Energy balance

Spontaneous long-term weight gain triggers a metabolic adaptation where increases in 24-hour energy expenditure and fat oxidation are overcompensatory (greater than predicted by changes in body composition), acting to oppose further weight gain.

Your body does try to defend its weight when you gain it, by burning slightly more calories and fat. However, this defense is very weak—roughly equivalent to the calories in half an apple. This means that while your body is trying to help, it is easily overwhelmed by small increases in food intake. Understanding this helps explain why weight gain is easy to sustain and why willpower alone is difficult; the biological 'brakes' are weak.