Submitted to: American Journal of Clinical Nutrition
Publication Type: Other
Publication Acceptance Date: 6/30/2009
Publication Date: 9/1/2009
Citation: Tang, G., Russell, R.M., Qin, J., Dolnikowski, G.G., Grusak, M.A. 2009. Reply to MB Krawinkel. American Journal of Clinical Nutrition. 90:696-697. Interpretive Summary:
Technical Abstract: We appreciate the opportunity to respond to the letter from Dr. Krawinkel regarding our recent publication on the vitamin A equivalency of Golden Rice. In this study we utilized stable isotope methodologies and a single serving (per subject) of Golden Rice (a transgenic rice that produces beta-carotene in the grain) to study beta-carotene absorption and bioconversion to vitamin A in five healthy, adult subjects in Boston, Massachusetts. We showed that Golden Rice beta-carotene in the dose provided (approx. 1 mg) was effectively converted to vitamin A. While Krawinkel acknowledges that our study provided evidence for beta-carotene uptake, he raises 2 concerns about the presented bioconversion results: one concern relating to the data analysis and the other relating to the selection of study subjects. In the writer's judgment, he believes that the reported 'effective' bioconversion efficiency of Golden Rice beta-carotene to vitamin A (mean of 3.8 to 1, by weight) was questionable, because only two of five bioconversion values were above the median value. First, this statement makes little sense, because by definition two of any five values will be above the median. However, it should be noted that the range of values observed in our five subjects was 1.9 - 6.4 to 1 (by weight); therefore, all five individuals demonstrated a better bioconversion of Golden Rice beta-carotene than the current bioconversion value (12 to 1, by weight) utilized by the Food and Nutrition Board, Institute of Medicine in its most recent Dietary Reference Intake recommendations for vitamin A. Additionally, the bioconversion values for Golden Rice beta-carotene in these five subjects were lower (i.e., more effective conversion) than the values of many foods previously studied, including fruits, sweet potato, carrots, and several green leafy vegetables. Regarding the selection of study subjects, the writer is concerned that this current study was not conducted with individuals from vitamin A deficient populations, or using diets that are more representative of these populations. As noted in our paper, we recognize that more studies are needed in at-risk populations, including those with children as the subjects, and in fact these studies are already in progress. Furthermore, we suggested that our bioconversion values could be used to provide estimates of what Golden Rice might provide to children, in terms of dietary vitamin A equivalents, but we also noted that longer-term feeding trials in which Golden Rice is incorporated into daily diets are needed to assess its efficacy in improving vitamin A status at the population level. Thankfully, with our current bioconversion estimates now determined, these types of studies can be designed in a sensible and scientifically informed manner.