Biological assimilation, or bio-assimilation, is the combination of two processes to supply cells with nutrients. The first is the process of absorption of vitamins, minerals, and other chemicals from food within the gastrointestinal tract. In humans, this is always done with a chemical breakdown (enzymes and acids) and physical breakdown (oral mastication and stomach churning). The second process of bio assimilation is the chemical alteration of substances in the bloodstream by the liver or cellular secretions. Although a few similar compounds can be absorbed in digestion bio assimilation, the bioavailability of many compounds is dictated by this second process since both the liver and cellular secretions can be very specific in their metabolic action (see chirality). This second process is where the absorbed food reaches the cells via the liver.

Most foods are composed of largely indigestible components depending on the enzymes and effectiveness of an animal's digestive tract. The most well-known of these indigestible compounds is cellulose; the basic chemical polymer in the makeup of plant cell walls. Most animals, however, do not produce cellulase; the enzyme needed to digest cellulose. However some animal and species have developed symbiotic relationships with cellulose-producing bacteria (see termites and metamonads.) This allows termites to use the energy-dense cellulose carbohydrate. Other such enzymes are known to significantly improve bio-assimilation of nutrients. Because of the use of bacterial derivatives, enzymatic dietary supplements now contain such enzymes as amylase, glucoamylase, protease, invertase, peptidase, lipase, lactase, phytase, and cellulase. These enzymes improve the overall bioassimilation in the digestive tract but are still not proven to increase bloodstream bioavailability. Assimilation in simple happens in human body happens in the small intestine when the villi absorb and transport the nutrients through blood after breaking them in simpler sbstances