This chapter describes the challenges faced when evaluating the bioavailability and bioequivalence (BE) of orally administered compounds that are endogenously present in the body.

The increased importance of biotechnology in the pharmaceutical industry has been attracting renewed attention to endogenous substances. Endogenous substances have been previously defined in a multiple of ways (1-3). However, the simplest definition describing these substances is that they are present and naturally synthesized in the body and are usually

controlled through mechanisms that maintain a state of equilibrium or balance in the body (called homeostasis) (1,3-5). Endogenous substances play important physiological and biochemical roles in the biological organism and when impaired can often be restored through the intake of appropriate foods containing specific exogenous compounds and/or dietary supplements or administration of drugs (5-7).