VO2 max is the maximum rate at which your body can use oxygen during intense exercise — a single number capturing the whole chain from lungs to heart to mitochondria. It's the best single measure of cardiorespiratory fitness, and it turns out to be an extraordinarily strong predictor of how long you'll live, rivaling or exceeding traditional risk factors like smoking and hypertension.
Training it well takes two ingredients. Zone 2 — sustained low-intensity work where you can still hold a conversation — builds mitochondrial density and fat oxidation, and it's where most endurance volume should sit. High-intensity intervals near your ceiling raise the ceiling itself. A common structure is a majority of easy Zone 2 volume plus one or two hard interval sessions weekly. The mistake most people make is training everything at a medium intensity that's too hard to accumulate volume and too easy to raise the ceiling.
Researchers at the Cleveland Clinic analyzed 122,007 patients who underwent treadmill testing between 1991 and 2014, then followed their mortality. Published in JAMA Network Open in 2018, the findings were unusually stark. The difference in mortality between the lowest-fitness group and elite performers was larger than the difference associated with smoking, diabetes, or coronary artery disease. Being in the bottom fitness quartile carried a risk comparable to or worse than having end-stage renal disease. And critically, they found no upper limit at which more fitness stopped helping — the long-standing worry that extreme fitness might be harmful found no support in the data. Lead author Wael Jaber remarked that being unfit on a treadmill test carried a worse prognosis than any of the standard cardiovascular risk factors they measured.