Properties of Food Folates Determined by Stability and Susceptibility to Intestinal Pteroylpolyglutamate Hydrolase Action

Abstract

The intestinal absorption of folate occurs at the monoglutamyl level, and an important measure of food folate bioavailability is how much folate from the food reaches the intestinal sites in forms that can readily be absorbed. In the absence of protecting agents, e.g., vitamin C and reduced thiols, many labile folates may be lost during cooking and during residence in the acid-peptic milieu of the stomach. On the other hand, the presence of polyglutamyl folate necessitates the action of intestinal hydrolases, which could be affected by food constituents. In this study, we developed an in vitro assay for the determination of an index of food folate availability. The index of folate availability in this study was defined as that proportion of folate that has been identified as monoglutamyl derivatives after tests for stability and susceptibility to an enzymatic hydrolysis. The index of folate availability varied widely among foods. The highest index was for egg yolk (72.2%), followed by cow's livers (55.7%), orange juice (21. 3%), cabbage (6.0%), lima beans (4.5%) and lettuce (2.9%). Yeast folate had the lowest index (0.3%). The availability indices generated by this study correlate with the indices of the bioavailability of the corresponding food folate observed in earlier studies, R2 = 0.529 (P = 0.068). Additional information is required on the bioavailability of other food products to test the usefulness of this in vitro approach for assessing food folate availability.

Fruit and Vegetable Intake during Pregnancy and Risk for Development of Sporadic Retinoblastoma

“…Although naturally occurring folate is found in leafy green vegetables, some fruits, egg yolks, and legumes, the bioavailability of dietary folates obtained from natural sources is highly variable (2.9-72.2%), and depends on gastric acidity, food preparation, and exposure to air oxidation (38). For legumes, the bioavailability is particularly low; for example, only 4.5% of the folate in lima beans is bioavailable after cooking them for 10 minutes.…”

L-ascorbic acid improves the serum folate response to an oral dose of [6S]-5-methyltetrahydrofolic acid in healthy men

“…In addition, 5-CH 3 H 2 PteGlu is rapidly degraded in the acid-peptic gastric environment, resulting in significant losses of folate during digestion (Lucock et al, 1995;Seyoum and Selhub, 1998). Since antioxidants such as L-ascorbic acid (L-AA) convert 5-CH 3 H 2 PteGlu into 5-CH 3 H 4 PteGlu (Donaldson and Keresztesy, 1962;Lucock et al, 1995) and protect [6S]-5-CH 3 H 4 PteGlu against oxidative degradation (Oey et al, 2006), it has been stated that normal endogenous gastric L-AA concentrations, as secreted in the gastric lumen, might be a critical factor in optimizing the bioavailability of natural folate (Thien et al, 1977;Lucock et al, 1995).…”

Molecular genetic analysis of the human dihydrofolate reductase gene: relation with plasma total homocysteine, serum and red blood cell folate levels

“…6 Moreover, an increase in tHcy increases the risk of cardiovascular disease and congenital abnormalities. 7 -9 Dietary folates, present in reduced and partially reduced form, 10 exist as polyglutamates and require hydrolysis to monoglutamates before absorption in the intestine. After release into the portal circulation, folate (mainly as 5-methyltetrahydrofolate) is distributed throughout the body and is used for homocysteine remethylation, leaving tetrahydrofolate (THF) and methionine.…”