In my previous post about weight control I noted that there was a glaring inconsistency that is inherent in the argument of oversimplifiers who say that to lose weight, a dieter need only carefully make sure that he takes in fewer calories in the future than he takes in now at his current stable weight.
The glaring inconsistency: Before the diet, the dieter’s weight is stable (if too high) without any careful attention.
Oh sure, there are plenty of people whose weight is increasing if they don’t pay attention, who have to carefully control their calories so that they don’t gain. Most of these are people who used to be fatter. (I’m one.) But the argument is generally about someone whose weight is stable before they begin weight loss. And there are lots of those people. Fat ones and thin ones. I was one for years. My weight was the same, slightly over the overweight/obese boundary, before and after three pregnancies. Never budged more than a pound on either side, without really paying any attention to it. My husband is one too. He does not have to do anything conscious to keep his weight constant. It’s always the same healthy weight.
That’s the set point idea that Megan was blogging about. The evidence for it is simple: Lots of us stay the same without paying careful attention to our calorie intake. But get this — if the assumptions of the oversimplifiers were correct, that without exercise rate of energy burned is essentially constant, it ought to be much more difficult to remain at a constant weight than to gain or to lose. It ought to require near-constant vigilance to remain precisely balanced on the knife-edge of the equation (rate of energy burned = rate of energy intake). And yet, it is easy. The hard part is falling off, at least to one side.
Clearly there are nonlinearities here. Rate of energy burned is some function of rate of energy intake, or else both are functions of some third thing.
So take the rate of energy burned apart. Let’s turn our spherical dieter into a spherical exerciser, and start thinking less about mass/energy balance than about thermodynamics.
Rate of burning = power exerted by body + rate of heat dissipated from body
Did I miss anything here? I don’t think so, but this doesn’t seem to be the most useful partitioning of the energy balance. Let’s think like engineers instead of physicists.
Rate of burning = power exerted by acts of will + (power exerted + heat dissipated) involuntarily
The first term is a function of things like how many steps we take over the course of the day, how often we go to the gym, how fast we move while we do our housework, that sort of thing. Also how heavy we are, because a lot of that power is the power necessary to accelerate and decelerate our own bulk! To a great extent, we can increase and decrease that first term. By definition, there’s nothing we can do directly to influence the second term. It includes stuff like how much heat our body puts out while it’s breathing and digesting. And there is no basis for believing that it is constant. What if our body had a secret mechanism that tunes the second term down when the first term goes up? Then taking the stairs instead of the elevator wouldn’t help much now, will it?
I happen to think it’s more likely that our body’s mechanisms tend to tune the second term up when the first term goes up — we’re told that muscles burn energy at rest, so anything that encourages your body to build muscle (cf. picking up heavy stuff and putting it down again) ought to cause your body to generate more heat.
But I also think it likely that the second term depends on other things. Like the difference between calories taken in and calories excreted — that whole idea that "your metabolism slows down when you eat less." Or on what macronutrients you’re eating: why wouldn’t the fat/carbohydrate/protein/fiber ratios that go into your mouth affect your personal engine efficiency?
So let’s update the equation.
-d(weight of fat)/dt = (1 lb fat/3500 kCal)*(power exerted by acts of will + involuntary power + involuntary heat + energy excreted – energy consumed)
where power exerted by acts of will is a knowable function of what you do, energy consued is a knowable function of what you eat, and — here’s the tricky part — involuntary power, involuntary heat, and energy excreted are essentially unknowable functions of what you eat and what you do.
And that’s not even getting into the idea that (what you eat) and (what you do) are themselves rather complicated functions of each other, some involving feedback loops. Who’s got less willpower when she’s tired? Anyone? Who’s got experience with the "you can’t eat just one" aphorism, besides Frito-Lay and Alcoholics Anonymous? Hmm?
So. No, it’s not that simple. And so people who have trouble losing weight are not stupid gluttons. They’re struggling with a complicated controls problem.
bearing blog 