The Calorie Deficit Equation
Weight loss requires consuming fewer calories than your body burns. This is not a theory or one approach among many — it is a thermodynamic fact confirmed by every metabolic ward study ever conducted. The question is not whether calorie deficits work, but how to create one that is sustainable and does not cost you muscle.
Step 1: Calculate Your Maintenance Calories
Your Total Daily Energy Expenditure (TDEE) is the number of calories you burn per day. Use the Mifflin-St Jeor equation:
- Men: (10 x weight in kg) + (6.25 x height in cm) - (5 x age) + 5
- Women: (10 x weight in kg) + (6.25 x height in cm) - (5 x age) - 161
Multiply by your activity factor:
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Desk job, no exercise |
| Lightly active | 1.375 | Light exercise 1-3 days/week |
| Moderately active | 1.55 | Moderate exercise 3-5 days/week |
| Very active | 1.725 | Hard exercise 6-7 days/week |
Step 2: Choose Your Deficit Size
| Deficit | Weekly Loss | Best For | Sustainability |
|---|---|---|---|
| 250 cal/day | 0.5 lb/week | Already lean, minimal weight to lose | Very high |
| 500 cal/day | 1 lb/week | Most people, most situations | High |
| 750 cal/day | 1.5 lb/week | Significantly overweight | Moderate |
| 1000 cal/day | 2 lb/week | Obese, under medical supervision | Low |
Real-World Examples
Here are calculated targets for three common profiles:
- 30-year-old woman, 165 cm, 75 kg, lightly active: TDEE = ~1,870 cal. For 1 lb/week loss: eat 1,370 cal/day.
- 35-year-old man, 180 cm, 90 kg, moderately active: TDEE = ~2,650 cal. For 1 lb/week loss: eat 2,150 cal/day.
- 45-year-old woman, 160 cm, 85 kg, sedentary: TDEE = ~1,650 cal. For 0.5 lb/week loss: eat 1,400 cal/day.
Why You Should Not Go Below 1,200/1,500
Eating below 1,200 calories/day (women) or 1,500 calories/day (men) without medical supervision risks:
- Micronutrient deficiencies (vitamins, minerals)
- Significant muscle loss alongside fat loss
- Metabolic adaptation — your body reduces TDEE to compensate
- Hormonal disruption (thyroid, cortisol, sex hormones)
- Binge eating episodes from excessive restriction
Protein Protects Muscle During a Deficit
The single most important dietary factor during weight loss is protein intake. Research shows that consuming 0.7–1.0g of protein per pound of body weight during a calorie deficit preserves lean muscle mass, keeps you fuller, and increases the thermic effect of your food (you burn more calories digesting protein than carbs or fat). In a 1,500-calorie diet, aim for at least 100–120g of protein daily.