Thursday, April 24, 2008

Stage One: Begin Poking at a Study

Some of you may have seen the reports of a new study which finds that the diet of a woman at conception affects the likelihood that the resulting child is either a girl or a boy:

The report, from researchers at Oxford and the University of Exeter in England, is said to be the first evidence that a child's sex is associated with a mother's diet. Although sex is genetically determined by whether sperm from the father supplies an X or Y chromosome, it appears that a mother's body can favor the successful development of a male or female embryo.

The study, published in the journal Proceedings of the Royal Society B: Biological Sciences, shows a link between higher energy intake around the time of conception and the birth of sons. The difference is not huge, but it may be enough to help explain the falling birthrate of boys in industrialized countries, including the United States and Britain.

The reason food intake may influence the development of one sex of infant rather than another isn't fully understood. However, in vitro fertilization studies show that high levels of glucose encourage the growth of male embryos while inhibiting female embryos.

Note how some of this cannot be understood without seeing the actual study, and most people don't have access to it. What does it mean that the sex is determined by the mother's diet? That bit "it appears that a mother's body can favor the successful development of a male or female embryo"? How does it favor this?

Most confusing. Because to me that would read as implying that the woman would miscarry more of either types of embryos, depending on her diet. I don't think the study included women who had conceived but who then miscarried. So how is it that the woman's diet has these effects?

What about the man's diet at the time of conception? Did they look at what he was eating in, say, the month before? I think lots of married couples, for instance, might eat very similar foods, as they tend to have meals together. To me it might make more sense that any effect found here would come from the one who makes the sperm, given that the sperm is made regularly and might be affected by the man's nutrition.

So I'm throwing out the thought that what might really affect the sex of the future child is what the guy eats, and that what the gal eats correlates in the study with the child's sex because the parents tend to eat similar diets.

Perhaps the study did look at the diets of the future fathers, too, and found that they had no impact. Perhaps. But I doubt that, very much, because studies of these types always focus on the women. It's most likely that we have no idea if the man's diet affects the sex of the future child or not.

The practical implications of a mistake like that (the one I'm speculating about here) might be serious, by the way. You'd be barking at the wrong tree if you tried to tell women to eat certain things to get a boy, if all the time it was what the man ate that made a difference. Then in the more traditional societies the woman would be blamed for not birthing more boys, even after all that extra good food was wasted on her.

This study sounds like those studies which try to find a sex-ratio change in human populations at birth as a response to environmental effects such as famine or war. Some other animal species give birth to more daughters when times are bad.

It would seem to me that if this is the hypothesis the researchers had in mind it would be much more relevant to look at the sex ratios at birth in a country which is suffering from famine, say, or to analyze the birth statistics from the era around WWII in the U.K.., compared to an era when war didn't make good food scarce.

I have just thought aloud here, asking the questions reading the study creates and wondering about the alternative ways results like the ones in the study might come about. To look at the study itself and the methods it used would be the next stage at the poking.