The relative effect of ascorbic acid on iron absorption from soy-based and milk-based infant formulas

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Abstract
The effect of varying concentrations of ascorbic acid on the absorption of iron from a soy-based infant milk formula containing 6 mg iron/100 g was examined in 64 adult Indian females using the extrinsic radioactive tag method. The corrected geometric mean absorption from the basic soy formula was only 1.8%. Addition of ascorbic acid in a concentration of 40 mg/100 g, did not significantly increase absorption (3.3%; t = 1.8, p > 0.07) but raising the concentration to 80 mg/100 g did so (6.9%; t = 2.4, p < 0.02). No further significant increase was noted when the concentration of ascorbic acid was increased to 160 mg/100 g (7.7%; t = 0.4, p > 0.7). The inhibitory effect of soy on iron absorption was further demonstrated by a direct comparison between the soy-based formula and a similar product based on cows' milk. The comparison was made at two concentrations of ascorbic acid. At 40 mg/100 g the geometric mean iron absorption from the soy formula was 2.4% compared with 5.3% from the milk formula (t = 2.8, p < 0.02), while the corresponding values at 80 mg ascorbic acid/100 g were 7.2 and 19.5%, respectively (t = 3.4, p < 0.02). The present results confirm the marked inhibitory effect of soy protein on iron absorption and calculations from the absorption figures suggest that such formulas should contain at least 12 mg/100 g iron together with ascorbic acid in a molar ratio of approximately 4:1 if they are to be adequate in terms of iron nutrition.