β-Hydroxybutyrate (BHB) is one of the three ketone bodies produced by the liver when glucose is scarce — during fasting, prolonged exercise, or a very low-carbohydrate diet. As insulin falls and fat is mobilised, BHB rises and becomes an important alternative fuel for the brain, heart and skeletal muscle.
BHB is not only fuel. It also acts as a signaling molecule: it inhibits certain histone deacetylases (HDACs), shifting gene expression in ways linked to oxidative-stress resistance, and it modulates inflammatory pathways. This dual role — energy source plus signal — is part of why the metabolic state of fasting differs from simply eating fewer calories at every meal.
Because rising BHB tracks the metabolic shift that accompanies ketosis and autophagy, it is sometimes used as a rough proxy for being in a fasted state. It is a proxy, not a direct measurement of autophagy. Work from Eric Verdin and colleagues has been central to mapping BHB's signaling effects.