In humans, essential amino acids (EAAs) stimulate muscle protein synthesis (MPS) with no effect on muscle protein breakdown. An insulin spike following the ingestion of carbohydrates can stimulate MPS, while insulin and carbohydrates can decrease the breakdown of muscle protein.
Net protein balance, which is an indicator of overall anabolism, is greatest when MPS is maximized and muscle protein breakdown is minimized. Erin Glynn and colleagues conducted a study to determine whether adding carbs to EAAs would improve net protein balance, compared to EAAs alone.
Young men and women ingested 10 grams of EAAs alone or 10 grams of EAAs plus 30 grams of sucrose (carbs). Although responses were more robust with the intake of EAAs plus carbs, compared to EAAs alone, MPS, muscle protein breakdown, and net protein balance were similar between the two. Because the overall muscle protein anabolic response was not improved with the intake of EAAs plus carbs, compared with EAA alone, this study suggests that nutritional strategies to enhance muscle protein anabolism do not require the additional energy from carbs.
Importantly, because there’s no specific muscle-building advantage to ingesting extra carbs and calories on top of EAAs, individuals on a calorie-restricted diet, such as physique competitors, bodybuilders, and weight class sport athletes, can simply supplement with (calorie-free) EAAs to keep them anabolic.
[Glynn, Erin L., et al. “Addition of carbohydrate or alanine to an essential amino acid mixture does not enhance human skeletal muscle protein anabolism.” The Journal of Nutrition 143.3 (2013): 307-314.]