Why VO2 Max Isn’t Always a Reliable Measure of Fitness

You Should Know: VO2 max — the maximum oxygen your body can use during intense exercise — has become one of the fitness world’s favorite numbers. Longevity experts call it one of the strongest predictors of long-term health, and smartwatches and rings now serve it up alongside your step count.

Going Deeper: Despite its popularity, VO2 max measurements from wearables may not be as reliable as many think. Cardiologist Eric Topol recently argued that most wearables can’t measure it with complete accuracy. True VO2 max testing requires a lab with gas exchange analysis, which is really only practical for elite athletes. Smart devices estimate it via heart rate algorithms, but studies show those estimates can be off by 7% to 16%. And landmark studies linking fitness to lower mortality didn’t even measure VO2 max — they tracked cardiorespiratory fitness using METs (metabolic equivalents of task), which compare how hard your body works during activity versus at rest.

Takeaway: The number on your wrist or finger shouldn’t be gospel. Performance trends, a 12-minute run test, or tracking METs may provide a more honest read on where your fitness stands.

Bottom Line: Don’t ignore VO2 max, but don’t obsess over it. The real goal is cardiorespiratory fitness, and you don’t need a lab to track that.