Insulin

Insulin is a peptide hormone released by the beta cells of the pancreas when blood glucose rises. It signals cells — muscle and fat especially — to take up glucose, and it promotes storage generally: glycogen synthesis, fat storage, and protein synthesis.

It matters because it is the central hormone of metabolic health, and also the most misrepresented. The claim that insulin causes obesity by driving fat storage — the carbohydrate-insulin model — has not held up in controlled feeding studies: when calories and protein are matched, low-carbohydrate diets do not produce greater fat loss.

In practice what matters is sensitivity rather than the presence of insulin. Insulin is also anabolic for muscle, which is why the demonisation of it in diet marketing sits awkwardly with its actual physiology.