Ascorbic Acid Intake And Nonheme-Iron Absorption From A Complete Diet - Brief Article

Nutrition Research Newsletter, Feb, 2001

Vitamin C is the only dietary constituent other than animal tissue that has been shown repeatedly to increase the absorption of nonheme iron in humans. In contrast with the effect of ascorbic acid on iron absorption, the improvements in iron status when the diet is supplemented with vitamin C has been minimal. The explanation for the disparity between the pronounced effect of ascorbic acid on iron absorption and its meager influence on iron status after prolonged supplementation is not apparent. The current study was recently published in the American Journal of Clinical Nutrition, and was undertaken to reexamine the importance of dietary ascorbic acid in iron balance by measuring nonheme-iron absorption from a complete diet before and after altering the diet to maximally increase or decrease the intake of dietary ascorbic acid.

Four separate measurements of iron absorption were performed in each subject by using dual radioactive iron labels sequentially. One of the initial pair of absorption tests included a standard hamburger meal that had been used in prior studies. Iron absorption from this meal was used as a reference to facilitate comparisons with published studies of iron absorption. For the remaining three iron absorption tests, the complete diet was tagged by having the subjects consume radioactive bread rolls with each of the three main meals of the day for five days. Snacking between meals was not allowed. During the initial period of dietary tagging, the participants were on their regular, self-selected (SS) diet. For the remaining two dietary periods, the subjects were required to modify their usual dietary intake to maximize (HC) or decrease (LC) their dietary intake of vitamin C. This was accomplished by providing a list of vitamin C-rich foods that the subjects had to either include or exclude from their diets. Energy, carbohydrate, protein, and fat intakes did not differ significantly among the three dietary protocols.

There was a five-fold difference found in mean dietary vitamin C intake between the LC and HC diets: 51 compared with 247 mg/d. Phosphorus was shown to have a significant inhibitory influence, an effect that was stronger than the enhancing effect of either ascorbic acid or animal tissue. The nature of the inhibitory effect of dietary phosphorus on iron absorption is unclear. Vitamin C accounted for only 8% of the variation in iron absorption as compared with 19% for phosphorus.

The reasons for the diminished influence of dietary factors when iron absorption is measured from a complete diet rather than from individual meals are unknown. One possibility is that residual gastric contents for meals eaten throughout the day dampen the influence of dietary factors compared with that in fasting subjects. For whatever reason, the findings of this study indicate that the influence of dietary ascorbic acid on iron absorption is substantially less than indicated by absorption studies with single meals.

J. Cook, M. Reddy, Effect of ascorbic acid intake on nonheme-iron absorption from a complete diet. Am J Clin Nutr 73:93-98 (2001) [Correspondence: J.D. Cook, Dept of Medicine, University of Kansas Medical Center, 3901 Rainbow Blvd., Kansas City, KS 66160. E-mail: jcook1@kumc.edu].