Boys and girls may get dfferent breast milk(27 Posts)
This is so strange,
Milk composition differs based on a baby's sex and a mother's wealth
Mother's milk may be the first food, but it is not created equal. In humans and other mammals, researchers have found that milk composition changes depending on the infant's gender and on whether conditions are good or bad. Understanding those differences can give scientists insights into human evolution.
Researchers at Michigan State University and other institutions found that among 72 mothers in rural Kenya, women with sons generally gave richer milk (2.8 percent fat compared with 0.6 percent for daughters). Poor women, however, favored daughters with creamier milk (2.6 versus 2.3 percent). These findings, published in the American Journal of Physical Anthropology in September, echo previous work that showed milk composition varying with infant gender in gray seals and red deer and with infant gender and the mother's condition in rhesus macaques. The new study also follows findings that affluent, well-nourished moms in Massachusetts produced more energy-dense milk for male infants.
Together the studies provide support for a 40-year-old theory in evolutionary biology. The Trivers-Willard hypothesis states that natural selection favors parental investment in daughters when times are hard and in sons when times are easy. The imbalance should be greatest in polygamous societies, in which men can father offspring with multiple wives, such as the Kenyan villages. In those societies, a son can grow to be a strong, popular male with many wives and children, or he can end up with neither. Well-off parents who can afford to invest in sons should do so because their gamble could give them many grandchildren. Conversely, poor parents should not heavily invest in sons because it is unlikely to pay offtheir offspring start at the bottom of the socioeconomic ladder. For those families, daughters are a safer bet because as long as they survive to adulthood, they are likely to produce young.
The new study is exciting and enthralling, says Robert Trivers, an evolutionary biologist at Rutgers University and co-author of the hypothesis, who was not involved in the recent work. It is a Trivers-Willard effect I wouldn't have the guts to predict.
Even beyond fat and protein, other milk components might vary in humans, says Katie Hinde, an assistant professor in human evolutionary biology at Harvard University. She has found higher levels of cortisol, a hormone that regulates metabolism, in rhesus macaque milk for male infants. Her work shows that milk differences could change infant behavior and might affect growth and development. Only half the story is what the mom's producing, Hinde says. The other [half] is how the infant uses the milk. These findings could have implications for formula, which could be tweaked to optimize development for both boys and girls.
So what happens when the breast feeding is shared between more than one woman and different children.
What happens with boy/girl twins?
Do toddlers put on weight during tandem nursing? I would have thought newborn milk to really fattening?
Yes milk reverts to newborn composition with tandem feeding. Anecdotally I've heard that a lot of toddlers wean themselves when their mother has another baby since the milk changes and they don't like it anymore. This is also why you can't really feed someone else's newborn, for instance, if you are feeding an older baby. I mean, you can, but they won't be getting newborn milk. The whole topic of wet nursing is fascinating.
Rosa I always thought milk differed as the baby aged which is why you can't bank milk when your baby is six months plus (?). So I assume with tandem feeding the milk reverts to newborn composition. But I don't know.
I'm a bit thrown by the causation hypothesis tbh. I thought there were stats available (going back forever) which showed a significant likelihood of gender based on father's (or parents, can't remember) employment? I may have dreamt this...
The study doesn't look at women in subsequent pg, does it? So it doesn't take into account an individual woman's propensity for type of dairy production, or whether this varies by gender.
I'm be willing to bet that it was more likely that maternal diet linked to social position influenced both the gender and milk composition, rather than the gender affecting the milk composition.
But it did tickle me that yonder Mr Trivers was a fan. imagine that!
Fascinating. Must have a proper read.
Also, what would happen when you are tandem feeding? Say your older child is a boy but new baby is a girl. I wonder what would happen then?
I found this bit a bit odd:
"Together the studies provide support for a 40-year-old theory in evolutionary biology. The Trivers-Willard hypothesis states that natural selection favors parental investment in daughters when times are hard and in sons when times are easy. The imbalance should be greatest in polygamous societies, in which men can father offspring with multiple wives, such as the Kenyan villages. In those societies, a son can grow to be a strong, popular male with many wives and children, or he can end up with neither. Well-off parents who can afford to invest in sons should do so because their gamble could give them many grandchildren. Conversely, poor parents should not heavily invest in sons because it is unlikely to pay offtheir offspring start at the bottom of the socioeconomic ladder. For those families, daughters are a safer bet because as long as they survive to adulthood, they are likely to produce young."
I can see that milk composition could be affected by diet, and at a push the body might know the sex of the baby and adjust composition accordingly. But for the human body to respond to the prevailing family construction in the society seems rather a push, to say the least. And doesn't seem to make sense - wouldn't the immediate family situation have more of an impact than society norms? And actually that little passage throws up a whole load of ideas which just seem odd and unlikely.
Did they think to test the milk composition of the same women for first and subsequent children? Were they making sure they tested at the same feed every day for each woman? did they take into account different cultural / individual patterns of sleep & feeding? And so on.
2.6 versus 2.3... That is tiny. 2.8 versus 0.6 is bigger, but still.
