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Nutrient Interactions and Toxicity

Dietary Interactions Influence the Effects of Bovine Folate-Binding Protein on the Bioavailability of Tetrahydrofolates in Rats^1,,2

Department of Biochemistry and Molecular Biology, The University of Queensland, St Lucia, Queensland 4072, Australia and ^* Centre for Food Technology, Department of Primary Industries, Hamilton, Queensland 4007, Australia

³To whom correspondence should be addressed. E-mail: p.nixon{at}mailbox.uq.edu.au.

The newborns of mammals have a high folate demand, yet obtain adequate folate nutrition solely from their mothers’ milk despite its low folate content. Milk folate is entirely bound by an excess of folate-binding protein (FBP), prompting speculation that FBP may affect the bioavailability of the limited folate supply. Previous research has shown that FBP-bound folic acid is more gradually absorbed, thereby reducing the peak plasma folate concentration and preventing loss into the urine. Natural folates are reduced derivatives of folic acid, with milk predominantly containing 5-methyltetrahydrofolate, yet little research has been carried out to determine the role of FBP in the bioavailability of reduced folates. We studied the effect of FBP on folate nutrition of rats in both single-dose and 4-wk feeding experiments. The effect of FBP was influenced by the presence of other milk components. FBP increased bioavailability of dietary folate when it was consumed with other whey proteins or with soluble casein. However, in the presence of acid-precipitated casein or a whey preparation enriched in lipids, bioavailability was decreased. These results highlight the difficulties of extrapolating from experimental results obtained using purified diets alone and of studying interactions among dietary components. They suggest that the addition of FBP-rich foods to folate-rich foods could enhance the bioavailability of natural folates, but that the outcome of such a combination would depend on interactions with other components of the diet.

KEY WORDS: • folate-binding protein • 5-formyltetrahydrofolate • 5-methyltetrahydrofolate • bioavailability • dietary interactions

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