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Human Nutrition and Metabolism
Research Communication

Use of an Oral/Intravenous Dual-Label Stable-Isotope Protocol to Determine Folic Acid Bioavailability from Fortified Cereal Grain Foods in Women¹

Nutrition & Consumer Science Division, Institute of Food Research, Norwich, UK and ^* School of Chemistry, University of Exeter, Exeter, UK

²To whom correspondence should be addressed. E-mail: paul.finglas{at}bbsrc.ac.uk.

Folic acid fortification, mandatory in the United States, is currently being considered by the UK. The hypothesis that the matrix of some cereal-product vehicles may result in low fortificant bioavailability was tested using a dual oral/intravenous (i.v.) isotopic-label approach, which was evaluated concurrently. Fifteen women received 225 µg oral folate (capsules, fortified white bread and fortified branflakes), mainly as folic acid labeled with ¹³C on 6 carbons of the benzoyl ring (¹³C₆-PteGlu), followed by i.v. injection of 100 µg folic acid labeled with ²H on 4 hydrogens of the glutamic acid group (²H₄-PteGlu). The urinary excretion ratio (UER) in intact folate of the percentage of labeled oral dose excreted divided by the percentage of i.v. dose excreted was used as the primary index of absorption. The geometric mean (95% confidence interval) UER for folic acid capsules was 3.68 (1.90, 7.14) at 24 h and 2.18 (1.24, 3.83) at 48 h. Because these were significantly in excess of 1.0, indicative of 100% absorption of the oral dose, it was concluded that oral and i.v. labeled folic acid are handled differently by the body and that "absolute" absorption cannot be calculated. Compared with the 48-h UER for folic acid capsules, the "relative" 48-h UER for white bread and branflakes was 0.71 and 0.37, respectively, indicating that some cereal-based vehicles may inhibit absorption of fortificant. However, even the validity of this "relative" approach is questioned.

KEY WORDS: • folic acid • bioavailability • absorption • stable isotope • cereal

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