Part of the problem is what scientists refer to as ‘compensation’.

There is no doubt exercise burns calories, but at the kind of level most of us work out, it also stimulates hunger.

The more moderate activity we do, the more we eat, effectively cancelling out the weight loss benefits.

Linia Patel, a sports dietician with the British Dietetic Association, says it’s a misnomer that exercise is a green light to increase the calories.

‘Appetite often soars when you exercise a lot,’ Patel says. ‘You need to make sure you  eat enough, but stop before you get to the point where you aren’t burning excess energy off.’

Your body is not a machine; it has complex feedback loops. This is why the "calories-in calories-out" hypothesis is naive, at best. Via Why going to the gym can make you FAT | Mail Online.