Folate in milk

Free folate, released by conjugase action, is absorbed by a carrier-mediated mechanism in the jejunum. However, the folate in milk is mainly bound to a specific binding protein (which has been used in radioligand binding assays for folate) the protein-folate complex is absorbed intact, mainly in the ileum, by a mechanism that is distinct from the jejunal transport system for free folate. The biological avculability of folate from milk, or of folate from diets to which milk has been added, is considerably greater than that of unbound folate, whereas that of folate from cereal foods, or of free folic acid taken with cereal foods, is lower. 

DL Holt, RL Wehling, MG Zeece. Determination of native folates in milk and other dairy products by high-performance liquid chromatography. J Chromatogr... 

K Wigertz, M Jagerstad. Comparison of a HPLC and radioprotein-binding assay for the determination of folates in milk and blood samples. Food Chem 54(4) 429-436, 1995. 

INDYK HE et al. (2000), Determination of biotin and folate in infant formula and milk by optical biosensor-based immunoassay , JAOCS, 83(5), 1141-8. 

Folate is a relatively unstable nutrient processing and storage conditions that promote oxidation are of particular concern since some of the forms of folate found in foods are easily oxidized. The reduced forms of folate (dihydro- and tetrahydrofolate) are oxidized to p-aminobenzoylglutamic acid and pterin-6-carboxylic acid, with a concomitant loss in vitamin activity. 5-Methyl-H4 folate can also be oxidized. Antioxidants (particularly ascorbic acid in the context of milk) can protect folate against destruction. The rate of the oxidative degradation of folate in foods depends on the derivative present and the food itself, particularly its pH, buffering capacity and concentration of catalytic trace elements and antioxidants. 

Folate is sensitive to light and may be subject to photodecomposition. Heat treatment influences folate levels in milk. Pasteurization and the storage of pasteurized milks have relatively little effect on the stability of folate but UHT treatments can cause substantial losses. The concentration of oxygen in UHT milk (from the headspace above the milk or by diffusion... 

Anon. 1982A. Folate binder in milk may facilitate folate absorption. Nutr. Rev. 40, 90-92. 

Bovine milk also contains binding proteins for vitamins B12, folic acid and riboflavin. It has been suggested that the folate-binding protein contributes to the absorption of folate in the intestines (Parodi, 1998). 

Occasionally, protein hinding of a vitamin on foods increases its absorption and hence its biological availability. For example, folate from milk is considerably better absorbed than that from either mixed food folates or free folic acid (Section 10.2.1). Folate bound to a specific binding protein in milk is absorbed in the Ueum, whereas free folate monoglutamate is absorbed in the (smaller) jejunum... 

Demethylated tetrahydrofolate monoglutamate is released hy extrahepatic tissues and is transported hound to a plasma folate binding protein similar to that in milk. It has a very low affinity for methyl-tetrahydrofolate and other one-carbon substituted derivatives. It functions mainly to return folate to the liver, where it is either conjugated for storage or methylated to 5-methyl-tetrahydrofolate that is secreted in the bile. 

Folale deficiency occurs in pregnancy on a wide scale. The increased utilization of the vitamin by die fetus and related tissues, as well as the secretion in milk during lactation, can place an increased demand on the dietary folates consumed by the mother. Sea ere folate deficiency leads to megaloblastic anemia. This disease, in which the synthesis of red blood cells is impaired, tends to occur with pregnancy in underdeveloped countries, but usually not in North America or Europe, Goat s milk is a poor source of folate and vitamin Bjj. Ovcrrchancc on goal s milk as a source of food for infants can result in a deficiency in these vitamins and in anemia. 

The function of folate binders in folate metabolism remains unclear, except in the case of milk where they have recently been shown to enhance the absorption of folate in breast-fed infants and possibly protect the folate against utilization by intestinal bacteria (C7). The binder may also have a function in sequestering folate from the mother s circulation. It has not been shown to have a role in polyglutamate synthesis nor does it appear to play a part in enabling cells to take up folate on the contrary, Waxman and Schrei-ber (W12) have shown that the binder prevents the uptake of folate by cells in tissue culture. 

Waxman, Schreiber, and Herbert first described a radioisotopic assay for the measurement of serum folate in 1971 (W14) which they claimed gave almost identical results to those obtained by with the L. casei microbiological assay and could separate low, borderline, and normal folate levels. Serum was mixed with PHjmethyltetrahydrofolic acid in a phosphate buffer and a folate binder was added which, in this case, was Carnation brand instant powdered milk. Following incubation, free vitamin was separated from that which was bound using hemoglobin-coated charcoal. The radioactivity of the supernatant was determined, and from this the folate concentration in the serum sample could be calculated using appropriate standards and blanks. 

Coat s milk is sometimes fed to infonts, particularly those who are sensitive to cow s milk. Coat s milk contains very little folate. Nicol and Davis (N4) found goat s milk to contain only 7 compared with 52 pg/liter in cow s milk. Heating the milk reduced the folate in the goat s milk to 1 p.g/liter and halved that in the cow s milk. 

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