Bioavailability of folate from fortified milk products

Summary

The gap between actual intake and recommended intake of folate could be bridged by the consumption of fortified food products. Milk is considered as a potential food matrix for folate fortification in countries (such as theNetherlands) witha highmilk consumption. The aim of the work described in this thesis was to study the bioavailability of folate from milk products to establish whether milk is a suitable matrix for fortification with folic acid or 5-CH ₃ -H ₄ folate. In addition, the role of folate-binding proteins (FBP) in the bioavailability of folate from milk was investigated.Studies with a dynamic in vitro gastrointestinal model showed that folic acid and 5-CH ₃ -H ₄ -folate are highly bioaccessible from fortified milk products. The bioaccessibility of folate from fortified milk products was lower in presence of additional FBP, with a more pronounced inhibitory effect for folic acid as compared with 5-CH ₃ -H ₄ folate. This was explained by the observed difference in extent of binding to FBP between folic acid and 5-CH ₃ -H ₄ -folate in the duodenal lumen. Before gastric passage, folic acid and 5-CH ₃ -H ₄ -folate were mainly bound to FBP (76-79%) while 7% was free. After gastric passage, folic acid remained bound to FBP to a similar extent (80-81%). For 5-CH ₃ -H ₄ -folate the FBP-bound fraction gradually decreased from 79% to 5% and the free fraction increased from 7% to 93%. So, while folic acid enters the proximal part of the small intestine bound to FBP, 5-CH ₃ -H ₄ -folate appears mainly to be present as free folate in the duodenal lumen. The intestinal absorption of folic acid and 5-CH ₃ -H ₄ folate was studied using monolayers of human colon carcinoma (Caco-2) cells. Only a small difference in transport, in rate and underlying transport mechanisms, across Caco-2 cells was found between folic acid and 5-CH ₃ -H ₄ -folate. In presence of FBP, the absorption of folic acid and 5-CH ₃ -H ₄ folate was found to be lower and dependent on the extent of binding to FBP at the luminal side of the intestinal cells.Results from a human intervention study showed that the consumption of 200mg of folic acid added to milk significantly increased folate concentrations in serum and red blood cells. Although only two fortified milk products were tested in a human study, several milk products fortified with folic acid or 5-CH ₃ -H ₄ -folate with or without additional FBP were tested in the in vitro studies with the gastrointestinal model. Finally, a kinetic model was used to integrate the in vitro results about the kinetics of folate bioaccessibility and intestinal absorption and to extrapolate the findings to the human situation. With this in silico approach, the blood folate levels in humans could be predicted accurately.In conclusion, the in vitro and in vivo studies described in this thesis show that milk is an appropriate food matrix for folate fortification. A dietary strategy with fortified milk products can be recommended to bridge the gap between actual and recommended folate intake to optimize the folate status of the population. Folic acid-fortified milk should, however, not be supplemented with additional FBP as this will lead to a lower bioavailability of folic acid.