Women who eat diverse, nutritious food are more likely to give birth to boys.
Yeah, yeah, we know sex is determined by the dad-to-be’s sperm but it looks like the maternal environment differentially influences the “viability” of male and female embryos. Yeah, blame mom again. According to Fiona Matthews and colleagues, it’s what she’s eating (or not eating) around the time of conception. The researchers from University of Oxford and the University of Exeter published the results of their study in an online report from the Proceedings of the Royal Society B: Biological Sciences.
So what we have here are 740 women recruited around the 14th week of pregnancy. The moms-to-be, who didn’t know the gender of their babies, reported what they’d eaten for the last year before they got pregnant, then kept a food diary through early pregnancy, then reported their later pregnancy diets around 28 weeks. I know, I can’t remember what I ate yesterday except for the two bags of M&Ms (they were almonds- protein at least) that I probably would not like to report. But, perhaps not perfect, food diaries and retrospective food questionnaires (what did you eat last year? over the last decade?) are very common in epidemiological studies and have proved to be reliable and valid.
So, did momma’s diet seem to make a difference? Yes, but only during the year prior to conception. What moms reportedly ate during pregnancy did not predict gender. Women who ate nutrient-rich diets were more likely to have sons. And, I can see the commercials already – eating breakfast cereal also predicted gender. Women who ate at least one bowl a day were more likely to have a boy than those who ate one bowl or less per week. No other foods were significantly related to gender.
Why cereal, you ask? The researchers took breakfast cereal as a proxy for breakfast – reasoning that people generally eat cereal only at breakfast and that cereal is the standard British breakfast. Eating cereal, and thus, breakfast, produces higher glucose levels – and it is this glucose that somehow influences the viability of the embryo. There is some evidence high glucose levels favor male embryos. Which makes me wonder – what about data from diabetics? Do they have more sons than daughters? Anyone know? I’d be more confident in this leap if women had reported how often they ate breakfast. Or better yet, if researchers measured their glucose levels from time to time, say in the morning, afternoon, and evening. I know, it fluctates – but still.
By the way, women’s weight and BMI were not significant factors – so it doesn’t seem to matter how much or how little women eat – it’s the total nutritional value of her food choices.
I do wonder if the the year-long preconception time frame could be shortened – if in fact it’s only a month or two before conception that really matters. It looks like the outcome measures varied in length- “preconception” diet included a whole year, the “early pregnancy,” maybe 4 months, and the “later pregnancy,” maybe 3 to 4 months. The investigators probably asked about the whole year in general, not segregated into separate months and this makes sense. So it would be impossible to get at this issue from this data. But let’s face it, most women probably don’t change their eating patterns much. But I suspect some women, who may be trying to get pregnant might eat better either to enhance their odds or for the health of their babies. So we can’t be sure that the year before conception diet looks similar to the one a month or two before conception. This study just begs for replication.
Now all this may sound strange at first, but there’s evidence other mammals with plenty of good food give birth to boys at a higher rate. It also makes sense from an evolutionary perspective that reproductive biology evolved to allow some parental control over offspring sex. We humans spend an inordinate amount of time and effort raising a relatively small number of offspring. Boys are in some ways a riskier investment when the goal is to get your genes into future generations. A small, weak male may not yield many if any offspring if he has to compete with the more robust, larger males who can have many. Yet, a sickly female, has a better chance of having a baby than her male counterpart. Why? Think about it.
An interesting point raised by the researchers – maternal diet may also help explain the decreasing male birth rate in the U. S. and other industrialized countries. I’ve written about the slight but significant decline in the XY babes before – one explanation being the increase in exposures to toxins that somehow harm male embryos more. Yet here’s another explanation. Take the U.S where we women and girls receive regular messages to be thin (“lose 10 pounds by summer”, “lose all your belly fat”) and let’s assume, many women diet. So, the more women diet, the more they skip breakfast, the more girls we have. Okay, so women who diet have more girls. Healthier-eating women, boys. Are we women, as a nation, always dieting? Of course, it’s also possible many of us are consuming lots of empty calories thereby reinforcing the stereotypical American diet of fast food, frozen dinners, and Big Gulps. This works with the findings too – all those empty calories translate into a disadvantage for boys.
Makes me wonder, what about the malnourished women in Africa? What’s the birth rate there? Makes me also want some further research – perhaps a replication here in the U.S. where we might get info on whether the women actually ate breakfast – the breakfast cereal may just be an artifact. Of course, it could also be about the all the added vitamins and minerals in cereals.
Judging by the flood of responses to Tara Parker-Pope’s post about the study on her Well blog at The New York Times – I’d say the issue strikes a chord with people. The discussion features a diverse mix of responses from mini-lectures on evolutionary biology and fertility to skinny moms who had all sons to conspiracy theorists (cereal companies rigged the study) and skeptics (all studies are flawed, no results can be trusted!). No wonder, it touches on evolution, the unborn, and motherhood.
I’m also struck by the number of people who either refute or accept the findings based on their own experience – if a skinny mom who never ate breakfast had 5 sons, she’s a skeptic. If a woman ate health food and power shakes and had 5 sons, she’s a believer. Many people seem to forget basic statistics – in general, over a population, this country, for example, women who eat better will be more likely to give birth to sons. Doesn’t mean all women eating well, doesn’t mean you will or might have a boy if you’re downing fruits, veggies, and brown rice. But if we had to make a guess about the gender of say, 1000 babies born to women we knew were healthy eaters, we’d be correct more of the time if every time we guessed boy.
You can read the study free online.
Mathews, F., Johnson, P.J., Neil, A. (2008). You are what your mother eats: evidence for maternal preconception diet influencing foetal sex in humans. Proceedings of the Royal Society B: Biological Sciences, -1(-1), -1–1. DOI: 10.1098/rspb.2008.0105