How VO2 Max Is Measured

The test

VO2 max is measured with a graded exercise test: a progressively harder effort to exhaustion while expired air is analysed for oxygen and carbon dioxide. The result is millilitres of oxygen per kilogram of body weight per minute. It is performed by a testing facility with metabolic cart equipment, not estimated from a wrist device.

Why it is in this project

VO2 max is the best single measure of cardiorespiratory fitness and among the strongest predictors of all-cause mortality in the literature. The obvious worry about a year of repeated fasting days is that fitness erodes. This is the measurement that answers it.

Why the number can move without fasting explaining it

• Training done, or not done, in the weeks before the test
• Body weight, since the result is expressed per kilogram — losing weight can raise it with no change in the engine
• Sleep, illness and caffeine on test day
• Motivation to push to genuine exhaustion, which varies

A watch-estimated VO2 max also appears in the wearable data. That figure is an algorithm's guess from heart rate and pace, and it is not comparable with the laboratory test. Where both appear they are labelled separately.

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.