Carbs alone have little impact on protein synthesis, but when combined with whey protein, the increase in insulin might have a synergistic or additive effect on muscle protein synthesis. However, surprisingly few studies have tested this hypothesis.
Canadian researchers assessed muscle protein synthesis and breakdown at rest and after resistance exercise in men on two separate occasions. During one trial, subjects consumed 25g of whey protein. In a second trial, subjects consumed 25g of whey plus 50g of carbs (maltodextrin).
Compared to the protein-only group, the protein-carb group experienced increased insulin response five-fold. However, both trials resulted in a significant increase in protein synthesis and protein breakdown in the rested and exercise-stimulated condition, with no difference between the groups.
This indicates that stimulating high levels of insulin after exercise by ingesting carbs does not have an impact on muscle protein synthesis and breakdown versus consuming an optimal level of protein alone. Therefore, you don’t need carbs to build muscle.
The potential downside to taking carbs post-exercise is that it shuts down fat oxidation and breakdown, which is why those trying to lose weight should avoid taking carbs post-workout.