How Resting Metabolic Rate Is Measured

The test

Resting metabolic rate is measured by indirect calorimetry: breathing into a metabolic analyser at rest while oxygen consumption and carbon dioxide production are measured, and energy expenditure is calculated from them. The result is calories per day at complete rest.

Conditions

• Fasted, in the morning
• Rested and still for a period before the measurement begins
• No caffeine, no exercise beforehand
• Same facility and same equipment across the year where possible

Why it matters here

The standing concern with any sustained reduction in energy intake is metabolic adaptation — the body spending less at rest. Repeating RMR across the year is the direct test of whether that happens under a three-day-a-week fasting pattern, rather than assuming it either does or does not.

Interpretation limits

RMR measurements have real day-to-day variability, and a difference of a few percent between tests should not be read as adaptation. Lean mass largely determines RMR, so any change is read next to the DEXA lean-mass figures rather than on its own.

What this measurement cannot tell you

A single reading is close to meaningless on its own. What matters is the shape of the series across a year, read alongside the fasting schedule, sleep, training and the notes for that week. Nothing on this site is presented as evidence that fasting caused a change: this is one person, with no control condition, and many things change over a year besides the fasting days.