Energy deficit
When calorie expenditure exceeds intake, forcing the body to draw on its reserves (fat, glycogen). It is the basic principle behind all weight loss.
An energy deficit (or calorie deficit) is the engine of weight loss: spending more than you take in forces the body to mobilise its reserves. A deficit of around 500 kcal a day classically aims for ~0.5 kg of loss a week.
The original angle (the deficit is not linear): the common mistake is to believe that a constant deficit produces constant loss. In reality, the body adapts
- Adaptive thermogenesis: basal metabolic rate drops (-10 to -20%) to save energy
- Hormones: leptin falls (less fullness), ghrelin rises (more hunger), the thyroid slows down
- Behaviour: you unconsciously move less (less spontaneous fidgeting, reduced NEAT)
The result: weight loss slows, then stalls (a “plateau”). This is not a failure of willpower but a biological defence by the body.
How to optimise a deficit:
- Moderate rather than aggressive: too large a deficit speeds up adaptation, muscle loss and regain. A gentle deficit is more sustainable
- High protein: preserves muscle (you lose fat, not muscle) and keeps you full
- Strength training: maintains muscle mass and metabolism
- Diet breaks: periods at maintenance interspersed with dieting limit metabolic adaptation
The importance of quality: for the same deficit, a diet of whole foods (filling and rich in micronutrients) makes the deficit “easier” to stick to than a diet of ultra-processed foods (which leave you hungry). The calorie deficit remains the basic principle, but how you create it determines how sustainable it is.