Bioavailability is the amount of and rate at which a drug or supplement enters into systemic circulation where it can act.[1] When it comes to supplements, bioavailability also looks at how much of the supplement is used by or stored within the body. Medications or supplements taken orally have a lower bioavailability than those delivered intravenously, and the process of absorption takes longer.[2] The substance can begin metabolizing before it is absorbed into the bloodstream, limiting its bioavailability.[1] The digestive system absorbs substances with more ease when they easily form charged molecules (ions).[2] Substances that do not dissolve well in water and are therefore slowly absorbed have a low bioavailability.[1] After the substance moves into the gut, transport proteins in the intestines are able to transport materials into the bloodstream.2 Once in the bloodstream, the substance can enter the portal system to go to the liver which is the main site of metabolism.[1]

Bioavailability can change between people based on factors such as GI tract health, genetics, pregnancy, stress, medication interactions, or interactions with other substances such as caffeine and alcohol. Certain ingredients function to enhance bioavailability when combined with a particular drug or supplement and are referred to as supporting ingredients. Other factors that can help enhance bioavailability are proper medication storage and taking the substance at a consistent time of day or with food, if applicable.[2] Drugs have the highest bioavailability in gas form, followed by liquid form and then solid form.[3]

References

1.     Le, Jennifer. Drug Bioavailability https://www.merckmanuals.com/professional/clinical-pharmacology/pharmacokinetics/drug-bioavailability (accessed May 5, 2021).

2.     Rowe, Keith. What is Bioavailability & Which Vitamins Are Absorbed Best in Your Body? https://brainmd.com/blog/what-is-bioavailability/ (accessed May 5, 2021).

3.     Bioavailability of Drugs http://howmed.net/pharmacology/bioavailability-of-drugs/ (accessed May 13, 2021).