Choosing a BMR Equation: Mifflin-St Jeor vs Katch-McArdle

The three equations at a glance

Mifflin-St Jeor: the default, and why

The Frankenfield 2005 systematic review (Journal of the American Dietetic Association) found Mifflin-St Jeor predicted resting metabolic rate within 10% of measured values more often than any competing equation, and the Academy of Nutrition and Dietetics still names it the default clinical equation when indirect calorimetry is unavailable. In that review it was accurate — within ±10% — for about 82% of non-obese and 70% of obese adults, versus roughly 69% and 64% for Harris-Benedict. That is strong, guideline-backed evidence for the general adult population.

The equation itself needs only weight, height, age and sex: BMR = (9.99 × weight in kg) + (6.25 × height in cm) − (4.92 × age in years) + s, where s is +5 for men and −161 for women. The result is the basal rate, before any activity multiplier is applied.

Unless there is a specific reason to switch — a known body composition, or a validation concern at BMI extremes — Mifflin-St Jeor should remain the starting point.

Harris-Benedict: legacy, and its bias

The original Harris-Benedict equation dates to 1919 and tends to over-estimate BMR by 5–15% in modern sedentary adults; the revised (Roza-Shizgal, 1984) form narrows but does not eliminate the gap. It remains useful mainly for comparability with older records — Mifflin-St Jeor is the more accurate general-population choice.

Katch-McArdle: when body composition is known

Katch-McArdle (mathematically the Cunningham form) predicts resting energy from lean body mass alone. Because it ignores fat mass, it is more accurate for lean and muscular individuals whom weight-based equations tend to under-feed. A 2023 systematic review and meta-analysis in athletes (O’Neill et al.) placed the lean-mass-based Cunningham equation among the group showing no significant bias versus measured RMR — but the single most accurate equation there was a weight/height/age model (Ten-Haaf), and the authors emphasise matching the equation to a population of similar athletes rather than assuming a lean-mass model always wins. So treat lean-mass prediction as a strong option when body composition is known, not a guaranteed upgrade.

The trade-off is an extra input: it needs a body-fat percentage (from DXA, BIA, or a validated skinfold/circumference estimate) to derive lean mass. Treat it as the “body-composition known” upgrade rather than a replacement default — and note that the same body-fat input also unlocks fat-free-mass-based protein dosing.

Where Mifflin-St Jeor degrades

Two caveats are worth surfacing in a clinical workflow. In severe obesity (BMI ~35–40 and above), MSJ predicts within 10% only about half to 58% of the time and can mis-estimate in either direction — measured or indirect calorimetry is preferable where available. And in some young, high-body-fat cohorts, predictive equations including MSJ tend to over-estimate resting metabolic rate. MSJ was also derived largely on white US adults, so ethnicity-stratified accuracy is thinner.

The activity multiplier is the bigger error source

BMR equations agree within a few percent of one another. The activity multiplier — the ×1.2 (sedentary) to ×1.9 (very active) factor applied to reach TDEE — is coarser, self-reported, and only loosely validated against doubly-labeled water. It swamps the difference between equations.

People routinely over-report activity, and moving one category (say, from lightly to moderately active) adds roughly 200–400 kcal/day. Physical activity level is a continuum, not four buckets. The practical fix is presentation: treat TDEE as an estimate with a band, and — where real weight is tracked over time — reconcile the estimate against observed weight change rather than trusting a single number.

A worked example: does the equation or the activity factor move the number more?

Take a 40-year-old woman, 70 kg, 165 cm. Mifflin-St Jeor puts her BMR at about 1,370 kcal/day; the revised Harris-Benedict puts it at about 1,430 — a gap of roughly 60 kcal, or ~4%. That is the entire cost of choosing the “wrong” equation.

Now apply the activity multiplier. At sedentary (×1.2) her Mifflin-St Jeor TDEE is about 1,640 kcal/day; bumping her one category to lightly active (×1.375) lifts it to about 1,890 — a ~250 kcal jump, roughly four times the difference between the two equations. Each further category step adds another ~200–250 kcal.

The lesson holds across inputs: the activity factor, not the equation, is where a total-energy estimate is won or lost. Pin the activity level honestly, present a band, and correct against observed weight trend.