Human Nutrition and Metabolism Research Communication

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 g oral folate (capsules, fortified white bread and fortified branflakes), mainly as folic acid labeled with 13 C on 6 car- bons of the benzoyl ring ( 13 C6-PteGlu), followed by i.v. in- jection of 100 g folic acid labeled with 2 H on 4 hydrogens of the glutamic acid group ( 2 H4-PteGlu). The urinary excre- tion 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 can- not 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 for- tificant. However, even the validity of this "relative" ap- proach is questioned. J. Nutr. 132: 936 -939, 2002.