The study, which links higher energy intake around conception to the birth of sons, provides the first explanation of why the number of boy babies is in decline in the west, suggesting it is the result of women consuming low fat foods and skipping breakfast, among other things.
The research shows a higher calorie intake around the time of conception can shift the odds of having a son from ten to 11 boys in every 20 births. The effect was such that the more women ate, the more likely she was to have a boy.
As well as consuming more calories, women who had sons were more likely to have eaten a higher quantity and wider range of nutrients, including potassium, calcium and vitamins C, E and B12.
In other words, women who want a son should eat a generous bowl of cereal for breakfast, munch bananas, use more salt and boost their overall daily calories by 400 calories - the equivalent of a meal.
Although the DNA in sperm determines sex, it seems that in the never ending battle of the sexes mothers can favour the development of one sex of infant rather than another, a faculty that nature uses to fine tune the sex ratio in Stone Age days to suit times of feast and famine, says the team from the Universities of Exeter and Oxford.
To reveal how you are what your mother eats, the team focused on 721 first-time pregnant mothers in the UK, who did not know the sex of their unborn child and were asked to provide records of their eating habits before and during the early stages of pregnancy.
They completed detailed questionnaires which asked about their usual consumption of more than 150 common items, and recorded details on the types and quantities of breakfast cereals, margarines and milk they used. During pregnancy they also kept food diaries.
They were split into three groups according to the number of calories consumed per day around the time they conceived, revealing that 56 per cent of the women in the group with the highest energy intake at conception had sons, compared with 45 per cent in the lowest group.
"The effect was linear, that is the more women ate, the more likely she was to have a boy - so the effect might be even larger if women had particularly high intakes" says Dr Fiona Mathews of Exeter, lead authorof the study in the journal Proceedings of the Royal Society B: Biological Sciences.
An additional son per 20 births was down to eating more than 2200 calories compared with fewer than 1850 calories of the low intake group and there was an average difference between the mothers of boys and girls of 130 calories per day.
Dr Mathews says this could explain why over the last 40 years there has been a small but consistent decline, of about one per 1000 births annually, in the proportion of boys being born in industrialised countries, including the UK, the US and Canada.
This decline mirrors the fall in average energy intake in the developed world (the obesity epidemic is driven by burning fewer calories in everyday life, due to less overall exercise, and eating high fat diets).
"This research may help to explain why in developed countries, where many young women choose to have low calorie diets, the proportion of boys born is falling," says Dr Mathews, adding that there is also a link between higher national incomes and fewer boys.
There is also evidence that skipping breakfast is now common in the developed world: in the USA, the proportion of adults eating breakfast fell from 86 per cent to 75 per cent between 1965 and 1991.
"Our findings are particularly interesting given the recent debates within the Human Fertilisation and Embryology Authority about whether to regulate 'gender' clinics that allow parents to select offspring sex, by manipulating sperm, for non-medical reasons. Here we have evidence of a 'natural' mechanism that means that women appear to be already controlling the sex of their offspring by their diet."
The work complements studies of other animals that shows that more sons are produced when a mother has plentiful resources or is high ranking, reflecting how boys are more taxing to raise.
"Boys breast feed for longer and for more," she says. "There is evidence from traditional societies that mothers invest more time in bringing up boys. And if a mother has plentiful resources then it can make sense to invest in producing a son because he is likely to produce more grandchildren than would a daughter. However, in leaner times having a daughter is a safer bet."
The phenomenon, where lean times are linked with daughters, has been most extensively studied in insects, but is also seen in horses, cows and some species of deer.
Dr Mathews adds: "Potentially, males of most species can father more offspring than females, but this can be strongly influenced by the size or social status of the male, with poor quality males failing to breed at all.
Females, on the other hand, reproduce more consistently. The mechanism is not yet understood in mammals, but it is known from IVF research that high glucose levels encourage the development of male embryos while inhibiting female embryos. In humans, skipping breakfast depresses glucose levels and so may be interpreted by the body as indicating poor environmental conditions and low food availability.
The findings showed no evidence of a link between the gender and a mother smoking and drinking caffeine prior to pregnancy. Nor was there a correlation between the body mass index (BMI) of a mother and the sex of her child.
Professor Stuart West, from the University of Edinburgh, comments: "This is an interesting result that is consistent with what appears to be going on some animals, such as red deer. However, I would be extremely cautious about using diet to try and influence offspring sex.
First, the effect appears to be relatively small, with the sex ratio varying from only 45 per cent sons with low calorie diets, to 55 per cent sons with high quality diets.
Second, similar data in animals such as non-human primates shows huge variation between studies, and so it would be key to determine the repeatability of these results. Third, diet will have other effects for both the parent and offspring."
The sex of a child is determined at the moment of fertilisation, when an egg containing an X chromosome encounters a sperm. That sperm can contain either a Y chromosome, in which case the embryo will be male, or an X, in which case it will be female.
This study was funded by the Sir Jules Thorn Charitable Trust.
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