what are often complex pathways. The various processes that influence what compound reaches the active site, and at what concentration, are collectively referred to as pharmacokinetics information. Knowledge of a dietary supplement ingredient’s pharmacokinetics and in vivo metabolism, if available from animal or human studies, will allow the most appropriate interpretation of the relevance of compound concentrations used in in vitro experiments to amounts ingested by humans.

As described above, in vitro assays are valuable because they allow scientists to answer questions that otherwise may not be answerable due to various constraints with whole-animal and human studies. It is not possible to describe the complete realm of in vitro assays in this report, but consideration of the value provided by those described here gives perspective to the value of in vitro assays as a scientific tool for predicting risk.

A number of different assays focus on examining effects of supplement ingredients on cellular receptors. Knowledge of which receptors are activated or inhibited, combined with general knowledge about what physiological processes are stimulated by different types of receptors, will increase understanding of how dietary supplement ingredients will affect humans. General types of assays for examining receptor activity are:

• Receptor binding assays—used to measure how strongly a substance (such as a dietary supplement ingredient or its constituent) binds to various receptor types to determine if the substance is likely to activate or inhibit cellular receptor activity.

• Cell function assays—used to measure agonism or antagonism of receptor activity in the intact cell by examining effects that are downstream of the receptor itself.

Receptor activation is often a critical event in biological pathways that leads to a functional effect, hence many manufactured pharmacological agents are selected for intentional receptor targeting. Strong agonist or antagonist action does not generally warrant particular attention in and of itself, but it may warrant attention when combined with general knowledge about the receptor. Many receptors are characterized well enough to know whether sufficient activation or inhibition in target tissues is associated