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The underweight brain

Okay, I give up. Dealing with the media’s breathless messes when reporting on science feels like an endless game of whack-a-mole, and I’d decided life is too short for that. But this one is so hysterical, it’s pulled me back in.

Obesity linked to stupidity, brain scans show shouts one headline. Obese People Have ‘Severe Brain Degeneration’ moans Yahoo News. Obesity Leads to Brain Degeneration says a copycat. I’m sure you’ve seen these things too.

Well, let’s plow through a small dose of the actual science because the neat thing about the truth is that it’ll set you free.

As one of my favorite bloggers, the AngryBlackBitch says, shall we?

Raji, Ho et al. Brain structure and obesity. Human Brain Mapping, 2009:


… [I]t is unknown whether these factors, specifically obesity and Type II diabetes, are associated with specific patterns of brain atrophy. We used tensor-based morphometry (TBM) to examine gray matter (GM) and white matter (WM) volume differences in 94 elderly subjects who remained cognitively normal for at least 5 years after their scan.

On a personal note, this made me feel all warm and fuzzy. I did some tensor-based morphometry as part of my Ph. D. work, so it was like meeting an old friend. It’s not a common technique, and it is a very sensitive way of measuring changes in shape.

Note, in particular, the phrase: who remained cognitively normal. This study had nothing — nothing — to do with intelligence or stupidity. They were not looking at that.

They were looking at changes in the volume of specific regions of the brain.

Changes in volume also do not relate to intelligence or stupidity. If size was that important, elephants would be rocket scientists. Size tells you about nothing except, well, size. It tells you how many cells occupy that volume, not how well-connected they are or what they’re connected about. Some birds, for instance, have large brains proportional to their weight. (For birds, that is.) That’s not because they’re thinking deep thoughts. It’s because flight requires enormously complex and rapid processing of balance and visual signals.

The abstract continues:

Bivariate analyses with corrections for multiple comparisons strongly linked body mass index (BMI), fasting plasma insulin (FPI) levels, and Type II Diabetes Mellitus (DM2) with atrophy in frontal, temporal, and subcortical brain regions. A multiple regression model, also correcting for multiple comparisons, revealed that BMI was still negatively correlated with brain atrophy (FDR <5%), while DM2 and FPI were no longer associated with any volume differences.

So, statistically, BMI had a strong correlation with certain changes in brain morphology. In two-way comparisons (bivariate analysis) other factors had some correlation, but in the multivariate case, which reflects our multifaceted reality more closely, BMI was the significant correlate.

(I know that in fat-positive writing, BMI is said to be bunk. Statistically, it is not bunk because it allows you to look at weight distribution in a population while taking height into account. This is a lot less bogus than trying to do a study that looks only at weight and ignores height, which is the other alternative given the data usually available. The main confounding factor in some populations is that highly muscular people have high BMIs, but that’s not a big issue in a study of elderly people.)

In an Analysis of Covariance (ANCOVA) model controlling for age, gender, and race, obese subjects with a high BMI (BMI > 30) showed atrophy in the frontal lobes, anterior cingulate gyrus, hippocampus, and thalamus compared with individuals with a normal BMI (18.5-25). Overweight subjects (BMI: 25-30) had atrophy in the basal ganglia and corona radiata of the WM. Overall brain volume did not differ between overweight and obese persons.

This is fascinating. Different regions were affected based on weight class. The first set are all gray matter, the second set are two regions of white matter that, between them, have a big role in connecting the first set. The fact that overall brain volume didn’t differ between the two groups means . . . what? That medium weight affects certain white matter, which then, as weight increases, affects the specific gray matter but the white matter recovers? That different physiological consequences operate at different weight levels, and affect the brain in unrelated ways?

Higher BMI was associated with lower brain volumes in overweight and obese elderly subjects. Obesity is therefore associated with detectable brain volume deficits in cognitively normal elderly subjects.

So, what does it all mean? It says nothing about intelligence. I hope I’ve made that clear. Nor does it say anything about “degeneration.” Think about the analogous case of muscles, with which we’re more familiar. A person with less muscle mass doesn’t have muscle degeneration. They just have fewer muscle cells. Depending on how they use those cells, they may be making lace.

What it does mean is that there are fewer cells “in the bank,” so to speak. The brain is a very adaptable organ, even in old age it turns out. After damage, such as from a small stroke, it’s capable of routing around the problem and eventually learning to use other cells to do the same task. But to do that, those other cells have to be available. The fewer there are, the higher the likelihood the brain can’t recover from increasingly minor damage.

The younger the person, the less this matters, because the natural loss of cells with age hasn’t yet had much of an effect. The older the person, the more it means greater susceptibility to anything that does cause problems for the brain. It still does not mean there necessarily will be problems. It just increases susceptibility because there’s less in reserve.

That’s not great news for fat people, but neither does it have anything to do with instant cretinism. However, that boring reality doesn’t sell any ad space.

brain, obesity, Thompson