Folate is classed as part of the complex of B vitamins, but is a generic term for a vitamin that functions as a coenzyme in methylation reactions. An examples of this is the methylation of homocysteine to form the amino acid methionine. The role of folate in this reaction has received a lot of attention in recent years because homocysteine is a risk factor for cardiovascular disease. Because folate is involved in the conversion of homocysteine to methionine, it is able to lower plasma levels of homocysteine. Higher folate levels are therefore associated with a reduced risk of cardiovascular disease and high homocysteine levels can be brought down to the normal range with addition of folate to the diet. In epidemiological studies, folate levels are also inversely associated with neural tube defects during pregnancy and certain forms of cancer.
Folate is found in supplements and fortified foods in the form of folic acid (pteroyl-L-glutamic acid). It is formed from pteridine, PABA and glutamic acid, all of which can be synthesised in humans. However, the enzymes are not present to couple the pteridine to the PABA and so folic acid is a vitamin. Naturally occurring folates in plants tend to be in the form of group of closely related chemicals called pteroylpolyglutamates, which are similar in structure to folic acid, but comprise of up to nine glutamate residues. The main pteroylpolyglutamate in food in N5 methyl tetrahydrofolate (THF) and N10 formyl THF. Mushrooms, green vegetables (e.g. spinach), brussel sprouts and broccoli are all good sources of folates, but fruit tends to be a poor source. Liver is also a good non-plant sources of folates
The absorption of folate from food varies greatly depending on both the food and its preparation methods. Folate losses tend to occur during heating and so raw foods have higher levels than cooked equivalents. Absorption of supplements is generally higher than from whole foods because before pteroylpolyglutamates can be absorbed, they must first undergo enzymatic deconjugation in the small intestine. This reaction is cataylsed by the conjugase enzymes (γ-glutamylcarboxypeptidase) from the brush border of the jejunum or via conjugase enzymes in the pancreatic secretions and bile. The resulting monoglutamyl folates are absorbed by a saturable transport process that is sodium, energy and pH dependent. Conjugase inhibitors are present in cabbage, lentils, legumes and oranges and consumption of these foods can impair folate absorption. Conjugase enzymes are zinc dependent and severe zinc deficiency can also reduce the absorption of folates.
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