Twins: blood, milk and society

Well...the title is a little creepy, but sums up the melange in this report, Study finds that a woman's chances of having twins can be modified by diet. But there is more than diet, researchers have long known that genetics plays a role in twinning rates, it is heritable in that some proportion of the population variation correlates with genotypic variation. And we also know that the rise of fertility medicine has resulted in a boom of multiple births in the modern world. Twinning then neatly encapsulates the manifold aspects of many phenomena of interest which exhibit a biological angle.

The researcher found that vegan women had 1/5 the twinning rates as non-vegan women. I don't have access to the original paper in The Journal of Reproductive Medicine, so I don't know how well variables were accounted for, but the magnitude of the difference seems great enough that given a decent sample size there is something there. Importantly, he also found that "The concentration of IGF in the blood is about 13 percent lower in vegan women than in women who consume dairy." IGF is a growth factor, and the article notes that the basal concentration of IGF is greatest in Africans, least in Asians and middling in Europeans, and this is exactly the rank order of fraternal twinning across the populations. So, not only do you have a suggestive correlation, but you have a suspect in this case for why that correlation might be cropping up. Also, northern Europeans have higher dizygotic twinning rates than southern Europeans. This is interesting because of course northern Europeans have a more dairy-rich diet, so the difference here might be accounted for by diet as opposed to genetics. One should also note that East Asians tend to be lactose intolerant, and traditionally do not consume dairy as adults, but sub-Saharan Africans also tend to be lactose intolerant, so it seems to be in this case that endogenous basal IGF levels may be implicated in this case (one could check the twinning rates of the Masai, who are lactose tolerant, vs. their Bantu neighbors who usually are not).

A final note, there is a locus, igf2, which is a major illustration of genomic imprinting in humans (it obviously has a role in the generation of IGF). Paternal copies tend to be expressed, and induce a larger offspring, and this is opposed by the action of another locus, igf2r. I don't know enough about molecular genetics, but there might be some epigentic process going on here where milk consumption is modulating these loci through some pathway.

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Heard an interview of the paper's main author on the BBC World. The paper evidently includes a comparison of twinning rates in the US and the UK. The rate of increase in the US has been faster than in the UK. Steinman suggests that this is consistent with the fact that Growth Hormone is used on cattle in the US but not in the UK - additional epidemiological evidence of causation.

It's important to note that the variability in twinning rates applies only to DZ twins. Thr MZ rate is pretty constant.

It's important to note that the variability in twinning rates applies only to DZ twins. Thr MZ rate is pretty constant.

yes, it does seem like something that should be noted. but why? (e.g., what is the importance?)

The answer is simple; for placental mammals in general, 'litter size' is under natural selection. Furthermore, the ability to change 'litter size' in response to environmental conditions is advantageous to the parents as it provides them a method by which only as many offspring as they can effectively take care of are born.

Let me explain in detail, though. Monozygotic Twinning is basically a biological accident. One egg and one sperm unite. But somewhere between that early fusion and later stages, what should be one collection of cells turns into two. If this happens early enough, both collections of cells become separate but identical zygotes. If it happens too late, you get no, or a malformed, baby.

In other words, monozygotic twinning isn't under any sort of biological control.

Dizygotic twinning, on the other hand, is. We forget this as humans because with the exception of the marmosets/tamarins, monkeys, apes, and humans are strictly one baby at a time animals. Of course, many prosimians produce litters, as do most other mammals outside the primate taxon. Producing litters is, in fact, the ancestral condition of placental mammals.

Since evolution is ultimately all about reproductive success, one could see why producing several offspring at a time is to one's advantage. However, a limiting factor is how much energy you can invest in the offspring. How much energy can you put into pregnancy, into lactation, into carrying, into provisioning? Animals thus have to strike the balance between their desire to make as many babies as possible with how much energy they can invest in each baby.

Anthropoid primates have chosen an extreme strategy of investing a large amount of energy on a severely reduced litter of a single offspring at a time. Many rodents, on the other hand, have chosen the strategy of large litters with minimal investment afterward. Such basal differences in reproduction (called the Intrinsic Rate of Increase) would probably be laid down in the genetic makeup of a given species. This probably explains why well-fed zoo monkeys and civilized humans alike still tend to produce only one baby at a time, even though they are clearly capable of raising at least two (and sometimes more) children in a given litter.

