What is the Mifflin-St Jeor equation, and how do you use it for clients?

BMR or RMR: what's the difference?

You'll see two terms for energy at rest. Basal metabolic rate (BMR) is the energy your body needs for basic life functions, measured under strict lab conditions: after an overnight fast, in complete physical and mental rest, in a neutral temperature. Resting energy expenditure (REE), also called resting metabolic rate (RMR), is the energy you burn at rest under looser, everyday conditions. It allows minor movement and only a short rest beforehand. The Mifflin-St Jeor equation was built from REE measurements, so we call its result REE here.

The formula

Weight goes in kilograms, height in centimetres and age in years. The only difference between the two versions is the constant at the end.

Men:    REE = (9.99 × weight) + (6.25 × height) − (4.92 × age) + 5
Women:  REE = (9.99 × weight) + (6.25 × height) − (4.92 × age) − 161

The answer is in kilocalories a day. Multiply by 4.184 if you work in kilojoules.

A worked example

Say your client is a 38-year-old woman who weighs 72 kg and is 165 cm tall. She works at a desk and trains three times a week.

REE = (9.99 × 72) + (6.25 × 165) − (4.92 × 38) − 161
    = 719.28 + 1031.25 − 186.96 − 161
    = 1403 kcal a day (rounded)

That's her energy use at rest. She doesn't lie still all day, so the next step is an activity factor.

From REE to a daily target

Total daily energy expenditure (TDEE) is REE multiplied by a physical activity level (PAL). A PAL of 1.2 fits someone seated nearly all day. Someone on their feet much of the day, or a desk worker who trains most days, sits nearer 1.7 or 1.8.

For our client, a moderate PAL of 1.76 gives about 2470 kcal a day to hold her weight. If she wants to lose around 0.5 kg a week, you'd set her target roughly 500 kcal below that, at about 1970 kcal.

Then split the target into protein, carbohydrate and fat. How to calculate macros for a client walks through that step.

Why most practitioners start with Mifflin-St Jeor

A 2005 systematic review in the Journal of the American Dietetic Association compared the common prediction equations against measured resting metabolic rate. Mifflin-St Jeor came out as the most reliable for healthy adults, both at a healthy weight and with obesity. The older Harris-Benedict equation tends to read high for many people today.

It's still an estimate. Any equation can be out by 10% or more for an individual client, and it can't see a very muscular build, an illness or a medication that changes metabolism.

• Use Mifflin-St Jeor as the default for healthy adults when you don't know body fat.
• Use Katch-McArdle or Cunningham when you have a reliable body fat measurement, especially for lean, muscular clients.
• Use measured data (indirect calorimetry) when it's available, or adjust the target from the client's weight trend after two to four weeks.

There is more on choosing between them in Mifflin-St Jeor, Harris-Benedict or Katch-McArdle.