Vitamin-like substances include a number of compounds that resemble vitamins in their activity but are normally synthesized in the human body in adequate amounts. They are often classified with the B vitamins because of similarities in function and distribution in foods. Their status as essential nutrients remains uncertain. Choline is found in all living cells and plays a role in nerve function and various metabolic processes. Myoinositol is a water-soluble compound; its significance in human nutrition is not established. Para-aminobenzoic acid is an integral part of folic acid but its role in human nutrition has not been documented.
Carnitine has an essential role in the transport of fatty substances. Lipoic acid seems to have a coenzyme function similar to that of thiamine; however, because it is synthesized in the human liver and kidneys, it is not considered a vitamin. Bioflavinoids are a group of substances that affect the permeability of capillaries but do not normally have to be added to human diets.