Biyoavailability

Biyoavailability, commonly known as bioavailability, is the proportion of an administered substance that reaches the systemic circulation or the site of action in an active form. It depends on the route of exposure and the physical and chemical properties of the substance. For drugs, biyoavailability is a determinant of dose, efficacy, and safety; for nutrients, it describes how much of a nutrient can be absorbed and utilized.

In pharmacokinetics, bioavailability is often quantified by comparing the area under the concentration–time curve (AUC) after non-intravenous administration with AUC after intravenous administration. Absolute bioavailability, F abs, equals (AUC_non-IV / AUC_IV) × (Dose_IV / Dose_non-IV). Relative bioavailability compares two non-intravenous forms. Bioequivalence studies assess whether two products have similar bioavailability.

Factors influencing bioavailability include solubility and dissolution rate, chemical stability in the gastrointestinal tract, intestinal permeability, and first-pass metabolism by the liver. Formulation aspects such as particle size, salt form, and use of absorption enhancers also play a role. Physiological factors include gastric pH, gastric emptying time, intestinal transit, microbiota, age, disease, and genetics. Food can increase or decrease absorption, depending on the substance.

For nutrients, bioavailability can be affected by dietary components (phytates, oxalates, fiber) and enhancers (vitamin C for iron, fat for fat-soluble vitamins). Processing methods, cooking, and meal composition alter liberation and absorption.

Understanding bioavailability guides dosing, product development, and dietary recommendations. It highlights why the same nominal amount of a substance may have different effects in different people or formulations, and why some products are optimized to improve absorption.