Concept
FATmax
The exercise intensity near a person's highest measured rate of fat oxidation. It varies between people and is not a universal heart-rate number.
What FATmax describes
As exercise intensity changes, the amount and proportion of energy supplied by fat also changes. FATmax describes the intensity where measured fat oxidation is highest for that individual.
Why it is not a fixed zone
Training status, food intake, testing method, and daily condition affect the result. A watch formula cannot identify a precise personal FATmax point.
How it relates to Zone 2
FATmax often overlaps with easy aerobic work, but it is not automatically identical to a watch's Zone 2 boundaries.
Use easy, repeatable effort as the training target. Treat FATmax as an explanation for why easy running can be useful, not as a number to chase.
The intensity with the highest fat oxidation rate does not guarantee the greatest long-term fat loss.