Presented by Barry Shane, Ph.D. at the December 15, 2003 meeting of the Advisory Committee for Reproductive Health
Folate nutrition and metabolism and influence on NTDs Metabolic roles of folate and factors that influence its requirement Polymorphisms in folate genes and/or poor folate status are metabolic and genetic risk factors for disease (cancer, vascular disease, birth defects) Daily recommended intakes (DRIs) for folate may be influenced by individual genetic profiles Peri-conceptual folic acid reduces incidence of birth defects
Folate transport and retention in the body Dietary folates, primarily polyglutamates, are hydrolyzed by a brush border hydrolase, prior to absorption in the gut. Monoglutamate forms circulate in plasma and are transported into tissues via a membrane carrier (RFC) and/or by a receptor- mediated process (FBP). Folate retention by tissues requires their conversion to long chain length polyglutamate forms. Incomplete conversion to polyglutamates by tissues such as intestinal mucosa and liver leads to release of folate, mainly as methylfolate, into plasma. Tissue folates turn over primarily by catabolism to pterins and aminobenzoylpolyglutamates. The half life of whole body folate is approximately 100-200 days, depending on folate intake.
Folate transport and retention in the body: effect of high folate intake Plasma folate levels increase as folate intake increases while tissue folate levels saturate at high folate intakes. Although folate absorption and tissue uptake are not limited, a decreased ability to convert the entering folate in tissues to retainable polyglutamate forms leads to release of much of the transported folate back to the plasma. At high folate doses, the capacity of the intestinal mucosa and liver to metabolize the dose to methylfolate is exceeded and unmetabolized folate is released into the circulation. The half life of whole body folate decreases with increased folate intake. At very high intakes, the initial loss is due to exceeding the renal capacity for reabsorption.
Consequences of impaired folate status or metabolism Metabolic Disruption Biochemical Markers Clinical Associations S-adenosylmethionine (MTHFR, MS, B 12 deficiency) Thymidylate Purines (A,G) Hypomethylated DNA Elevated homocysteine Reduced methylation Decreased DNA synthesis & reduced cell division Cancer CVD demyelination NTDs Cancer NTDs Anemia Anemia Increased uracil in DNA Decreased DNA synthesis & reduced cell division
Neural tube defects (NTDs) <ul><li>The most common congenital abnormality at birth in US </li></ul><ul><li>Neural tube closes during fourth week post conception </li></ul><ul><li>Embryo nutrition dependent on yolk sac during this period </li></ul><ul><li>Peri-conceptual folic acid decreases the incidence of NTDs </li></ul><ul><li>Disruption of some folate genes in mice produces a NTD phenotype: in some cases preventable by additional folate </li></ul>
Relationship of early pregnancy maternal red cell folate to risk of NTD Daly et al JAMA 1995; 274:1698-702 12 10 8 6 4 2 0 453 (200) 906 (400) 1360 (600) 1813 (800) Red Cell Folate, nmol/L (ng/mL) NTD Risk per 1000 Births
Folate and the etiology of NTDs <ul><li>Mechanism(s) for protective effect of folate not established </li></ul><ul><li>Etiology is both environmental and genetic </li></ul><ul><li>Not all NTDs are preventable by additional folate </li></ul><ul><li>Common polymorphisms in some folate genes influence NTD risk </li></ul><ul><li>MTHFR 677 C->T (case risk) </li></ul><ul><li>MTHFD1 R653Q (maternal risk) </li></ul><ul><li>Folate fortification is the first nutritional intervention to expose the entire population to target distinct genetic populations </li></ul>
A common polymorphism in the MTHFR gene ______________________________________________________ Normal allele Gene sequence …..GCG GGA G C C GAT……………… Protein Sequence ……Ala Gly Ala Asp………………. 677 C->T allele Gene Sequence …..GCG GGA G T C GAT………………. Protein Sequence ……Ala Gly Val Asp ………………. ______________________________________________________ Associated with elevated homocysteine when folate (and riboflavin) status is poor, and decreased cancer risk. Elevated homocysteine is a risk factor for vascular disease.
Increasing intake UL Risk of inadequacy Risk of excess 0.5 0.5 EAR 1 EAR 2 RDA 1 RDA 2 Common genetic variation may influence the requirement for folate