Folic acid plays an important role in protein metabolism as a component of the coenzyme tetrahydrofolic acid. Tetrahydorfolic acid effects the transfer of one-carbon units. It is therefore required for the synthesis of hemoglobin, glycine, methionine, choline and purines and for the metabolism of phenylalanine, tyrosine and histidine.
The interrelationships with other nutrients are more or less established. With regard to vitamins C, B12 and choline, results have been inconsistent. However, lack of any of these nutrients further increases folate requirement (Duncan and Lovell, 1994b).
Intestinal uptake of folic acid is carrier-mediated in rainbow trout (Casirola and Ferraris, 1997) and occurs by simple diffusion in catfish (Casirola, et al., 1995). In either fish, folic acid uptake is not influenced by the presence of 5-methyltetrahydrofolate.
Dietary supplementation of folic acid beyond that required for optimal growth was found not to enhance the immune system of chinook salmon (Oncorhynchus tshawytscha) (Leith et al., 1989).