Energy balance

Caloric restriction induces a metabolic adaptation (reduction in energy expenditure greater than expected from loss of fat-free and fat mass) and a behavioral adaptation (reduction in physical activity) that together reduce total daily energy expenditure, thereby creating resistance to further weight loss and promoting weight regain.

When you restrict calories, your body fights back by lowering your resting metabolism and making you move less (fidgeting, posture, steps). This is a biological survival mechanism, not a personal failure. To maintain weight loss, you must account for this drop in energy expenditure. Combining diet with exercise may help prevent this metabolic slowdown, but you must also be aware that your natural daily activity levels may drop unconsciously as you lose weight.