A worked example
A 30-year-old woman, 170 cm and 75 kg, has a BMR of about 1,500 kcal a day by Mifflin-St Jeor, and Harris-Benedict puts it at 1,540 kcal. Add a mostly seated day and she burns around 1,800 kcal; with a moderately active one, about 2,330 kcal.
What BMR is, and what it isn’t
BMR, basal metabolic rate, is the energy your body uses lying still and awake: breathing, circulation, keeping warm, running the brain. For most people it’s 60 to 70% of what they burn in a day.
It isn’t your daily burn. Walking about, working, training and even digesting food come on top, which is what the rows under “With your day added” show. For a calorie target, that daily figure is the one to start from.
Mifflin-St Jeor, Harris-Benedict and Katch-McArdle
Mifflin-St Jeor, from 1990, is the formula dietitians use most, and the one Ampsu uses. The revised Harris-Benedict equations are older and usually come out a little higher. In a review of the common formulas, Mifflin-St Jeor was the one most often within 10% of a measured rate.
Katch-McArdle works from lean mass instead of weight, height and sex, so it needs a body fat figure. It tends to do better for muscular people, whom the other two underestimate.
| Example | Mifflin-St Jeor | Harris-Benedict | Katch-McArdle |
|---|---|---|---|
| Woman, 30, 165 cm, 60 kg, 28% body fat | 1,320 | 1,380 | 1,300 |
| Woman, 55, 165 cm, 75 kg, 36% body fat | 1,350 | 1,410 | 1,410 |
| Man, 30, 180 cm, 75 kg, 16% body fat | 1,730 | 1,790 | 1,730 |
| Man, 55, 180 cm, 90 kg, 25% body fat | 1,760 | 1,850 | 1,830 |
Can you eat below your BMR?
Plenty of people on a calorie target do, for a while, because BMR is only part of what you burn. A target under your BMR isn’t a problem in itself; what counts is the gap to your whole day’s burn, and whether you can keep it up. Ampsu never sets a target under 1200 kcal a day.
Questions people ask
Is BMR the same as RMR?
Nearly. BMR is measured under strict conditions, after a night’s fast and full rest. RMR, resting metabolic rate, is measured more loosely and comes out slightly higher. The formulas here estimate both about as well, and the two names are often used for the same thing.
Does BMR drop when you lose weight?
Yes, because there’s less of you to keep running. That’s one reason the same target works less well after a few months. Recalculate every 5 kg or so, or let Ampsu adjust it from your weigh-ins.
Why is my BMR different from a friend’s?
BMR follows size, muscle, age and sex. A bigger or more muscular body burns more at rest, and in these formulas BMR drops by about 5 kcal a day for every year of age.
Sources
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr, 1990
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated. Am J Clin Nutr, 1984
- Frankenfield D, et al. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults. J Am Diet Assoc, 2005