Bioavailability

BIOAVAILABILITY

Bioavailability refers to the difference between the amount of a substance, such as a drug, herb, or chemical, to which a person is exposed and the actual dose of the substance the body receives. Bioavailability accounts for the difference between exposure and dose. A drug's therapeutic action or a chemical's toxicity is determined by the dose received at the target site in the body. 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. Understanding bioavailability is critical to determining the amount of a drug to administer or the level of chemical exposure that is likely to produce toxicity.

The bioavailability of drugs depends on their formulation, which determines the rate at which they dissolve in the gastrointestinal tract. Although not legally considered to be drugs, the bioavailability of vitamin, mineral, and herbal supplements obey the same principles. For example, calcium (calcium bound to an organic acid such as citrate) is more easily absorbed by the gastrointestinal tract than calcium carbonate. Similarly, the bioavailability of chemical contaminants in the environment depends on the nature of the medium in which they are found. For example, the soil at locations of former manufactured gas plants can be very contaminated with chemicals (such as polycyclic aromatic hydrocarbons) that were produced by burning fuels, although very little of those chemicals is bioavailable because they are bound very tightly to the soil itself. The toxicity level of the chemicals in the soil, if measured in the laboratory, would be much greater than the toxicity level that would be experienced by someone exposed to the soil itself.

Questions of bioavailability are sometimes at the root of disagreements about what are the appropriate actions to take to protect public health and the environment from environmental contaminants. For example, sediment at the bottom of the Hudson River is contaminated with polychlorinated biphenyls (PCBs) due to past industrial disposal practices. Some argue that the PCBs in the sediment pose an unacceptable risk to the health of humans, fish, and other wildlife, and should be removed. Others argue that the PCBs are not a health hazard because of their low bioavailability in the sediment, and thus should be left in place because disturbing the sediment might make them more bioavailable. The bioavailability of chemical contaminants is often poorly understood, so it is sometimes not taken into account when the health risks from chemical exposures are assessed.

GAIL CHARNLEY

(SEE ALSO: Environmental Determinants of Health; Toxic Substances Control Act)