[ajuc]

Well one obvious adaptation is losing fat = less energy needed to move the body.

[csee]

We saw a very large reduction in caloric expenditure in people who were already very fit runners. So body weight changes over the course of the experiment are unlikely to explain it.

[magicalhippo]

So if they just kept doing the experiment they would end up not needing to eat?

Clearly there are some details that matter here...

[necovek]

And in most places in the article calorie expenditure is taken as "corrected for no-fat weight".