[hellodevnull]

One of the reasons people regain is that they've gone on diets with a calorie intake that is too low (or lacking in protein) and experienced muscle atrophy rather than fat loss. A decrease in muscle mass also decreases their metabolism, which means their BMR is now lower, but because they don't know, when they return to their seemingly normal calorie intake, they'll quickly put the weight they lost back on.

[billmalarky]

Muscle loss from calorie restriction is much less than most believe (marginal until you get around sub 10% bodyfat). Same goes for metabolism slowdown. You will lose strength on a calorie deficit, but the muscle tissue remains.

Google "starvation mode myth"

See also this military study where men on extreme calorie deficits and high activity levels did not lose lean muscle tissue until hitting extremely low (sub 5%) bodyfat levels.

http://jap.physiology.org/content/88/5/1820

[graeme]

What about people who don't have much resistance activity? That's the more typical case.

[dragonwriter]

That's a factor, but even if you just lose fat, you calorie expenditure from the same activity pattern, ceteris paribus, will be reduced (because you are carrying less weight), so if you return to the eating pattern which at which you were stable at the higher weight (assuming no other systemic/metabolic changes, just the fat loss) you will gain weight asymptotically approaching the the weight at which that "normal" calorie intake had you stable before.