But beyond gross differences in Intrinsic Rate of Increase, it would also be to a given individual's benefit if they could further modulate their litter size from year to year. A dietary control mechanism (using IGF-1 as a signal) would be ideal in this respect. When resources are plentiful, IGF-1 levels rise, which make it more likely that a given oocyte would be fertilized and/or a zygote implant. In this way, within the litter size bounds laid down by basic species-level genomic characteristics, the number of offspring produced in a given cycle could decrease or increase as resources (and thus available energy) changed from year to year.

Decided not to post it at the blog after I wrote it up, so sorry for the length.

1) yes, dietary control is important. moderns forget that humans in the "wild" space births a lot, at least 4-5 years. in the plentitude of the modern situation we can (and sometimes) breed at 2-3 year intervals.

2) but it is interesting in the context of milk, since this is after all a novel non-"wild" situation. vegans in the USA aren't starved are they? this is a separate issue from caloric intake (though i'd have to see if the paper, if someone has access to this journal, could you email me the paper at contact -at- gnxp -dot- com?).

3) so there is, it seems, genetically controlled variation across populations. are the differences in dizygotic twinning rates large enough to suggest different reproductive strategies?

it seems to me that #1 is the ideal for humans in most situations. twins are not as healthy and fit as singletons, so it seems like twinning should be selected against (in the "wild" it seems that infanticide of at least one twin would often be obligate).

on point 2, actually you'd be surprised at what veganism (especially raw food veganism) does to the body. It's pretty ugly. They are VERY deprived when it comes to the EAAs and other areas of nutrition. I'm too lazy to dig through my grad shcool notes for sources and everything, but this was actually a big deal in Germany at one point because the children of raw food vegans (who were forced into the same diet) were stunted in growth, showed reduced intelligence, and were in general less healthy than vegetarian or omnivorous children. They were trying to determine if they should legislate against forcing children into such a diet by considering it child abuse/neglect.

As to whether it's just the effect of milk, I unfortunately couldn't say. For vegetarians a lot of the ill health effects of veganism are avoided because most get plenty of animal-type protein through both milk and eggs. I actually made my biggest strength gains while on a vegetarian diet. So it may very well be that it's correlated with intake of animal-derived protein rather than just milk.

Although I do know for a fact that milk is high in IGF-1, i'm pretty sure it'd be broken down during digestion and would have to be reconstituted in the body de novo, but am not certain of that. IGF-1 is also tightly linked to growth hormone production, IIRC (should know this since i just finished hormonal physiology two weeks ago, tells you how I did on finals, lol).

On point 3, if as I'd expect IGF-1 doesn't directly impact number of implanted zygotes, but rather simply makes it more likely that a given zygote would implant, than the dizygotic twinning is a whatchamacallit, consequence of hte increased likelihood of implantation period.

In other words the population-level differences in constitutive IGF-1 expression are more about the likelihood that a given fertilization event will result in pregnancy. NOthing more, nothing less.

We see this in several taxa, where variation in fertility itself is maintained across a species due to differences in habitat quality etc. Wolves I believe are an example. The further north, the bigger the prey and the lower the success rate. This lower success rate also correlates with a longer interbirth interval despite similar frequencies of copulation. Don't hold me to the wolves thing though, that was about 1.5 years ago that I researched that, using data from the 1970's.

*shrug*

Thorny problem, one that this paper alone can't explain. But it does provide tantalizing avenues of further research.

Hey Razib, you look a lot better in your current photo than any of the others. Perhaps it is the smile ...

By The Real Richa… (not verified) on 20 May 2006 #permalink

I have the notion from somewhere that yams and other vegetable sources, like soy products, contain vegetable hormones that mimic hormones that have impacts on fertility -- thus yams cause twins. No doubt an overstatement, but still, I believe there are studies.

Prairie Mary

Mary, don't know about twinning specifically, but you are absolutely right about certain vegetables having mimic hormones. They tend to be estrogens, FWIW.

vegetable hormones having a measurable effect on people is well known too. Soy in perimenopausal symptom relief...or male gynecomastia for that matter.