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Coach Sheet

Glossary entry — nutrition

Basal Metabolic Rate BMR

The calories your body burns at rest to keep basic life functions running. Mostly determined by lean body mass, age, and sex. 

10 × weight_kg + 6.25 × height_cm − 5 × age + s (Mifflin-St Jeor)

What BMR measures

BMR is the energy your body burns at complete rest. To keep your heart pumping, brain running, and cells operating. It excludes movement, digestion, and exercise. For a typical adult, BMR accounts for 60-75% of total daily calorie burn.

BMR is mostly a function of lean body mass (the more muscle and organs, the more cells doing metabolic work) plus age (slows ~1% per decade after 25) and sex (males average ~5% higher than females at same lean mass due to androgenic factors).

The standard formula

Mifflin-St Jeor (1990) is the most accurate equation across NHANES populations:

Male:   BMR = 10 × weight_kg + 6.25 × height_cm − 5 × age + 5
Female: BMR = 10 × weight_kg + 6.25 × height_cm − 5 × age − 161

It outperforms the older Harris-Benedict equation by 2-5% in modern populations. ACSM and most working sports nutritionists adopted it as the practical default in the 2000s.

Worked example

A 28-year-old male, 178 cm, 68 kg:

BMR = 10 × 68 + 6.25 × 178 − 5 × 28 + 5
    = 680 + 1112.5 − 140 + 5
    = 1657.5 kcal/day

Round to 1658. This is the floor. What the body burns just keeping the lights on.

When Mifflin-St Jeor underestimates or overestimates

The formula is calibrated to populations of average lean body mass. It systematically misses for:

  • Above 30% body fat: overestimates BMR by 5-10% because the formula weights total body mass, but fat tissue is metabolically inactive. Use Katch-McArdle (370 + 21.6 × LBM_kg) instead.
  • Highly trained athletes: underestimates by 5% or so because their lean mass exceeds the population average. Bump activity multiplier one tier or use Katch-McArdle.
  • Pregnant or breastfeeding women, recovery from significant illness: not applicable. Defer to clinical guidance.
  • Very short or tall stature: works but gets noisier at the extremes.

How BMR fits into TDEE

TDEE (Total Daily Energy Expenditure) is BMR multiplied by an activity multiplier reflecting how much you move during the day:

TDEE = BMR × Activity Multiplier

Activity multipliers range from 1.20 (sedentary) to 1.90 (very active). The TDEE calculator handles the full lookup.

Why BMR matters for coaching

It’s the floor for any nutrition prescription. You can’t put a client into a cut that drops calories below BMR for sustained periods without metabolic adaptation, hormonal disruption, and loss of lean mass. Most aggressive cuts target a 20-25% deficit from TDEE, which still keeps calories well above BMR.

For a client with BMR 1,658 and TDEE 2,570:

  • Maintenance: 2,570 kcal
  • Cut (-20%): 2,056 kcal (still 25% above BMR. Safe)
  • Aggressive cut (-30%): 1,799 kcal (only 8% above BMR. Caution territory)
  • Crash diet (-40%): 1,542 kcal (below BMR. Don’t prescribe this)

Common confusion

“My BMR feels low, can I increase it?” Slightly. Adding lean mass via training raises BMR proportionally. A 5 kg lean mass increase adds ~100-150 kcal/day to BMR. Mostly, BMR is what it is.

“BMR varies day to day.” Yes, ±3-5% based on sleep, stress, recent meal composition, menstrual phase. The number from a formula is a population-average estimate, not a daily reading.

“DEXA measured my BMR at X. Is that more accurate?” DEXA measures body composition (lean mass, fat mass), not BMR directly. BMR is measured via indirect calorimetry (analysing oxygen consumption at rest), which most facilities don’t offer. Formula estimates are within 5% of calorimetry measurements, which is good enough for prescription work.

In Coach Sheet

The Profile tab takes Age, Sex, Height, Weight once. The Quick Stats tab below auto-calculates BMR via Mifflin-St Jeor with the formula visible in a hover tooltip. When weight changes, BMR updates in real time. The same BMR is the input to TDEE, the macro prescription, and the energy balance projection chart.

Sources

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