When you combine this will all the other factors which influence babies, children, adolescents, and ultimately adultsanother their lifetime, can this ever be a deciding factor?
It's sort of like the recent alcohol consumption in pregnancy findings in relation to IQ. Assuming you even agree that IQ is a good measure of intelligence - what difference will a couple of IQ points make, really, in the general scheme of things?
It's not as if IQ is the only way to get ahead in life - when you take social skills, emotional intelligence, charisma, hard graft, etc - into account. And even if it was the only indicator (which is so clearly isn't), then what difference is 2 IQ points going to have on a person's outcomes? Is it actually going to affect the education they have? The degree they get? Their profession? Their income? To any statistically significant degree? Hardly.
Same with this. Or at least, that's my gut feeling.
Personally, I have hopefully offset DD's 'disadvantage', as I fed her for 16 months, compared with 13 months for DS. Plus she was EBF for near to 7 months compared with his 6. None of those decisions made deliberately to be in favour of one or the other, mind.
How very interesting.
(Also, maytheoddsbe, I am pregnant, and now I am wondering if my cereal eating will have won over my DH's cycling in the making a girl or a boy task).
And what about exact baby age?
<happily designs studies for some other poor person to do>
I'd also love to know if birth order has any effect on milk composition.
And was the milk expressed for analysis at a consistent time of day? Or tested at all times of the day?
That makes sense doctrine ... so does the body have a way to change it if you feed other people's babies? Or if you move from a high-income context to a low-income one sometime during pregnancy?
And I would love to know how it works with twins.
It may well be I'm asking daft questions with so small a study but I would really like to work it all out, as may says, with a bigger group.
My hunch would be that perhaps the way women are treated has an impact as well as the gender of the baby, but I would like to know.
I thought breast milk was around 4% fat, same as cows milk, so those figures seem strange. Interesting but I'm not sure how relevant or useful it is. I've also heard of a study which showed that giving high calorie supplements to women in Ghana made no difference to milk content
The body will "know" the gender of the baby because of the hormonal balance in the womb, I think.
But was it corrected for maternal diet eg does the woman get a greater proportion of the family's meat/grain if the child is whatever is deemed to be the desirable gender in specific economic circs.
And yes, those are very small studies.
I can't say I've tried it, but it does link in with other studies such as the one that showed women who ate cereal for breakfast have more boys and women who are on very low calorie diets or who have lost lots of weight have more girls, I think studies have also found the age of both parents to be relevant as well as things like cyclists with lower sperm quality having more girls
But as you say I'd love to see more studies done with much more information and not starting off with a biased assumption. Some of the studies into gender have control groups of twenty or thirty- what bloody good is that?!
Goodness! I can't imagine that being terribly effective.
This is the sort of thing I find so fascinating but also so frustrating, because I would love to really understand how it all happens, and what the different pressures are that lead to these differences, and so often it seems the reports in the media aren't very exciting because they're assuming from the outset that any research is simply confirming natural differences between the sexes.
The Travers Willard theory is being used by people to try and select the sex of their babies, I don't know how effective it is but it is becoming popular. The theory goes that the best circumstances ie young fit mother, very fertile,well nourished in a stable relationship etc etc lead to more boys being born and in woman who are in less ideal circumstances ( biologically speaking) more girls are born
It's very interesting
Oh, that's a pity - I didn't realize it was such a small study. I would love to see results with a bigger one but I suppose there must be ethical issues, if you realize a child is being malnourished or the mother is not getting enough food.
sweet - I wondered that!
Very intersting but raises loads of questions. For example what happens in the case of boy/girl twins?
So rich/poor, girl/boy - 18 mothers (roughly) per grouping. I want to see their error bars, distributions, standard deviations within each group compared to differences between means, results of significance testing... I've just been discussing with colleagues a set of results where we got one signal with a sample size of 20 (in each of control and test case) and the opposite signal with a sample size of 40 - neither of which was statistically significant.
It is odd, but so interesting, isn't it?
I'd hope this sort of thing could help people work out how to prevent disadvantage to girls, maybe work out more about the causes of baby boys' vulnerability, and so on - it could be really useful (or am I naive? Not a scientist!).
Weird study. I wonder how the body knows whether it is breastfeeding a boy or a girl.
I admit that I breastfed my son longer than my daughter. Developmentally they were very different. Ds was much slower developmentally and was weaned at 2 and half years old. DD self weaned at 22 months.
Prehaps girl babies and boy babies have different appertites and the milk adapts appriopiately. Or maybe there are cultural influences affecting how responsive a mother is to her baby. Girls are often favoured in the first world, but boys are often favoured in the third world.
Btw - I don't know if it is depressing for girls?
Because aren't baby boys slightly more vulnerable to neonatal mortality, and we don't know why? I wonder how all of this is connected.
Do we know why?
I mean, are women with sons getting access to better food, or something? How does her body know it's a boy? I'm really interested by this.
Does anyone remember reading something - I'm sure it was linked to here - saying that there are dozens of things in breastmilk that we still don't even know the function of? It was saying a woman's breastmilk is very specific to her, which I also found fascinating.
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