Last updated: 3 October 2026
Reviewed by Jordan Morley, FA Level 2 Football Coach
A calorie deficit occurs when energy intake is less than total daily energy expenditure (TDEE). The body responds by oxidising stored energy — primarily fat. Approximately 7,700 kcal deficit = 1 kg of fat lost. A 500 kcal/day deficit produces approximately 0.5 kg/week. Deficits above 750 kcal/day risk muscle loss, hormonal disruption and metabolic adaptation. The maximum sustainable fat loss rate is approximately 0.5-1% of bodyweight per week. Adequate protein (1.6-2.2g/kg) is essential to preserve muscle during a deficit.
- Energy intake < TDEE = fat loss
- ~7,700 kcal deficit per kg fat
- 500 kcal/day deficit = ~0.5kg/week
- Protein 1.6-2.2g/kg preserves muscle
In practice
A moderate calorie deficit of 300-500 calories per day (roughly 0.5-1% of bodyweight loss per week) is generally recommended over aggressive crash deficits, because very large deficits increase muscle loss, hormonal disruption and the likelihood of regaining the weight once normal eating resumes — slow, sustainable deficits preserve more muscle and are easier to maintain long-term.
FAQ
How big should my calorie deficit be?
A moderate deficit of 300-500 calories per day, producing roughly 0.5-1% bodyweight loss per week, is generally considered sustainable and preserves more muscle than aggressive crash deficits.
Why does too large a deficit cause muscle loss?
Severe, prolonged energy restriction signals the body to conserve energy by breaking down metabolically costly muscle tissue alongside fat, especially without adequate protein intake and resistance training.