folic acid An <em>N</em>-acyl-amino acid that is a form of the water-soluble vitamin B<small>₉</small>. Its biologically active forms (tetrahydrofolate and others) are essential for nucleotide biosynthesis and homocysteine remethylation. An N-acyl-amino acid that is a form of the water-soluble vitamin B9. Its biologically active forms (tetrahydrofolate and others) are essential for nucleotide biosynthesis and homocysteine remethylation. LanguaL curation note: Used when folic acid is added at any level. Always use this term for enriched flour or rice, enriched bakery products, and enriched macaroni or noodle products. (21 CFR 137. 610(a), 21 CFR 136.115(a)(1), and others). folic acids N-acyl-amino acid