Calorie deficitCaloric deficit · Energy deficit

A calorie deficit exists when energy intake is lower than total daily energy expenditure. The body makes up the shortfall from its own stores, predominantly fat if protein intake and resistance training are adequate, and body mass falls.

It matters because it is the one requirement common to every method that produces fat loss. Fasting windows, low-carb, high-protein and meal-timing approaches all work by making a deficit easier to sustain — none of them bypasses the requirement, and any that claims to is selling something.

In practice, the size of the deficit is a trade-off. Aggressive deficits lose weight faster but cost more lean mass, more hunger, worse training quality and a higher drop-out rate. A moderate deficit that you can hold for months usually beats a severe one you abandon in three weeks.