Published online 22 April 2008 | Nature | doi:10.1038/news.2008.769

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Calorie intake before conception may mean the difference between boys and girls.

Eating breakfast cereal might make it more likely you have a boy. Punchstock

The sex of new babies is influenced by the mother's diet before she conceives, a new study suggests. According to a survey of 740 British mums to be, a high-calorie diet is more likely to lead to a baby boy in nine months' time.

Researchers led by Fiona Mathews of the University of Exeter collected data on the pre-conception dietary habits of pregnant women, and found that 56% of women in the highest one-third of calorie intake had male fetuses. In the lowest third, only 45% bore boys.

The women, who were attending maternity clinics, were asked to compile a 'retrospective diary' of their food intake in the weeks before they fell pregnant. Mathews and her colleagues then analysed the results to look for a relationship between food intake and the sex of their offspring.

The level of calorie intake was the main dietary factor that affected offspring sex, say the researchers, who report their research in Proceedings of the Royal Society B1. Overall, women in Matthews’s study who produced sons ate an average of 180 calories more per day than those who had daughters — "the equivalent of eating a banana", she says.

Intriguingly, a major difference seemed to be between women who routinely ate breakfast cereals and those who didn't, Mathews adds. Among those who ate breakfast cereals almost every day, 59% gave birth to boys. In contrast, 43% of those who claimed to eat breakfast "rarely or never" produced sons.

Sugar babies

The researchers are unsure of why this happens, although they suggest that it could be to do with levels of glucose in the blood. Male test-tube embryos generally require a more glucose-rich growth medium to survive in the lab — Mathews suspects that a similar process may operate in the uterus, so women with higher sugar levels are more likely to nurture a male embryo as it is implanted in the womb lining.

Mathews even suggests that the prevalence of dieting in young Western women might be skewing the sex ratio towards more baby girls. She cites studies from the United States showing that average energy intake in adolescent women has declined by 17% between 1965 and 1996.

It is not clear how overeating may affect gender selection, however — the new study excluded obese women because of the other health complications involved with being severely overweight.

The results seem to fit with the biological theory that many animals, perhaps including humans, tend to produce male offspring when food and resources are in plentiful supply. This is thought to be a strategy to maximize genetic returns, as successful, healthy sons can go on to produce far more grandchildren than daughters.

What mum eats

It's an interesting theory, although far from proven, says Paul Haggarty, an expert on nutrition and reproduction at the Rowett Research Institute in Aberdeen, UK. “There are some plausible biochemical mechanisms, although what they have done is pretty limited,” he says — it was a rather small study, he points out.

Haggarty adds that his own database of dietary habits in some 1,500 Scottish mothers shows no evidence that diet can affect the gender of their offspring. “But who knows, we may well come up with something very interesting in bigger data sets,” he says.

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Nevertheless, evidence is emerging that not only is it true that ‘you are what you eat’ — but also that your mother’s or even your grandparents’ diet can affect your health. Previous studies of mice have shown that coat colour can be determined by a mother's diet during pregnancy, and that children’s risk of diabetes can be raised by chemical changes to DNA that resulted from their grandparents’ poor diet when pregnant with the children’s parents.

Haggarty would also like to see more conclusive, large-scale studies that address the effect of food supplements such as folate, which is crucial for healthy embryonic development, on the gender or health of babies. Pregnant women, and those hoping to get pregnant, tend to take far more supplements than the general population, he says.

Although Mathews and her team did not collect data on food supplements, she suggests that trace nutrients such as potassium in the diet might also affect the sex of babies, perhaps by raising the acidity of the womb, which might make it more welcoming for a male embryo.