So, I've witnessed a lot of discussions about obesity in my day (I, uh, might be spending too much time on the internet). Some of these discussions have been very nuanced and largely reasonable, while others have been...not so much those things. But no matter what the level of discourse is, it is an inviolable truth of the universe that at some point during the debate, something like the following must be said:
"Weight loss is just a matter of calories in versus calories out. If you expend more calories than you consume, you'll lose weight. It's simple thermodynamics."
-Random made-up internet person, who is my foil for the dayThat these talismanic words be invoked by someone is, for all intents and purposes, an ironclad rule - sooner would the actual laws of thermodynamics be violated than they not be said somewhere along the line.
And it annoys me. It annoys me not because the statement is false (it's true, of course - well, I would rather say that mass in versus mass out actually determines weight loss, but that's quibbling). No, it annoys me precisely because it's true. The fact that it's true means that people can keep saying it in debates and feel justified in doing so, when in fact it's a profoundly useless truth.
Let me explain why I think so.
(Oh, and I should probably emphasize here that being a borderline-unhealthily-skinny person, these debates have an abstract/academic quality to me that they may not have to other people. So: thin-privilege alert or whatever)
So as I said, the caloric-balance model of weight loss is true. In fact it's trivially true (or at least the mass-balance version is trivially true, but mass and energy are pretty fungible in the body, so that's fine). But true doesn't mean useful, and the problem starts when people assume that caloric intake and output are easily controllable.
The classic caloric-balance argument would go something like this: the body takes in a certain number of calories per day, and expends a certain number of calories per day. When excess calories are consumed (that is, more calories are consumed than expended), these excess calories are stored in the body as fat or muscle mass. We know these excess calories have to be stored in the body somewhere, because feces and urine contain almost no calories, and something something conservation of energy. So, you should gain weight if you take in more calories than you put out, because the extra calories stay in your body, and the extra calories have weight. Similarly, if a person expends more calories than they consume in a day, then again that energy has to come from somewhere, and that somewhere has to be energy stores in the body. These energy stores, consisting of either fat or muscle, have mass. Therefore, if you burn more calories than you take in, you'll end up using some of your energy stores, and so you have to lose mass. There is no other way you could have a calorie deficit in your diet, short of violating conversation of energy (which most people are for some reason reluctant to do).
So, okay. Huh. When you put it that way, it actually sounds pretty airtight. Consuming less calories than you expend really should lead to weight loss. Why would anyone ever object to such a sensible-sounding statement?
The classic rebuttal to this argument would go something like this: the body is extremely complicated. Certainly someone can decide to lower their caloric intake below the level of their normal caloric output. However, if they do that, there's no guarantee that their caloric output will stay the same. People are not in control of their metabolism, for instance. When faced with very low intake of calories, the body can decide to lower its metabolism (read: make you feel extremely tired), which will cause you to expend less calories than you otherwise would have. This might foil your plan to consume fewer calories than you expend. Or, alternatively, since the body is in control of your hunger sense, it's entirely capable of making you feel very hungry. Unbearably hungry, in fact. So hungry that you wind up eating enough to bring you back to a calorie-neutral diet (or even an excess-calorie diet). The point is: assuming perfect control over one's intake and output of calories, the energy/mass-balance point of view would make sense, and yes, a calorie deficit would lead to weight loss. However, biochemically speaking caloric intake and output are not under our control, and indeed to control them would require almost superhuman restraint. Therefore, whatever the logical merits of the calories in/calories out model, it's simply not a useful way of thinking when we take into account human psychology/physiology - people just don't have the willpower required to guarantee weight loss in this manner.
That's the classic rebuttal to the caloric-balance argument.
I'm not going to give the classic rebuttal, though. I'm going to do something different. As a physicist, I like thought experiments, so I'm going to try one of those. Let's forget about willpower limitations entirely. In fact, let's imagine a person with literally perfect willpower. Whatever they decide to do, they can do (up to the physical limits imposed by their body). So if they decide they want to...I don't know, stop themselves from peeing, they can easily do so up until the point where their bladder explodes.
