Biovailability

Bioavailability is the fraction of an administered dose of a substance that reaches the bloodstream or the site of action in an active form. In pharmacology, it is denoted F and describes the rate and extent of systemic exposure after administration.

For a substance given intravenously, F is 1 (100%). Other routes vary widely; oral bioavailability is commonly less than 100% due to incomplete absorption and first-pass metabolism in the liver and gut wall. Non-oral routes such as sublingual, transdermal, inhalation, or intramuscular administration may bypass some first-pass effects and exhibit higher or more consistent F.

Several factors influence bioavailability. The compound’s solubility, stability, and permeability interact with formulation factors such as release mechanisms and excipients. Physiological conditions, including gastric pH, intestinal transit time, and the presence of food, can alter absorption. Patient factors like age, disease states, genetic polymorphisms affecting metabolic enzymes, and interactions with other drugs also play a role, as can the gut microbiota.

Bioavailability is typically quantified by comparing the area under the concentration-time curve (AUC) after a route of administration to that after intravenous administration: F = (AUC_oral / AUC_iv) × (Dose_iv / Dose_oral). It is a central parameter in pharmacokinetics, dose optimization, and bioequivalence testing.

In nutrition, bioavailability describes the portion of a nutrient that is absorbed and utilized. Food matrix, competing nutrients, and chemical form influence uptake; for example, iron absorption is enhanced by vitamin C and inhibited by phytates.