Joel S comments

The genetic argument doesn’t hold water: how many of our grandparents were obese? Not many, and they had the same genes.

We search endlessly for a cause for obesity when it is common for an adult man in America, whose caloric requirement (to maintain the same weight with a sedentary lifestyle) might be 2000 or 2500, to be ingesting 4000 or more per day.

There is no mystery: a person who is ingesting only their maintenance number of calories per day will not be obese. Our caloric intake, combined with sedentary lifestyles, explains the obesity epidemic entirely. It’s not sugar, per se, and it’s not fats, per se.

I appreciate Joel bringing these issues up. I never know whether discussing these basics will insult my readers or not.

On genetics: How is it possible that obesity can be strongly genetic if our grandparents weren’t obese?

The nerdy argument is that while the incidence of obesity has risen over time the variability in obesity attributable to genetics has actually remained constant. This is one of the remarkable stylized facts that a Theory of Obesity would need to explain.

In more common terms it works like this. In our grandparent’s day very few people were obese but a few were overweight and in general some were pudgier than others. Well, it turns out that if you carry the genes that produced overweightness in our grandparents time then you are almost certainly obese today. If you carry the genes that produced pudigness in our grandparent’s day then you are very likely overweight today.

The general level of fatness is rising but the relative fatness of people in the population is as genetically determined as ever. Moreover, genetics has a lot to say.

There is a very interesting parallel with height. In the same way as obesity, height has been increasing over time. Yet, height is strongly genetic. Indeed, height seems to be only slightly more genetic than obesity.

Even more fascinating, the rise in height slowed down just as the rise in obesity was speeding up. If you look at the rise in body mass you actually see a smooth trend that extends at least back to the early 20th century. However, for the first part of that trend people were getting heavier because they were getting taller. Now they are getting heavier because they are getting fatter.

One radical hypothesis that I have toyed with is that obesity is the result of improving nutrition. In short, we see that height, intelligence and obesity are rising over time at similair growth rates. We think the first two are due to increases in nutrition. Is it possible the same is true for the third?

Its clear that being obese is undesirable in the modern world but its not immediately clear that its less evolutionarily maladaptive than being 6’2” or having an IQ of 135.  In all cases you have resources going to create tissue that probably wouldn’t have conferred much of an advantage in our evolutionary past.

I don’t endorse this hypothesis, but I think its important to keep even radical suggestions in mind.

On Calories: Don’t we just eat more and exercise less?

We definitely eat more. Its not clear that we exercise less or that sedentary behavior can explain anything. In fact households that make their living doing manual labor are more likely to be obese than those who make their living as professionals.

We do eat more though. Doesn’t that explain it?


The question is why do we eat more. Eating behavior in all animals is actually fairly tightly regulated by numerous feedback loops. Most animals do not “watch what they eat” and most do not get obese. At least not those who are not genetically prone to do so.

More importantly humans in the 1950s and 1960s were mostly normal weight yet most of them were not on tightly regulated diets. Most thin people today probably could not tell you how many calories they ingested each day nor how many they expended. This regulation is carried on subconsciously by hormone mechanisms through out the body.

The regulation mechanism is also highly fine tuned. In order to stay within one pound of your current weight over a year, calories-in have to match calorie-out with 99.7% accuracy. The calorie testing equipment that determines what goes on food labels is not even that accurate, so it seems implausible that people are achieving this through conscious equation of calories-in with calories-out.

So, for thousands of years the regulation mechanism worked despite people living in environments that were quite different from our hunter-gatherer past. However, in the past 30 years it has broken down completely. That begs for an explanation.

For the nerdier, I would also suggest that the sudden breakdown is why I am skeptical of “many independent causes of the obesity epidemic” theories. Why should it be the case that all of sudden lots of independent forces came together all to breakdown the calorie regulation mechanism in the same direction?

That is, if we were seeing just as much spontaneous anorexia as spontaneous obesity then maybe I could buy the “things just went haywire” hypothesis. However, it looks like something particular went wrong with the down-regulation mechanism of calorie management. This was probably a single cause or at least a single complex of causes. Not independent events.

There is a lot more to say but this is enough for one post.