Obesity and IQ
Previously, I intimated that the connection between obesity and cardiovascular disease and death from such diseases may not be what is commonly believed:
I don’t dispute that overweight and obese people are in poorer health on average, and perhaps do have shorter lives. However, I don’t think it’s at all clear that this is caused by the obesity. It is quite likely, given what we know, that the poorer health results from a higher mutational burden in the overweight (i.e. genetic load), which contributes to both the obesity and the poor health.
In any case, impact from obesity on say cardiovascular health fairly small, less than a factor of two, which, for a wholly correlation result is pretty weak.
The idea is that people who are in poorer health have all around poorer genetic quality – in other words, they tend to have higher levels of genetic load (see here at West Hunter, or my HBD Fundamentals page for some discussion on genetic load). And thanks to pleiotropy – that is, genes that impact multiple systems in the body – we tend to see a lot of bad things going together.
Possibly evidencing that in the case of obesity is the connection between obesity and low IQ. It is commonly known that poorer (i.e., stupider) people tend to be fatter. It is commonly believed that this stems from things associated with low IQ, like poor impulse control and low future-time orientation, and indeed those things likely play a role. But it’s telling to see just how stark the relationship is.
Satoshi Kanazawa (who, despite the removal of his Big Think page, still very much publishes; see his website) did just that. Looking at a nationally representative primarily White British sample (n just under 10,000 in the last waves, 97% White), he found that IQ measured in childhood excellently predicts obesity at age 51:
We see a steady drop in average body mass index (BMI) as IQ rises.
This really does seem to turn the conventional wisdom on its head. We know that low-IQ individuals are in poorer health and live shorter lives than do higher IQ individuals. Hence, one cannot help but wonder if the associations between obesity and poor health and cardiovascular risks stems entirely from low IQ.
The association between health and IQ is quite strong, and there is evidence that suggests that it stems not just from the fact that high-IQ individuals take better care of themselves. Higher-IQ individuals just appear to be all around “healthier”, on average, than their lower-IQ counterparts.
IQ is a huge target for genetic load, thanks to the number of genes involved in the brain. Most of the within-population variance in IQ likely stems from overall levels of these deleterious mutations. An individual with a large amount of genetic load is likely to demonstrate fitness deficits in a wide variety of systems. We see this with physical attractiveness, as there is a positive relationship between IQ and attractiveness. We also see this between IQ and height, as taller people are more intelligent on average. Indeed, a recent model indicated that pleiotropy is likely in good part responsible for the connection between height and IQ.
In fact, it has been said that – at least within a population – there aren’t so much “genes for intelligence” as there are “genes for stupidity“. That is, much of variation in IQ stems from these deleterious mutations, each one individually rare and of typically weak effect, but collectively powerful to the hapless soul that happens to have lot of them.
Obesity appears to be somewhat similar. Many of the identified genes that appear to influence obesity seem to operate in the brain, perhaps indicative of the pleiotropic nature of the genetic roots of both low IQ and obesity. (Edit: See this paper by Tyler, Asselbergs, Williams, and Moore (2009) for a technical discussion on the nature of pleiotropy and epistasis.)
Hence, we may be looking at the wrong place when looking at the somewhat poorer outcomes of obese people, and indeed might be foolish to assume that these are outcomes are everybody’s problem; they are primarily a low-IQ problem. I proposed a test that – while not being definitive – might shed some light on this:
A simple test would be [to] compare the health of obese people to thinner people controlling for IQ (a good proxy for genetic load). I’d wager that the ill effects are significantly attenuated.
This would determine which of the two factors – obesity or IQ – had the stronger “impact” (remember, this would be correlational) on negative health outcomes. If it turned out that it was IQ, then it would be much harder to seriously assume that obese people would live longer if they simply lost weight. I look forward to such a study.
Edit: (I forgot to add this when I wrote the post) Kanazawa also noted the same in his paper:
One alternative explanation for the significantly negative association between childhood intelligence and obesity is the ‘‘general fitness factor’’ approach (26), which suggests that both higher intelligence and health (including the ability to stay within the normal weight range) reflect underlying genetic quality. In this context, it is interesting to note that general intelligence is significantly positively associated with BMI at Age 7 … and at Age 11 … and it is not correlated with BMI at Age 16 … In other words, general intelligence is significantly negatively associated with BMI only in adulthood … when individuals have complete control over what to eat and not to eat
The study randomly assigned 5,145 overweight or obese people with Type 2 diabetes to either a rigorous diet and exercise regimen or to sessions in which they got general health information. The diet involved 1,200 to 1,500 calories a day for those weighing less than 250 pounds and 1,500 to 1,800 calories a day for those weighing more. The exercise program was at least 175 minutes a week of moderate exercise.
But 11 years after the study began, researchers concluded it was futile to continue — the two groups had nearly identical rates of heart attacks, strokes and cardiovascular deaths.
But the outcome is clear, said Dr. David Nathan, a principal investigator and director of the Diabetes Center at Massachusetts General Hospital. “We have to have an adult conversation about this,” he said. “This was a negative result.”
This strongly supports an earlier proposition of mine, that assuming that promoting diet and exercise will ameliorate cardiovascular risk is foolhardy. At least in this group, it did not. Sure, one could argue that these patients were already diabetic, and hence maybe the damage was already done, but, I’m not holding my breath on this one.
In the end, the problems of obesity and cardiovascular disease may just be something we just need to learn to live with – a serenity moment.
Edit, 2/11/15: See also this study by Kanazawa (Intelligence and obesity: which way does the causal direction go?, 2014), which reviews other studies which replicate the findings of this one.