Several factors can affect how much of a nutrient contained in a supplement becomes actually available for the body to absorb. One of the most important ones is the drug delivery method.
Supplements that are administered orally must pass through the intestinal wall to reach the liver through the portal circulation. Both these sites are called sites of “first-pass metabolism,” meaning that the active component of the supplement might be metabolised before it can reach the systemic circulation.
Therefore, many orally administered supplements might struggle to reach an adequate plasma concentration that is going to be translated into a beneficial, therapeutic effect.
Formulation is also another important factor affecting bioavailability. For example, a supplement formulated into a tablet or capsule format will disintegrate in the stomach within a reasonable amount of time to release the active ingredient. If this does not happen at an optimal rate, the nutrient won’t be absorbed. To address this issues, newer liquid-based supplements seem to be more effective than capsules or tablets because they are more rapidly absorbed.
Due to poor bioavailability, in many cases, the active ingredients can be destroyed in the gut or flushed away before delivering the intended benefits.