Women who consume a lot of calories when trying for a baby are more likely to give birth to a son, they said. Women who ate around 2,200 calories a day were 1.5 times more likely to have a boy than those who ate less than 1,850 calories a day.
The eating of breakfast cereals before and around the time of conception was also "strongly associated" with women producing sons, the researchers said. They worked out that women eating breakfast cereals were 1.89 times more likely to have a boy than those who did not consume them.
Diets high in a number of nutrients, including potassium, calcium and vitamins C, E and B12, were also linked to male births.
The study was led by experts from the University of Exeter and the University of Oxford. Lead author, Dr Fiona Mathews, from the University of Exeter, said, "I think if you are wanting to conceive a boy, then the breakfast-cereal finding is the main thing that popped out.
"Women who had boys also had a 300mg higher daily intake of potassium so foods like bananas are good." Dr Mathews said it did not seem to matter which food groups those consuming 2,200 calories a day relied on. "It does not seem to matter whether you get most of your energy from carbs or fat; it's about the total amount of calories consumed," she said.
The research focused on 740 first-time pregnant mothers in the U.K., who did not know the sex of their fetus. Nutritional data was collected for three time periods: usual intake before conception (preconception); intake at around 16 weeks' gestation (early pregnancy) and usual intake between 16 and 28 weeks' gestation (later pregnancy). The results showed a link between food intake and gender around the time of preconception but not at other times in pregnancy.
"In 56% of women, the highest third of preconceptional energy intake bore boys, compared with 45% in the lowest third," the study said. "Intakes during pregnancy were not associated with sex, suggesting that the fetus does not manipulate maternal diet."
The researchers said the mechanisms behind the determination of sex in humans are not well understood. "However, a pathway has been proposed that could explain our associations of fetal sex with energy intake and breakfast cereal consumption around conception," they said. "In vitro, glucose enhances the growth and development of male conceptuses while inhibiting that of females. "Skipping breakfast extends the normal period of nocturnal fasting, depresses circulating glucose levels and may by interpreted by the body as indicative of poor environmental conditions."
The researchers said that over the last 40 years, there has been a small "but highly consistent" decline in the proportion of boys born in industrialized countries. Changes in the diets of young women may explain the pattern, they added.
Dr Mathews said, "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. "Our findings are particularly interesting given the recent debates within the Human Fertilisation and Embryology Committee 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," she said. Dr Fiona Mathews added, "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.
"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 study was published in the journal Proceedings of the Royal Society B: Biological Sciences. Professor Stuart West, from the University of Edinburgh, said, "This is an interesting result that is consistent with what appears to be going on some animals, such as red deer.
"The basic idea here is that sons have more to gain from being better quality and larger, and so a mother should preferentially produce sons if they are going to have more resources to put into reproduction.
"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% sons with low-calorie diets, to 55% 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."
Louise Silverton, deputy general secretary of The Royal College of Midwives, said, "We do empathize with mothers who may want a baby of a particular sex. "However, having a good diet before conception and during pregnancy can lay the foundations for a healthier life for the unborn child and the mother.
"Midwives are there to advise and support mothers about a healthy diet that produces a healthy child and one that will also improve and sustain the health of the mother.
"We would be concerned for the health of mothers and babies if women were adapting to a less healthy diet -- for example, stopping having a breakfast or reducing the range of nutrients they eat - in order to produce a child of a specific sex. "For midwives, the health and safety of the mother and child is and has to be paramount, not the sex of the baby."
From the April 28, 2008, Prepared Foods e-Flash