*Geek Box: Bioavailability
Bioavailability is the amount of a compound that reaches circulation, generally expressed as a percentage of the total intake. In this regard, it is not quite the same as absorption, because a compound is only bioavailable after it has been not only absorbed, but passed through the liver. In nutrition, the bioavailability for macronutrients is generally very high, because there are specific transporters in the gut for fats, sugars, and amino acids. For micronutrients, it can differ, as often a number of minerals may share the same transporter, thus competing for absorption. In addition, homeostasis for minerals and trace elements is maintained at the level of the gut, which can upregulate or downregulate absorption based on levels in the body. The bioavailability of polyphenols was originally believed to be quite low overall, but also variable, with different compounds showing bioavailability of anywhere from 1 to 60%. However, it is now known that the bioactivity if polyphenols is from metabolites, not the parent compounds, and metabolites may circulate in concentrations up to 100-fold greater to the parent compounds, and much greater bioavailability.