Measured versus estimated metabolic rate

An estimated metabolic rate comes from an equation that returns the average for people of your age, sex, height and weight. A measured rate comes from indirect calorimetry, which records your own oxygen use and carbon dioxide output at rest, after an overnight fast.

Almost every calorie target in existence comes from an equation. It is possible to measure the thing the equation is guessing at, in about ten minutes.

How accurate are metabolic rate calculators?

A predictive equation takes your age, sex, height and weight and returns the average resting energy expenditure of people who share those characteristics. That is a genuinely useful starting point and it is a population regression, which means it carries the variance of the population it was built on.

For someone near the center of that population it works acceptably. For anyone away from it — a lot of muscle, very little muscle, a large weight loss in the past, thyroid disease, advanced age — the estimate diverges from reality, and the divergence is not detectable without measuring.

What happens during a resting metabolic rate test?

You breathe normally for about ten minutes, at rest, after an overnight fast. Oxygen consumption and carbon dioxide production are recorded, and from those two figures the device calculates your resting energy expenditure and your respiratory quotient. Results are available within minutes.

What does a resting metabolic rate test measure?

Resting energy expenditure

The energy your body uses at rest, which is the large majority of what most people use in a day. This is the number a nutrition plan should be built on. It is also the number that changes as lean mass changes, which is why a target set at the start of a program may no longer be right six months in.

Respiratory quotient

The ratio of carbon dioxide produced to oxygen consumed. Because fat and carbohydrate are oxidized with different proportions of oxygen consumed to carbon dioxide released, this ratio indicates which fuel is predominantly being used at rest — lower for fat, higher for carbohydrate. It is a description of that moment under the recorded conditions, not a fixed trait, which is why the fasting requirement is absolute.

Read it against lean mass

Resting energy expenditure is produced overwhelmingly by lean tissue, so the absolute number means little on its own. A low value in someone with low muscle mass is a finding about muscle. That is why body composition is measured first.

How should you prepare for a metabolic rate test?

A recent meal, a cup of coffee, a cigarette or a brisk walk from the parking lot will all change the result. So will talking, dozing off or a leaking mask. The measurement is easy; getting the conditions right is the part that requires discipline. How to prepare.

What people usually find out

Most often, that the equation was wrong in one direction or the other, and that a plan built on the equation was therefore aimed at the wrong target. That is a more useful answer than either “you must be eating more than you think” or “your metabolism is broken,” and it is testable rather than assumed.

Frequently asked questions

Is a resting metabolic rate test worth it?

For most people, yes. The usual finding is that the equation behind their calorie target was wrong in one direction or the other, so the plan built on it was aimed at the wrong number. A measurement replaces both “you must be eating more than you think” and “your metabolism is broken” with an answer that can be tested rather than assumed.

What is a good resting metabolic rate?

There is no single good number, because resting energy expenditure is produced overwhelmingly by lean tissue. The absolute figure means little until it is read against your lean mass. A low value in someone with low muscle mass is a finding about muscle, not about metabolism, which is why body composition is measured first.

How accurate is a resting metabolic rate test?

It measures your own oxygen use and carbon dioxide output instead of guessing from age, sex, height and weight, but only if the conditions are right. A recent meal, a cup of coffee, a cigarette, a brisk walk, talking, dozing off or a leaking mask will all change the result, which is why the overnight fast is absolute.

How do I check my resting metabolic rate?

Measure it rather than calculate it. An online calculator runs an equation that returns the average for people of your age, sex, height and weight. Indirect calorimetry records your own oxygen use and carbon dioxide output while you breathe normally at rest for about ten minutes after an overnight fast. Results are available within minutes and are read against your body composition.