Physiological availability is the proportion of an ingested nutrient that is actually absorbed and becomes available for the body’s physiological functions, synonymous with bioavailability. It is invariably lower than the amount on the label: iron from spinach is poorly absorbed thanks to oxalates, folate from food is less available than folic acid from supplements, and fat-soluble vitamins need dietary fat for uptake. Availability depends on the nutrient’s chemical form, the food matrix, interactions with other dietary components, and the individual’s status, with absorption often rising in deficiency. The concept is central to setting dietary requirements, which build in availability factors, and to formulation, where choosing bioavailable forms like ferrous sulphate over elemental iron determines whether a fortified product actually nourishes. Label values without availability are promises, not deliveries.