The physics of weight loss is quite simple. It’s the first law of thermodynamics: Energy is conserved. Our body stores energy as fat, which has mass and thus weight, when we’re in the presence of gravity, as I assume we all are. Burning more calories as you consume results in an overall trend of less weight, if the two are equal the weight stays the same, and more calories in adds to the weight. Now, this discounts fluctuations you may have, due to things such as how much water (zero calories, but still has mass) or food is currently being processed by your body, so this is something that needs to be measured over a longer time frame than a day or two.

That’s all there is to it. Eat less and/or exercise more and you’ll tip the balance, as it were, toward losing weight.

But it’s never quite that simple in practice, now is it? There was a recent article in the NY Times, “In the Fatosphere, Big Is In, or at Least Accepted” in which the author states (about some overweight people): “And they reject a core belief that many Americans, including overweight ones, hold dear: that all a fat person needs to do to be thin is exercise more and eat less.” Well, they key here is a subtle distinction — “be thin” rather than “lose weight.” I agree that not everyone is going to be thin — hell, I’m never going to be thin, but that’s a separate issue. “Thin” is an ideal that doesn’t necessarily equate with “not overweight;” it’s a comparison of a physical dimension, a size and shape, compared with a mass. But the “core belief” is not a belief, it’s science. Eat less and exercise more, and you will lose weight. The barriers to that are largely psychological, it seems to me (what motivates you eat, or eat too much, and what prevents you from prioritizing exercise, and there’s a lot that can be written about that. And I’m sure a lot has been).

But there’s a little more physics, or at least math, that confounds the issue. Continue reading →