This paper establishes a statistically significant association between maternal diet and subsequent hypospadias in boys born to vegetarian mothers. Hypospadias is a structural defect in the penis that has been increasing in frequency in several countries over the past 20 years, including the United States [More...]

An important aspect of this study is that it was prospective. Mothers were interviewed during pregnancy and thus the study avoids the problem of biased recall. The study examined 7928 boys, of whom 51 had hypospadias.

Of vegetarian mothers, 2.2% gave birth to a boy with hypospadias, compared to 0.6% of mothers who were omnivores. The adjusted odds ratio of hypospadias for vegetarian mothers was 4.99 (with 95% confidence intervals from 2.10-11.88).

North et al. consider three possibilities for the association with vegetarianism. One is the likelihood that vegetarianism will result in increased intake of phytoestrogens, particularly in soy products. A second is that an increased vegetable diet may expose the mother to more pesticide residues. They are unable to resolve this with the data they gathered because the sample of vegetarian mothers who bought only organic food was too small. The authors note in passing that none of these mothers (163 in total) gave birth to a hypospadiac, but that only one would have been expected based on the overall rate in the population. A third hypothesis is that the vegetarian diet is more likely to lack some nutrient essential for male reproductive tract development.

The authors refer to the higher use of soy in the Japanese diet, but make no reference to data indicating that the rate of hypospadias in Japan is less than one-tenth the overall rate reported in this study. This apparent conflict with their "soy hypothesis" might reflect metabolic differences among populations, given that soy has been part of the traditional diet in Japan for many generations.

The study controls for a wide variety of variables that might confound the relationship. They did take blood samples and expect to measure levels of phytoestrogens, pesticide residues and essential nutrients in a later study. This may allow a more definitive test of the three hypotheses.

The authors also noted an association with iron tablets: 1% of mothers who had taken iron tablets during pregnancy had boys born with hypospadias, a statistically significant finding compared to omnivorous mothers who had not taken iron (0.6%). Because iron supplements are common in vegetarian diets, North et al. examined this more closely. They found that vegetarian mothers who did not take iron were more likely to have hypospadias, and that non-vegetarian mothers who took iron were also more likely to give birth to boys with hypospadias.

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