Skip Breakfast for a Daughter, Eat Cereal for a Son

THERE are numerous old wives' tales about how a couple can increase their chances of having a boy or a girl. For a son, make love only on odd days of the month, eat plenty of meat and be sure the father keeps his genitals cool by wearing boxer shorts and loose-fitting trousers; for a daughter, put a wooden spoon under the bed and eat plenty of yogurt.




Although a child's sex is genetically determined by the father, mothers can influence the development of one sex rather than another. Studies in animals and humans suggest that there are links between the sex of a child and the mother's diet and her levels of stress. Although the mechanisms are not well understood, this appears to have evolutionary roots which favour greater reproductive success. Hence in hard times when food might be scarce, daughters were more valuable because their chances of providing offspring would have been greater than sons, who might get killed or fail to find a mate. But in good times sons were a better bet because they could father more children.

Those same evolutionary influences persist in modern life. New research shows a clear link between higher energy intake around the time of conception and the birth of sons--especially by mothers who eat cereals for breakfast.

The study, by a team of researchers from the Universities of Exeter and Oxford, looked at the eating habits of 740 British mothers expecting their first child. The overall sex ratio of their children was close to 50:50. But when split into three groups according to the number of calories the mothers consumed around the time of conception, the picture changed. Of those with the highest energy intake, 56% had sons, against 45% in the group with the lowest calorie intake. Moreover, besides consuming more calories the women with sons were more likely to have eaten a higher quantity and range of nutrients, especially breakfast cereals.

This could help to explain why there has been a small but consistent decline over the past 40 years in the proportion of boys being born in relatively well-off industrialised countries, says Fiona Mathews of the University of Exeter, the lead author of the group's paper, which was published this week in the Proceedings of the Royal Society B: Biological Sciences.

Big dietary changes have taken place in developed countries. Yet despite rising levels of obesity and a decline in physical activity, the group could find no evidence of a link between the body-mass index of a mother and the sex of her child. But worries about weight have led many women to eat low-calorie diets. Moreover, says Dr Mathews, skipping breakfast has become far more common.

So what could be happening? It is known from in vitro fertilisation research that high levels of glucose can enhance the growth and development of male embryos but inhibit female ones. Skipping breakfast extends the normal period of nocturnal fasting and depresses glucose levels, which the group thinks could be interpreted by the body as indicative of hard times. So, prospective parents now know what to do first thing in the morning.