Insulin is often thought to be the master regulatory hormone of metabolism while glucagon plays a secondary role, but this assumption is not exactly accurate. Here, Drs. Mary Dan and Michael Eades describe the body’s metabolic processes and how the body reacts to different energy sources. They also discuss the etiologies for Type 1 and Type 2 diabetes mellitus and propose a unique cause for the latter: lipotoxicity of the islets, driven by excess dietary carbohydrate consumption. They argue restricting carbohydrates is the most effective way to treat and reverse the cascade of hormone-driven processes that underlie Type 2 diabetes mellitus (T2DM). They also elucidate why treating T2DM with insulin can lead to elevations in blood glucose and make the problem more severe.