How the Mifflin–St Jeor Formula Works
The Mifflin–St Jeor equation, published in 1990 and validated across multiple populations, is the gold standard for estimating Basal Metabolic Rate (BMR) — the number of calories your body burns at complete rest. It outperforms the older Harris-Benedict equation by approximately 5% accuracy in both normal and overweight individuals.
Male BMR
(10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
Female BMR
(10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
Your TDEE (Total Daily Energy Expenditure) = BMR × Activity Multiplier. The macronutrient split then distributes those calories into protein, carbohydrates, and fats based on your goal.
Activity Multipliers Explained
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | × 1.2 | Desk job, little or no deliberate exercise |
| Light | × 1.375 | Light exercise 1–3 days per week |
| Moderate | × 1.55 | Moderate exercise 3–5 days per week |
| Active | × 1.725 | Hard exercise or sports 6–7 days per week |
| Very Active | × 1.9 | Physical labour job AND heavy training |
Macro Splits by Goal
| Goal | Calorie Adjustment | Protein | Carbs | Fat |
|---|---|---|---|---|
| Fat loss | −15–20% deficit | 35% | 35% | 30% |
| Maintenance / recomposition | TDEE | 30% | 40% | 30% |
| Muscle building | +10–15% surplus | 30% | 45% | 25% |
Visualising a Fat Loss Macro Split
Protein (35%)
Preserves muscle during a deficit. Target 1.6–2.2g per kg of body weight.
Carbohydrates (35%)
Primary fuel for training. Prioritise complex sources: oats, rice, sweet potato.
Fat (30%)
Essential for hormonal health. Never drop below 20% of calories.
Protein: The Non-Negotiable Macro
Protein is unique among macronutrients — it has a significantly higher thermic effect (25–30% of calories burned in digestion vs 6–8% for carbs and 2–3% for fat), builds and preserves lean muscle mass, and is the most satiating macronutrient per calorie.
| Goal | Protein Target | Evidence Basis |
|---|---|---|
| General health maintenance | 0.8g / kg body weight | WHO minimum — insufficient for active people |
| Body recomposition | 1.6–2.0g / kg | Morton et al. 2018 meta-analysis of 49 RCTs |
| Fat loss (muscle preservation) | 2.0–2.4g / kg | Higher target recommended when in caloric deficit |
| Muscle building | 1.6–2.2g / kg | No benefit observed beyond 2.2g/kg in most studies |
| Older adults (65+) | 1.2–1.6g / kg | Higher needs due to anabolic resistance |
Common Macro Counting Mistakes
Ignoring protein quality
Not all protein is equal. Leucine content, DIAAS score, and digestibility matter. Animal proteins and soy consistently outperform other plant sources for muscle protein synthesis.
Dropping fat too low
Fat below 20% of calories disrupts testosterone, oestrogen, and the absorption of fat-soluble vitamins (A, D, E, K). Low-fat diets impair hormonal health.
Eating back all exercise calories
Fitness trackers overestimate calorie burn by 30–100%. Eating back all exercise calories can eliminate a caloric deficit entirely.
Not accounting for cooking methods
Food scales should weigh raw ingredients. Cooked weight varies significantly based on water lost during preparation.
Neglecting fibre
Fibre is a carbohydrate that is poorly tracked but critical — it feeds the gut microbiome, regulates blood sugar, and improves satiety. Target 25–38g daily.