[ForrestN]

My doctors have always said that losing weight too quickly results in the metabolization of one's muscles and other important, healthy tissue in addition to fat. Could it be that crash dieting works narrowly to solve, for example, diabetes via the loss of fat, but has other negative consequences for the body? Maybe it's not a "doesn't work" thing, but a "collateral damage" thing or a "makes it harder to keep the weight off" thing, which is another old saw of why you shouldn't lose weight too fast?

[nradov]

Weight loss always involves some loss of lean tissue in addition to fat. Dieters can mitigate this to an extent by protein intake high and maintaining an exercise program.

[radu_floricica]

And sleep. It's rarely mentioned, but the difference between good and bad sleep may mean the difference of 80/20 fat/muscle loss, and 20/80. If the numbers are true (I know of at least one study, but no meta stuff) it means that crash dieting with bad sleep is actually counter productive, as you lose a lot more muscle than fat.