title: Unmetabolized folic acid in plasma is associated with reduced natural killer cell cytotoxicity among postmenopausal women.
abstract: Folic acid (FA) supplements and food fortification are used to prevent neural tube defects and to lower plasma homocysteine. Through exposure to food fortification and vitamin supplement use, large populations in the United States and elsewhere have an unprecedented high FA intake. We evaluated dietary and supplemental intakes of folate and FA in relation to an index of immune function, natural killer cell (NK) cytotoxicity, among 105 healthy, postmenopausal women. Among women with a diet low in folate (<233 microg/d), those who used FA-containing supplements had significantly greater NK cytotoxicity (P = 0.01). However, those who consumed a folate-rich diet and in addition used FA supplements > 400 microg/d had reduced NK cytotoxicity compared with those consuming a low-folate diet and no supplements (P = 0.02). Prompted by this observation, we assessed the presence of unmetabolized FA in plasma as a biochemical marker of excess FA. Unmetabolized folic acid was detected in 78% of plasma samples from fasting participants. We found an inverse relation between the presence of unmetabolized FA in plasma and NK cytotoxicity. NK cytotoxicity was approximately 23% lower among women with detectable folic acid (P = 0.04). This inverse relation was stronger among women >or= 60 y old and more pronounced with increasing unmetabolized FA concentrations (P-trend = 0.002). Because of the increased intake of FA in many countries, our findings highlight the need for further studies on the effect of long-term high FA intake on immune function and health.
authors.affiliation: Jean Mayer United States Department of Agriculture Huamn Nutrition Research Center on Aging at Tufts University, Boston, MA, USA.
authors.forename: Aron M
authors.initials: AM
authors.lastname: Troen
authors.forename: Breeana
authors.initials: B
authors.lastname: Mitchell
authors.forename: Bess
authors.initials: B
authors.lastname: Sorensen
authors.forename: Mark H
authors.initials: MH
authors.lastname: Wener
authors.forename: Abbey
authors.initials: A
authors.lastname: Johnston
authors.forename: Brent
authors.initials: B
authors.lastname: Wood
authors.forename: Jacob
authors.initials: J
authors.lastname: Selhub
authors.forename: Anne
authors.initials: A
authors.lastname: McTiernan
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Yasui
authors.forename: Evrim
authors.initials: E
authors.lastname: Oral
authors.forename: John D
authors.initials: JD
authors.lastname: Potter
authors.forename: Cornelia M
authors.initials: CM
authors.lastname: Ulrich
mesh_terms: D004032:Diet; D005260:Female; D005492:Folic Acid / Q000008:administration & dosage / Q000097:blood* / Q000378:metabolism; D006397:Hematinics / Q000008:administration & dosage / Q000097:blood* / Q000378:metabolism; D006801:Humans; D007694:Killer Cells, Natural / Q000276:immunology*; D008875:Middle Aged; D017698:Postmenopause / Q000097:blood*
chemical_list: D006397:Hematinics; D005492:Folic Acid