Milk Folates: Characterisation and Availability

Abstract: With the ultimate goal to study milk folate bioavailability, a reverse-phase HPLC technique was developed and compared with a radioprotein binding assay. All methods showed similar ranges for folates in cow’s milk, with the variation attributed to seasonal variations and the use of different starter cultures. Most cheese varieties contained slightly higher values for whey cream cheese. Studies using HPLC indicated 5-methyltetra- hydrofolate (5-CH3THF) as the major folate form in milk, but more studies are needed concerning folate forms in other dairy products. Due to the suggested role of folate-binding proteins in facilitating the absorption of folates from milk, new data on actual concentrations in different dairy products are now available and presented in this study. These data clearly showed that folate- binding proteins are present in unprocessed milk, pasteurised milk, spray-dried skim milk powder, and whey. In contrast, UHT milk, fermented milk, and most cheeses contained only low or trace amounts. Among the factors responsible for incomplete bioavailability is the possible degradation of labile tetrahydrofolate forms in the acidic gastric environment. In order to investigate this possibility, an in vitro method which simulated conditions prevailing in the stomach and in the small intestine was developed. The retention of 5-CH3THF in the dairy products was close to 90% in the presence of 0.01% sodium ascorbate, as compared to ordinary food folate analysis using HPLC. Without ascorbate, the retention of native 5-CH3THF was lower, generally between 40% and 90%. Thus, provided that the environment in the stomach and small intestine offers a sufficiently reducing environment, only minimal losses of dietary 5-CH3THF due to degradation would be expected. Interestingly, milk did increase the apparent absorption of dietary folates significantly (P < 0.05) in a balance study with ileostomy patients, however, there was no difference between fermented and unfermented milk. Therefore, the suggested role of folate-binding protein in the absorption of dairy folates did not seem to affect the total amounts of dietary folates absorbed, at least not in this model, for adults, where milk constitutes only a part of the total folate intake.