Not all nutrients present in a feed will actually be used by the animal. The part that really is absorbed and utilized by the animal, we call the bioavailable nutrients. There are multiple reasons why some nutrients are unavailable. Some nutrients might physically not be reachable for digestion and absorption, others are chemically blocked or are degraded by micro-organisms in the gastro-intestinal tract before they reach the absorption sites. Also animal related causes like diseases can result in a lower bioavailability.
Especially for feed additives, it is essential to consider if these supplemented nutrients are good bioavailable. Production techniques or the chemical molecular structure are typical product parameters which influence bioavailability and thus efficiency of the product used.
Bio-availability is a pronounced issue in trace mineral supply in relation to dietary interactions, solubility characteristics and other items. In vitamin nutrition, enzymatic transformation steps may be overpassed leading to products with higher biological activity. In amino acids, chirality can play a distinguished role.