Basal Metabolic Rate (BMR)
The energy required to keep you alive. We compare the Mifflin-St Jeor, Harris-Benedict, and Cunningham equations for metabolic tracking.
The Baseline of Energy Expenditure
Basal Metabolic Rate (BMR) is the amount of energy (calories) your body requires to perform its most basic life-sustaining functions while at complete rest. This includes breathing, circulation, nutrient processing, and cellular production.
BMR accounts for roughly 60% to 75% of your Total Daily Energy Expenditure (TDEE). If you want to lose fat or build muscle, you must calculate an accurate baseline.
The Standard: Mifflin-St Jeor
Introduced in 1990, the Mifflin-St Jeor equation replaced the older Harris-Benedict formula (which tended to overestimate BMR by about 5%). It relies on total body weight, height, and age.
- Men: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161
Pros: It is the most accurate formula for the general population. Cons: It does not account for body composition. It treats 100kg of fat and 100kg of muscle as the exact same metabolic tissue, which is completely false.
The Athlete's Choice: Cunningham Equation
Muscle tissue is metabolically active. Adipose tissue is essentially metabolically inert. If you are a heavily muscled athlete, Mifflin-St Jeor will heavily underpredict your BMR.
The Cunningham equation (1980) requires you to know your Lean Body Mass (LBM), making it far superior for athletes.
- BMR = 500 + (22 × LBM in kg)
If a 90kg bodybuilder has 10% body fat (81kg LBM), their Cunningham BMR is ~2282 kcal. If a sedentary 90kg man has 35% body fat (58.5kg LBM), his Cunningham BMR is ~1787 kcal. Mifflin-St Jeor would assign them the exact same number.
Total Daily Energy Expenditure (TDEE)
BMR is only your baseline. To find how many calories you burn in a day, you must multiply your BMR by an Activity Multiplier (PAL - Physical Activity Level).
- 1.2: Sedentary (desk job, no exercise)
- 1.375: Lightly active (light exercise 1-3 days/week)
- 1.55: Moderately active (moderate exercise 3-5 days/week)
- 1.725: Very active (hard exercise 6-7 days/week)
- 1.9: Extra active (very hard physical labor or two-a-day training)
Warning: The most common mistake in nutrition planning is overestimating the activity multiplier. Lifting weights for 45 minutes a day does not make you "Very Active" if you sit at a desk for the other 23 hours.