From an evidence-based perspective and as it pertains to the field of performance and image enhancing (P&IE) drugs, insulin has quite possibly the most unfavourable risk to benefit ratios. Often erroneously cited as the ‘most anabolic hormone’ the application for means of augmenting skeletal muscle hypertrophy/hyperplasia is actually without well-substantiated or defined pharmacological mechanism and unfortunately the postulated activity may not be as so widely propagated or assumed.
For the more common short acting preparations of illicitly used insulins (such as those known by proprietary names; NovoRapid, Humalog or Apidra) the insulin peptide is not actually homologous to that endogenously produced. These preparations are modified with amino acid substitutions to the active peptide for purposes of conformational and chemical stability in therapeutic use for the intended subjects; insulin-dependent diabetics.
In healthy persons these modifications can mean that the hypoglycaemic activity of insulin can be much more abrupt, exaggerated or staggered than that of endogenous fluxes, thus engendering the associated danger and scope for unpredictability.
Whilst there are many hypothesized and empirically assumed mechanistic actions in which users base their P&IE use of insulin, the most frequently cited appears to stem from ‘the most anabolic hormone’ theory. Users will attempt to exploit indiscriminate facilitation of intracellular storage. This practice may have been perhaps extrapolated from older landmark research that suggested very high levels of blood insulin can stimulate muscle protein synthesis- as measured by the utilisation of essential amino acids in experimental conditions1,2. These findings have been later analysed and further clarified whereby it was actually found that skeletal muscle perfusion would be the overlying factor that determines amino acid delivery to the muscle cell as well as resultant muscle protein turnover3-6. Although hyperinsulinemia (as pharmaceutically induced) may in fact augment this process, as found in all corollary research7, there appears to governing feedback mechanisms which see this potentially reaching a ceiling effect that could otherwise be achieved through non-pharmaceutical intervention6-8.
1Bennet, W. M., Connacher, A. A., Scrimgeour, C. M., Jung, R. T., & Rennie, M. J. (1990). Euglycemic hyperinsulinemia augments amino acid uptake by human leg tissues during hyperaminoacidemia. American Journal of Physiology-Endocrinology And Metabolism, 259(2), E185-E194.
2Biolo, G., Fleming, R. D., & Wolfe, R. R. (1995). Physiologic hyperinsulinemia stimulates protein synthesis and enhances transport of selected amino acids in human skeletal muscle. Journal of Clinical investigation, 95(2), 811.
3Fujita, S., Rasmussen, B. B., Cadenas, J. G., Grady, J. J., & Volpi, E. (2006). Effect of insulin on human skeletal muscle protein synthesis is modulated by insulin-induced changes in muscle blood flow and amino acid availability. American Journal of Physiology-Endocrinology and Metabolism, 291(4), E745-E754.
3Timmerman, K. L., Lee, J. L., Dreyer, H. C., Dhanani, S., Glynn, E. L., Fry, C. et al. (2010). Insulin stimulates human skeletal muscle protein synthesis via an indirect mechanism involving endothelial-dependent vasodilation and mammalian target of rapamycin complex 1 signaling. The Journal of Clinical Endocrinology & Metabolism, 95(8), 3848-3857.
5Dillon, E. L., Casperson, S. L., Durham, W. J., Randolph, K. M., Urban, R. J., Volpi, E., … & Sheffield-Moore, M. (2011). Muscle protein metabolism responds similarly to exogenous amino acids in healthy younger and older adults during NO-induced hyperemia. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 301(5), R1408-R1417.
6Barazzoni, R., Short, K. R., Asmann, Y., Coenen-Schimke, J. M., Robinson, M. M., & Nair, K. S. (2012). Insulin fails to enhance mTOR phosphorylation, mitochondrial protein synthesis, and ATP production in human skeletal muscle without amino acid replacement. American Journal of Physiology-Endocrinology and Metabolism, 303(9), E1117-E1125.
7Abdulla, H., Smith, K., Atherton, P. J., & Idris, I. (2016). Role of insulin in the regulation of human skeletal muscle protein synthesis and breakdown: a systematic review and meta-analysis.
8Host, H. H., Hansen, P. A., Nolte, L. A., Chen, M. M., & Holloszy, J. O. (1998). Glycogen supercompensation masks the effect of a training induced increase in GLUT-4 on muscle glucose transport. Journal of Applied Physiology, 85(1), 133-138.