Association of serum folate concentrations with serum homocysteine and serum methylmalonic acid concentrations in older adults from the NHANES 1999–2002 cohort. Serum homocysteine (A) and serum methylmalonic acid (B) concentrations decreased with increasing serum folate in vitamin B-12–replete subjects (serum vitamin B-12 ≥148 pmol/L), but increased with increasing serum folate in vitamin B-12–deficient subjects (serum vitamin B-12 <148 pmol/L). Symbols represent geometric means. vitamin B-12 <148 pmol/L: n = 32, 33, 32, and 33 for lowest to highest folate categories, respectively; vitamin B-12 ≥148 pmol/L: n = 1032, 1152, 892, and 2267 for lowest to highest folate categories, respectively. B-12, vitamin B-12. Reproduced from reference 40 with permission. Copyright (2007) National Academy of Sciences.