Most dieters are so concerned about second-order effects, such as daily fluctuations in weight and changes in metabolism, that they lose track of the first law of thermodynamics: conservation of energy.

Want to lose a pound of fat? You can work it off by hiking to the top of a 2,500-story building. Or by running 60 miles. Or by spending 7 hours cleaning animal stalls. […]

Exercise is a very difficult way to lose weight. Here’s a rule of thumb: exercise very hard for one hour (swimming, running, or racquetball)– and you’ll lose about one ounce of fat […]

If you run for an hour, you’ll lose that ounce of fat and also a pound or two of water. By the next day, when you’ve replenished the water, you might think, “the weight came right back!” But you’d be wrong—you really did lose an ounce. It is hard to notice, unless you keep running every day for a month or more, and don’t reward yourself after each run with a cookie.

There is a much easier way to lose weight, as we can learn from the first law of thermodynamics. Eat less.

The rest can be found at The MIT Technology Review : The Physics Diet