Apr 28 2008
Can a Mother’s Diet Affect the Sex of Her Child?
Researchers from the universities of Exeter and Oxford in England have published a study that they claim shows that a woman’s diet around the time of conception can influence the sex of their child. At first glance this claim sounds implausible, since in mammals the male sperm entirely determines the sex of a child, not the female egg. But the story is more complicated than it at first may appear.
The study involved having 740 women who were trying to get pregnant for the first time keep track of their daily diet. What they found is that those women who had a higher calorie diet overall, and those who ate at least one bowl of breakfast cereal per day, had a 24% greater chance of having a boy than those who skipped breakfast or consumed lower total calories.
What this type of study shows is a potential correlation. Before we can conclude that the correlation is reliable the study should be replicated. Also, correlation does not equal causation – it may be due to causation, but we cannot be sure. There may be some other factor involved that was not measured or controlled for in the study. So, in addition to replication, the hypothesis that diet can affect the sex of a child should be tested prospectively. This means that women who are trying to get pregnant can be randomized to various diets and then followed prospectively to see if there is any difference in the chance of conceiving a boy or a girl.
What about the plausibility of the claim? As I said, the female eggs always contain an X chromosome. The male sperm are 50% X and 50%Y. If an X sperm fertilizes the egg, the resultant child is female, and if a Y sperm wins the race, then the child will be male. So how can the mother’s diet affect the outcome?
Well, there are a two main hypotheses: either the female environment is more friendly to Y sperm, giving them an advantage, or it is more friendly to XY embryos, allowing them to survive better. There is evidence to suggest that the latter case may be true. In vitro, XY embryos tend to survive better in high glucose environments. Women who skip breakfast may have low blood sugar, which may in turn lower the survival of male embryos.
There is also some indirect evidence from animal husbandry for a dietary effect on male births. Well fed animals are more likely to give birth to males than females. This observation does not necessarily translate to humans, but it shows that such an effect is plausible.
Why would this be the case from an evolutionary point of view? Well, first, we must listen to the warning of Stephen J. Gould against hyperadaptationalism. We do not necessarily need to find a specific evolutionary advantage for every single trait. Many traits may be epiphenomena – they occur for no particular reason, or as a side consequence of some other trait that was selected for. So it may be that male sperm of male embryos do better when women have a superior diet, but for some idiosyncratic reason not specifically selected for.
However, it may also be that there is an advantage to birthing slightly more males when food is plentiful, and fewer males (or more females) when food is scarce. When it comes to reproduction, and slight statistical advantage will be subject to direct and powerful Darwinian selective pressures. So it is plausible from this perspective that there would be mechanisms to tweak the male-female ratio in response to the environmental situation.
This study is an excellent example of science at work. The initial study was well designed and executed but has limitations on what it can actually tell us. Scientists are in the process of debating the merits and implications of the study – but all agree that follow up experiments are necessary to answer the questions raised by these data. Meanwhile, there is also sharp debate about the plausibility and possible mechanisms of such an effect in the context of multiple disciplines within biology. And in the end, evidence will rule the day.
9 Responses to “Can a Mother’s Diet Affect the Sex of Her Child?”
Leave a Reply
You must be logged in to post a comment.
Interesting, but it needs to be remembered that in virtually all species where gender is determined genetically and where both genders have the same number of chromosomes (unlike ants and bees for example), the gender ratio is always pretty close to 50:50.
When the population is stable, the average male and the average female have the same number of offspring survive and reproduce. That number is 2. If the number is different than 2, then the population isn’t stable.
Once the gender ratio gets a little off of 50:50, then there is a strong evolutionary pressure for the less frequent gender to become more frequent (because if the population is remaining stable, then the less frequent gender is having more descendents than the more frequent gender).
If the ratio is 75:25, then the more frequent gender is having 1 descendent while the less frequent gender has 3 descendents. There is strong pressure for the traits of those with 3 descendents to become more abundant than the traits of those with 1 descendent. If there is any heritability to gender (and there kind of has to be if it is genetically determined), then the gender ratio will evolve back to 50:50.
We are seeing this happen in China and India where sons are favored over daughters. In some parts of China the gender ratio is such that there are millions more males than females. In those areas, a daughter is a better reproductive “bet” than a son because there are many males who will be unable to find mates.
What about that urban myth (or not) about X sperm swims faster, but Y sperm lives longer, so you can try to influence the gender of the baby by carefully timing your sex? (sex the verb, may get confusing in this discussion…)
Any truth to that claim? The explanation is plausible, but is there any evidence that X and Y sperm actually have these traits?
Nice post except would you like to take another shot at this sentence?
“What they found is that those women who has a higher calorie diet overall, and those who more ate at least one bowl of breakfast cereal per day, had a 24% greater chance of having a boy than those who skipped breakfast or consumed lower total calories.”
I read this story a few days ago. The way it was portrayed was ridiculous. The author didn’t take any time to explain the reality behind the situation. Thank you for taking a moment to cover this. I was curious.
dennisr – that’s a perfectly cromulent sentence
The perils of daily blogging – thanks, I fixed it.
Ah, you embiggened your post with that fix, Steve.
It’s an interesting story, for sure. I’m sure that there are cdesign proponentsists who are dying to ask “But how does evolution know how to steer reproduction during times of famine? Doesn’t it seem like it’s been guided by some, I don’t know, intelligence?” Where’s Ben Stein when you need him?
Steve and Steve:
Cartoon references? Niiiicccee
#
# petrucioon 28 Apr 2008 at 2:50 pm
What about that urban myth (or not) about X sperm swims faster, but Y sperm lives longer, so you can try to influence the gender of the baby by carefully timing your sex? (sex the verb, may get confusing in this discussion.)
Any truth to that claim? The explanation is plausible, but is there any evidence that X and Y sperm actually have these traits?
When I was studying pharmacy we had a physiology lecture that went along those lines. This website gives a brief rundown of my understanding of making boys and girls – http://wiki.answers.com/Q/Is_there_a_difference_in_how_long_XY-chromosome_sperm_for_boys_and_XX_sperm_for_girls_live
Does anyone know whether or not the study controlled for the timing of intercourse and ovulation etc.?
I heard something similar in animal studies and it was thought to be an evolutionary trait. Basically, if the body senses a famine state, then one more sex is likely than the other. I can’t remember which it was exactly, but it would make sense for the females to be produced more in a famine state since a male can impregnate several females. The limiting factors for humans in terms of number of offspring are females because a man can have an many more children than any female. However, maybe this study also shows that feast atmospheres, males will be produced more because this increases competition and strengthens the gene pool. In short, if times are hard, produce more females since few males are needed to produce offspring, but in a feast, try to improve the overall gene pool by increasing competition.