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With thanks to vantid, I've just watched Why We Get Fat: Adiposity 101 and the Alternative Hypothesis of Obesity. It's not a very good lecture: the speaker gabbles a bit, and spends fully half his time on his introduction, but it gets into interesting stuff at about fifty minutes in.

Anyway.

When you discuss obesity with anyone these days, the argument always seems to reduce to one simple principle. Expend more energy than you eat, and Thou Shalt Lose Weight. Which is undeniably true—it's the First Law of Thermodynamics, probably the most single fundamental law in physics. Nothing goes faster than light; toast always lands on the buttered side; and energy is conserved. When it comes to diet: you take in energy as food, you expend it as heat and effort, and any imbalance between the two is buffered in the body's fat reserves. Muster your willpower, eat less food, expend more energy, and you'll feel a bit uncomfortable but your weight will go down.

This is true insofar as it goes—I'm not about to start arguing with physical inevitabilities. But the trouble with this view is that it's desperately oversimplified. For a start, it implies that fat is a passive reservoir, quietly soaking up excess energy from the bloodstream and releasing it again on demand, like a sort of indefinitely rechargeable battery. But, as the lecturer in the video demonstrates, fat tissue isn't passive. It exists in a dynamic equilibrium of its own, continually seeking to sequester excess energy from the bloodstream, and releasing energy again if needed badly enough. The basic biology behind this process has been very well understood for decades, and the video goes into a modest amount of detail about the biochemistry of glucose, triglycerides, fatty acids, and the various hormones (chiefly insulin) that mediate them.

The chief point that the author makes, is that, under certain circumstances, fatty tissue can begin to sequester energy very aggressively. He demonstrates this from three different angles: biochemistry, animal models in laboratory rats, and studies of various poor communities where adults get fat while their children are malnourished. Under these conditions, where the body's fat is continually sucking energy out of the system, there are only two alternatives: either reduce activity to a minimum, or eat more to compensate. (There's that First Law of Thermodynamics again.)

In effect, he inverts the whole causality of obesity. People, he says, don't get fat because they're eating too much. They're eating too much because all their energy is going to fat.

A dear friend of mine, with whom I was discussing this, said something like, "It just comes down to willpower." And yes, ultimately it does: willpower and thermodynamics. If hunger-strikers can will themselves to die, then fat people can will themselves to be thin: eat less, and less and less, until finally even their tenacious fat cells begin to surrender the goods. But we could be talking a lot of willpower here.

As a relatively habitually mostly-thin person, with a small number of placidly compliant fat cells, it doesn't cost me much effort to lose weight; I don't have to apply much willpower to it. I just let myself get a bit hungry, and the weight recedes. That's why it's seductively easy for thin people to think fat people are just idle or weak-willed. But, as this video implies, it ain't like that. I've always believed that it's harder for my fat acquaintances to lose weight, but now it looks like it's profoundly, qualitatively harder. To lose weight, it appears, a fat person has to go through privations that, to me, would represent starvation. It seems unjust to expect them to go the whole Bobby Sands.

So I think I have another incentive not to let myself get any fatter than I already am.