Calories in = Calories out?

There is variability in the response to overfeeding. Commenting on a seminal series of experiments published in the 1990s by Canadian researchers Claude Bouchard and Angelo Tremblay, Parker-Pope writes:

That experimental binge should have translated into a weight gain of roughly 24 pounds (based on 3,500 calories to a pound). But some gained less than 10 pounds, while others gained as much as 29 pounds.

Note that eating a pound’s worth of calories typically led to something like a half-pound of weight gain; this shows that weight increases lead to energy expenditure increases. This was in a study in which the subjects were prevented from exercising. Likely the weight gain would have been generally lower if the subjects had been free to move as they wished.