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. A pound of fat is the equivalent of around 3600 Calories (that Calories with a capital “C” which denotes that it’s really a kilocalorie from physics, in case you want to do a careful accounting of how much energy you’re burning). So if you change your intake vs output by just 120 Calories a day, so that you burn more than you consume, you’d expect to lose a pound in a month. But there are two problems with that: 1) It’s too slow for most people (psychology again) and 2) there’s the fine print. We burn calories just by being alive; there’s some basic motion, involuntary muscle movement, etc. A sedentary lifestyle will have you burn about 12 Calories per pound. If you weigh 200 lbs, then you should burn about 12 x 200 = 2400 Calories a day. Now, you try to lose your pound a month, more or less, by averaging only 2280 Calories a day. After a while — more than a year — you’re closing in on 190 lbs, and you’ve gotten discouraged because you stopped losing weight. Now lets do the math again: 12 x 190 = 2280. Wait a minute! That’s what the intake is, that’s supposed to be our key to a thinner me! But no, that’s the catch — your caloric demands are weight-dependent. What you actually need to do is assess your calorie intake in terms of your target weight. A constant calorie intake or extra exercise will give you a decaying-exponential reduction to a new weight. Go back to the old intake/exercise, and you get the inverted form of that change, i.e. the weight goes up, leveling off at a new value. I think one of the main problems is that many people view a diet as something that is temporary, but it can’t be looked at that way. Your weight is dependent on your calorie intake and your burn rate. Energy is conserved.