To spike insulin or not to spike insulin… that is the question!! Over the last few years, it has been a topic of confusion and debate. Many studies have reported that adding a carbohydrate beverage to a protein powder has very little effect on stimulating protein synthesis directly, but a new study suggests that insulin may enhance protein synthesis through other mechanisms. Insulin has a broad range of physiological functions, which also contribute to an anabolic state.
1) You Get A Pump
One of the biggest complaints among bodybuilders using a low-carbohydrate diet is that they have difficulty getting a good muscle pump. Insulin stimulates vasodilation by activating endothelial nitric oxide synthase (eNOS), which increases capillary recruitment and nutritive flow to skeletal muscle in healthy young adults. Vasodilation or the ability to open up muscle capillary beds cannot only contribute to better muscle pumps, but also increased protein synthesis. For example, a previous study reported that the ability of insulin to promote muscle protein synthesis correlates positively with muscle blood flow and amino acid delivery in young and older subjects. So basically, more blood flow is correlated with better protein synthesis and amino acid uptake. Interestingly, skeletal muscle protein synthesis is resistant to insulin in healthy non-diabetic older subjects; a defect associated with both reduced vasodilation and blunted Akt/mTORC1 signaling. This shows that being diabetic and being insulin resistant reduces muscle blood flow and protein synthesis.
2) It Stimulates Protein Synthesis
Amino acids activate the mTORC1 signaling pathway, which stimulates protein synthesis, and also muscle vasodilation, which can enhance protein synthesis by opening capillary beds, making it very difficult to determine the main mechanism through which insulin stimulates human skeletal muscle protein synthesis, e.g., directly, via Akt signaling, or indirectly, via endothelial-dependent vasodilation increasing tissue exposure to insulin and amino acids.
3) The Science Backs It Up
Some interesting research was published in Journal of Clinical Endocrinology and Metabolism, which reports that vasodilation from insulin and mTOR signaling pathways is increased by insulin. In the study, men received insulin or insulin plus L-NAME (i.e., a drug that blocks nitric oxide function which reduces muscle vasodilation). The researchers measured blood flow, anabolic signaling pathways, and protein synthesis rates. Interestingly, the researchers found that increases in blood flow from insulin were fundamental mechanisms in which insulin stimulates protein synthesis. They reported insulin-stimulated vasodilation, which promotes muscle protein synthesis by increasing nutritive flow and, consequently, mTORC1 signaling, whereas direct markers of protein synthesis such as Akt/PKB signaling were not involved.
Collectively, these data suggest that insulin-induced increases in blood flow, microvascular flow, and amino acid delivery are key, interconnected, components of insulin’s anabolic properties on skeletal muscle proteins.