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Nutritional Methodology

Folic Acid and 5-Methyltetrahydrofolate in Fortified Milk Are Bioaccessible as Determined in a Dynamic In Vitro Gastrointestinal Model

Miriam Verwei^*,2, Karin Arkbåge, Robert Havenaar^**, Henk van den Berg, Cornelia Witthöft and Gertjan Schaafsma^**

^* TNO-WU Center for Micronutrient Research, 3700 AJ Zeist, The Netherlands, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden, ^** TNO Nutrition and Food Research, 3700 AJ Zeist, The Netherlands and The Netherlands Nutrition Center, 2508 CK, Den Haag, The Netherlands

²To whom correspondence should be addressed. E-mail: Verwei{at}voeding.tno.nl.

Dairy products are a potential matrix for folate fortification to enhance folate consumption in the Western world. Milk folate-binding proteins (FBP) are especially interesting because they seem to be involved in folate bioavailability. In this study, folate bioaccessibility was investigated using a dynamic computer-controlled gastrointestinal model [TNO gastrointestinal model (TIM)]. We used both ultrahigh temperature (UHT)-processed milk and pasteurized milk, differing in endogenous FBP concentrations and fortified with folic acid or 5-methyltetrahydrofolate (5-CH₃-H₄folate). To study FBP stability during gastrointestinal passage and the effect of additional FBP on folate bioaccessibility, FBP-fortified UHT and pasteurized milk products were also tested. Folate bioaccessibility and FBP stability were measured by taking samples along the compartments of the gastrointestinal model and measuring their folate and FBP concentrations. Folate bioaccessibility from folic acid-fortified milk products without additional FBP was 58–61%. This was lower (P < 0.05) than that of the 5-CH₃-H₄folate-fortified milk products (71%). Addition of FBP reduced (P < 0.05) folate bioaccessibility from folic acid-fortified milk (44–51%) but not from 5-CH₃-H₄folate-fortified milk products (72%). The residual FBP levels in the folic acid- and 5-CH₃-H₄folate-fortified milk products after gastrointestinal passage were 13–16% and 0–1%, respectively, of the starting amounts subjected to TIM. In conclusion, milk seems to be a suitable carrier for folate, because both folic acid and 5-CH₃-H₄folate are easily released from the matrix and available for absorption. However, our results suggest that folic acid remains partly bound to FBP during passage through the small intestine, which reduces the bioaccessibility of folic acid from milk in this model.

KEY WORDS: • folate • fortified milk • folate-binding protein • in vitro gastrointestinal model • bioavailability

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