Measured versus estimated metabolic rate
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.
What a prediction equation is doing
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 indirect calorimetry does instead
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.
The two outputs
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.
Why the preparation is unforgiving
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.
Related reading
Medically reviewed by Dr. Gurpreet Singh Padda, MD, MBA, MHP, medical director of Measura. Last reviewed .