The evidence for this adaptive thermogenesis is scant and inconclusive. Even if it were true, we're talking about a difference of 100 kcal/day, which doesn't mean one can eat significantly less.

It doesn't make sense that our bodies would be capable of doing this. Evolutionary, if our bodies could do the same tasks for a lower energy expenditure, we'd all do that. But you simply can't keep a body at 37.5°C for half the calories