Now let's say this person decides to adopt a diet with a calorie deficit. Hunger pangs and tiredness be damned, they eat their watercress and complete their daily jog. They take in 1500 calories per day, say, and put out 2000. What will happen to such a person?
This isn't a trick question. I'm literally asking what you would expect to happen in this situation.
Okay, so I usually try to avoid asking leading questions, but I hope most people (including those who accept the calorie in/calorie out viewpoint) will agree: our hypothetical person in this case should lose weight. It would be super weird if they didn't: willpower normally limits someone from achieving a negative-calorie diet in the first place, but assuming that they could, surely they would lose weight, right?
Are you ready for the punchline?
This isn't a thought experiment.
I refer you to the case of Michael Edelman (#4 on the linked page - and NSFW, I guess? Depending on your workplace's position on extremely obese people?)
Michael Edelman was about 1200 pounds or so at his heaviest. That is...fairly heavy. Because of his extreme obesity (and the death of another, also extremely obese, friend) he ended up developing a severe fear of eating. This severe fear of eating - while debilitating for Michael - is convenient for us, because we will use it as a stand-in for extreme willpower. As a result of his fear, Michael didn't want to eat, no matter how hungry he felt. He wound up, not surprisingly, adopting a negative-calorie diet. And because of his (tragic) phobia, he was able to stick to that diet in spite of the pain it caused him.
So, dear readers: what do you suppose happened to him? This is our thought experiment brought to life - a person with essentially perfect willpower, choosing to consume fewer calories than he expends. Place your bets: did he lose all of his extra weight, or did he instead wind up going off the diet? Which gave in: the irresistible force or the immovable object?
Neither. He starved to death at 600 pounds.
Let me repeat that:
At 600 pounds.
Did anyone predict that? I for one didn't. And I definitely don't think the calorie in/calorie out people did, because I seem to remember them saying something about a negative-calorie diet, properly maintained, inevitably leading to weight loss - not leading to death by starvation while plenty of calories remained in the body to be made use of. So that's kind of weird.
What went wrong with the seemingly airtight caloric-balance argument? How can you have a deficit of calories and still not lose weight?
As I see it, the problem was with that sneaky word, you. Sure, assuming that you want a you to keep existing, you better either lose mass or take in as many calories as you put out, lest you violate the laws of thermodynamics. But the laws of thermodynamics are also perfectly happy with you outputting no calories - that is, with you being dead. This scenario completely satisfies conservation of energy as well. There is certainly no law of thermodynamics that says that your cells have to release any energy they have stored in order to prevent your death. They can just not release the energy.
But of course, the point here isn't that most people should avoid a negative-calorie diet for fear of starving to death. That's ridiculous. Most people don't have a phobia of eating, so most people don't have the willpower required to starve to death while still obese. Most people facing that level of hunger will just start eating, and either gain or not lose weight.
No, the point is this: people generally think that it's willpower that limits someone from losing weight. That if the person in question could just put up with a little hunger, a little suffering, they would shed pounds easily. But Michael Edelman straight up disproved this - he suffered through as much hunger as a person can possibly suffer through, and still died while obese. He showed that the body, if sufficiently messed up (biochemically speaking), does not have to release its stored energy. He showed that, in the limit of infinite willpower, a negative calorie diet does not have to lead to weight loss.
Instead, it can just lead to death.
Be honest here: did this surprise you? Would you have thought that a person could starve to death while being extremely obese? I don't think most caloric-balance people would have predicted this. I think most caloric-balance people would have predicted (rather strongly) that he would either go off the diet or lose weight - not starve to death. And when your model predicts something strongly, and it turns out to be falsified, it's usually time to update your model.
So maybe you should stop saying "It's all a matter of calories in, calories out."
Maybe you should instead look for more nuance.
Maybe you should start saying, "Damnit. Obesity is hard."