Basal Metabolic Rate (Mifflin-St Jeor)

Mark Mifflin and Sachiko St Jeor published this in 1990 after measuring resting energy expenditure by indirect calorimetry in 498 healthy adults, half of them with obesity, specifically because the 1919 Harris-Benedict equations were drifting high on modern bodies. The result is a clean linear fit: ten times the mass in kilograms, plus 6.25 times the height in centimetres, minus five times the age in years, plus 5 for men or minus 161 for women. An 80 kg man of 180 cm aged 30 gets $800 + 1125 - 150 + 5 = 1780$ kcal/day.

What this number is not is your daily calorie requirement. Basal metabolic rate is what you burn lying still and awake in a thermoneutral room, having not eaten. Total daily expenditure is this figure multiplied by an activity factor, usually between 1.2 for sedentary and 1.9 for very active, and that multiplier is a much cruder guess than the BMR itself. Treating the BMR as an intake target is the classic mistake and produces a deficit far larger than intended.

The fit is the best of the common ones, and it is still a fit. Validation studies put it within 10 percent of measured expenditure for roughly 70 to 80 percent of people, which means one person in four or five sits outside that band, and the residual error is systematic in ways the equation cannot see. Body composition is the big one: the equation knows mass but not how much of it is muscle, and lean tissue burns several times more than fat at rest, so two people with identical inputs can genuinely differ by 200 kcal a day. Thyroid disease, some medications, and the metabolic adaptation that follows sustained weight loss all shift it further.