Absorption, distribution and residue evaluation of active substances used as feed additives

Administration of feed additives is based on the necessity of satisfying a need in human nutrition. Oral feed additive intake results in an equilibration level which is followed by a steady state after absorption. The equilibrium point of the blood (tissue) level can be calculated mathematically. 70 Per cent of the plateau of the steady state is reached within four times the half-life. Some compounds give rise to an "initial peak" with higher concentrations than the expected plateau level (phase of adaptation). For residue evaluation these effects have to be considered. The relevance of different metabolic processes and the impact on a traditional monofactorial evaluation, neglecting the incidence of combination, is mentioned. Commonly used residue assessment is discussed and the safety margin of a toxicological threshold elucidated.