormone that helps us build muscle too.
Elevated levels of insulin help prevent muscle from breaking down , and furthermore, muscle only builds when insulin is present. For instance, it has been well-observed that insulin assists MPS by recruiting more blood and thus more quickly transferring nutrients to skeletal muscle tissue. Even though dozens of studies confirm this, insulin’s exact role is still being debated .
Is insulin capable of triggering and increasing MPS on its own, or is it simply an “ingredient” that must be present?
Enter Dr. Stuart Phillips, Ph.D., and Professor in the Department of Kinesiology at McMaster University; a name I will bring up a lot due to his MAJOR contributions to the study of protein metabolism.
Phillips cited compelling research along with his conclusion: blood amino acids alone will maximize MPS at even baseline levels of blood insulin, and higher insulin levels add no effect. On the other hand, elevated blood insulin levels are needed to maximize suppression of MPB, but the amount of elevation needed is very low, and higher levels add no benefit [1, 3].
In conclusion, if you consume enough protein, you won’t need to consume more than a healthy bite of… anything with carbohydrates, to minimize how much your muscles break down, and to maximize how much they can build .
With older adults, however, research shows that much higher levels of insulin are needed to promote MPS [5, 6, 7].
Fortunately, Jorn Trommelen produced a review of all the data and concluded :
1. Amino acids alone trigger MPS, independent of (administered) insulin.
2. Only extraordinarily high levels of (administered) insulin could potentially increase MPS.
3. In older adults, higher levels of (administered) insulin may assist MPS, but this effect is quickly diminished.
4. Higher levels of insulin are associated with lower levels of amino acids in the bloodstream.
We still did not directly answer the question of whether mixing carbohydrates with protein is necessary to maximize MPS.
René Koopman, Ph.D., works in the Department of Physiology at the University of Melbourne, with a specific interest in clinical nutrition and muscle metabolism. A study of his was published in 2007, where he and his team conducted a randomized crossover trial in 10 healthy and fit men. Participants were divided into 3 groups, and after 60 minutes of resistance training, they received enough protein to maximize MPS and were monitored for 6 hours. One group received ~.07g/pound of body weight of carbohydrates, another group received ~.27g/pound, and the third group consumed no carbohydrates (the control). Whole body protein breakdown, synthesis, oxidation rates, and protein balance, did not statistically differ between the groups .
A similarly designed study was published more recently, one that Stuart Phillips was involved with, and came to the same conclusion that the combination of carbohydrates with whey protein had no extra impact on MPS or MPB, when compared to whey protein alone .
At this point the data seems pretty clear that, nutritionally speaking, MPS is regulated by protein consumption only!
1. Phillips SM. Insulin and muscle protein turnover in humans: stimulatory, permissive, inhibitory, or all of the above? Am J Physiol Endocrinol Metab. 2008 Oct;295(4):E731. doi: 10.1152/ajpendo.90569.2008. Epub 2008 Jul 15. PubMed PMID: 18628353.
2. Trommelen J, Groen BB, Hamer HM, de Groot LC, van Loon LJ. MECHANISMS IN ENDOCRINOLOGY: Exogenous insulin does not increase muscle protein synthesis rate when administered systemically: a systematic review. Eur J Endocrinol. 2015 Jul;173(1):R25-34. doi: 10.1530/EJE-14-0902. Epub 2015 Feb 2. Review. PubMed PMID: 25646407.
3. Greenhaff PL, Karagounis LG, Peirce N, Simpson EJ, Hazell M, Layfield R, Wackerhage H, Smith K, Atherton P, Selby A, Rennie MJ. Disassociation between the effects of amino acids and insulin on signaling, ubiquitin ligases, and protein turnover in human muscle. Am J Physiol Endocrinol Metab. 2008 Sep;295(3):E595-604. doi: 10.1152/ajpendo.90411.2008. Epub 2008 Jun 24. PubMed PMID: 18577697; PubMed Central PMCID: PMC2536736.
4. Holt SH, Miller JC, Petocz P. An insulin index of foods: the insulin demand generated by 1000-kJ portions of common foods. Am J Clin Nutr. 1997 Nov;66(5):1264-76. PubMed PMID: 9356547.
5. Fujita S, Glynn EL, Timmerman KL, Rasmussen BB, Volpi E. Supraphysiological hyperinsulinaemia is necessary to stimulate skeletal muscle protein anabolism in older adults: evidence of a true age-related insulin resistance of muscle protein metabolism.Diabetologia. 2009 Sep;52(9):1889-98. doi: 10.1007/s00125-009-1430-8. Epub 2009 Jul 9. PubMed PMID: 19588121; PubMed Central PMCID: PMC2843438.
6. Meneilly GS, Elliot T, Bryer-Ash M, Floras JS. Insulin-mediated increase in blood flow is impaired in the elderly. J Clin Endocrinol Metab. 1995 Jun;80(6):1899-903. PubMed PMID: 7775638.
7. Rasmussen BB, Fujita S, Wolfe RR, Mittendorfer B, Roy M, Rowe VL, Volpi E. Insulin resistance of muscle protein metabolism in aging. FASEB J. 2006 Apr;20(6):768-9. Epub 2006 Feb 7. PubMed PMID: 16464955; PubMed Central PMCID: PMC2804965.
8. Koopman R, Beelen M, Stellingwerff T, Pennings B, Saris WH, Kies AK, Kuipers H, van Loon LJ. Coingestion of carbohydrate with protein does not further augment postexercise muscle protein synthesis. Am J Physiol Endocrinol Metab. 2007 Sep;293(3):E833-42. Epub 2007 Jul 3. PubMed PMID: 17609259.
9. Staples AW, Burd NA, West DW, Currie KD, Atherton PJ, Moore DR, Rennie MJ, Macdonald MJ, Baker SK, Phillips SM.Carbohydrate does not augment exercise-induced protein accretion versus protein alone. Med Sci Sports Exerc. 2011 Jul;43(7):1154-61. doi: 10.1249/MSS.0b013e31820751cb. PubMed PMID: 21131864.