Bioavailability is a term used by several branches of scientific study to describe the way chemicals are absorbed by humans and other animals. Bioavailability was once strictly ascribed to pharmacology, but now has broad applications in environmental sciences as well. Examining a substance’s bioavailability in pharmacological studies helps to determine dosages of particular medications. A bioavailability measurement of a medication, when it reaches circulation in the body, describes aspects like absorbency and half-life. It can evaluate medication delivery as well.
Intravenous administrations of medications are considered to have 100% bioavailability because they do not pass through the stomach. They are immediately in the circulatory system. However, other medications administered at the same time may reduce the effects of an intravenous administration and affect its bioavailability.
Pharmacology also distinguishes between absolute bioavailability and relative bioavailability. Absolute bioavailability is the measurement of a medication once it passes through the gut and is released into the circulatory system. Relative bioavailability is a term used to compare different formulations of the same medication, for example brand name versus generic.
Some studies have found that certain generic preparations are not equivalent in bioavailability to brand name versions of medications. One example of this is the drug Synthroid, which is usually marketed in generic form as thyroxidine. Many patients who use thyroid replacement therapy find that thyroxidine is not as effective as Synthroid. Technically, the two medications should be equivalent, but differences in the bioavailability of the two forms have been noted in studies. This does not, however, change most insurance coverage in regards to this medication. Generally, one will still have to pay extra to purchase Synthroid.
Other factors can affect bioavailability when drugs are taken orally. Slow or quick metabolism can change the absorption rate of medication. Factors outside of the body, like improper storage of medications, can lead to less than expected bioavailability results.
Bioavailability, when used in environmental sciences, evaluates the rate and amount of toxic substances that may occur in the body. One example of this is the recent concern over mercury levels in fish. Some fish contain high levels of mercury, a poison, which can lead to severe illness when ingested in high quantities. People who eat a lot of fish may be subject to mercury poisoning. Recent recommendations by the US FDA suggest consuming mercury-high fish no more than once a week.
Another bioavailability study that has raised concern about our relationship to the environment is the discovery of small amounts of jet fuel secreted in human breast milk. At this point, no one has suggested that mothers stop breastfeeding their children, but the study raises concerns about many things we take for granted, like flying planes or rockets, which may in fact increase risk of health problems to children and adults.
Bioavailability studies may also compare animal or plant populations to the presence of toxic chemicals in an environment. Of concern is the degree to which these chemicals are present in animals when they reach the circulatory system, and what possible effects this may have on animal populations.
For instance, animal populations, which were later consumed by humans, consumed plant populations covered with DDT. Some effects in humans were increases in certain cancers and autoimmune disorders. Humans could also consume DDT through plant material, creating similar issues. These studies resulted in bans on DDT, which was once commonly used as a pesticide.
The bioavailability of toxins in our environment, when studied, may make significant impacts on our life quality and choices. These studies are invaluable, as they can help us approach our ecology with a sense of responsibility and a view to the future.
By Tricia Christensen