Bioavailability is a critical pharmacokinetic parameter that refers to the extent and rate at which an active drug ingredient or therapeutic moiety is absorbed from a drug product and becomes available at the site of action. It is a key concept in both pharmacology and nutrition as it helps in understanding how substances, including nutrients, drugs, and other compounds, are absorbed and utilized by the body. Bioavailability can be influenced by various factors including the physical and chemical properties of the drug, the formulation of the drug product, and the patient's physiological conditions such as gastric pH, presence of food in the stomach, and intestinal transit time.
The measurement of bioavailability is particularly important in the development of pharmaceuticals. For oral medications, not 100% of the administered dose reaches the systemic circulation due to factors such as degradation in the stomach, incomplete absorption by the intestines, or first-pass metabolism in the liver. The bioavailability is expressed as a percentage and is calculated by comparing the area under the plasma concentration-time curve (AUC) of the administered drug with that of its intravenous formulation (which is assumed to have 100% availability). Drugs administered intravenously are considered to have a bioavailability of 100%, as they are introduced directly into the bloodstream.
For drugs that are administered via other routes such as oral, topical, or inhalation, the bioavailability generally decreases due to the barriers posed by the body's natural defenses. Enhancing bioavailability is a major focus in drug development and can involve techniques such as particle size reduction, use of surfactants, complexation, and the use of permeation enhancers. Moreover, advanced delivery systems like liposomes and nanoparticles have been designed to improve the delivery of drugs with poor natural bioavailability, thereby increasing their effectiveness.
Nutritional bioavailability differs but is equally important, particularly in the case of essential nutrients like vitamins, minerals, and trace elements. Factors such as the food matrix, preparation methods, and interactions with other nutrients can significantly influence the bioavailability of these nutrients. For example, the presence of phytates and polyphenols in plant-based foods can inhibit the absorption of minerals like iron and zinc. Understanding and improving the bioavailability of nutrients can help in formulating dietary guidelines and supplements that are more efficacious in addressing nutritional deficiencies and promoting overall health.