Bioavailability

Bioavailability describes the extent to which a substance becomes available for biological use after it is ingested or otherwise administered. In nutrition, it generally refers to the proportion of an ingested nutrient that is absorbed and becomes available for physiological functions or storage. Thus, the amount of a nutrient present in a food is not necessarily the same as the amount the body can use .¹

Nutrient bioavailability depends on several steps, including release of the nutrient from food during digestion, its chemical form and solubility, absorption across the intestine, and subsequent transport and metabolism. It can therefore vary among different forms and sources of the same nutrient. Other components of a meal and characteristics of the individual can also increase or decrease how much of an ingested nutrient ultimately becomes available .¹

Bioavailability is related to, but broader than, absorption. Absorption describes movement of a substance across a biological barrier, such as the intestinal lining. Bioavailability additionally considers whether the absorbed substance becomes available for physiological use or storage. Bioaccessibility is another related term that usually refers to the fraction released from a food or other matrix and made potentially available for absorption, without requiring that absorption actually occur .¹

The term has a more specific definition in pharmacology. For drugs, bioavailability describes the rate and extent to which an active ingredient is absorbed from a drug product and becomes available at its site of action. For many systemically acting drugs, this is assessed by determining the amount and time course of active drug reaching the systemic circulation .²