What is Bioavailability

Written by OmegaXL on May 9, 2014

Bioavailability describes the difference between how much of a substance (drug, herb, or chemical), entering the body is actually absorbed and available to various tissues or to metabolism. In other words, bioavailability accounts for the difference between exposure and effective dose. A drug’s therapeutic action or a chemical’s toxicity is determined by the dose received at the target site in the body, not necessarily how much was ingested. The dose at the target site is determined by the amount of the substance absorbed by the body, which depends on its bioavailability. If a substance is ingested, for example, its bioavailability is determined by the amount that is absorbed by the intestinal tract. If a substance is inhaled, its bioavailability is determined by the amount that is absorbed by the lungs.

The ability to absorb nutrients varies by gender, physiologic condition (e.g., illness, pregnancy, aging). The bioavailability of a nutrient can also increase or decrease if other substances are present. For example, calcium and magnesium lose much of their effectiveness if taken with fatty foods. The intestines themselves may also regulate the amount of a mineral that enters the bloodstream. For these reasons, taking high-potency vitamin supplements does not guarantee that all of the included nutrients will end up in body tissues.

Understanding bioavailability is critical to determining the amount of a drug or dietary supplement that is likely to produce effectiveness at the low end and toxicity at the high end. Although not technically drugs, the bioavailability of vitamin, mineral, or herbal supplements obey the same principles. For example, calcium citrate is more easily absorbed in the intestines than calcium carbonate. Hence, calcium carbonate is considered to me more bioavailable than calcium carbonate.

In nutritional science, which covers the intake of nutrients and non-drug dietary supplements, the concept of bioavailability lacks the well-defined standards associated with the pharmaceutical industry. The pharmacological definition cannot apply to these substances because utilization and absorption is a function of the nutritional status and physiological state of the subject,resulting in even greater differences from individual to individual (inter-individual variation). Therefore, bioavailability for dietary supplements can be defined as the proportion of the administered substance capable of being absorbed and available for use or storage.