Exercise has health benefits that can't be measured by VO2max.
Being physically fit is good for your health. Being physically active is good for your health. Those two sentences sound the same, but they're not. Physical activity is how much exercise you get, while physical fitness (more specifically cardiorespiratory or aerobic fitness, i.e. VO2max) is a measure of how much oxygen your heart and lungs can to deliver to be used by your muscles.
Of course, those who get lots of exercise tend to be the most fit. But that's not always the case: there's a significant genetic contribution (around 50 percent, by some estimates) to both your baseline fitness and how your fitness responds to training. That means you can have two people who do the same exercise but have very different fitness, or two people who have the same fitness but do very different amounts of exercise. The question is: which of these factors matters more?
A neat study by Matt Laye and his former colleagues at the University of Copenhagen, published last month in the journal Disease Markers, offers some interesting insights. The study design was pretty remarkable. Through a rather complex process, they assembled a group of 10 consistent marathon runners (all of whom had either run 10 marathons or averaged more than 50 km/week over the previous five years) and matched them individually with 10 sedentary controls with similar age, gender, BMI, and VO2max. Finding such people (i.e. high VO2max without exercising plus the right combination of age, BMI, etc.) is a pretty impressive feat!
Then they did the same thing with another group of 10 consistent marathon runners, this time matching them with controls of similar age, gender and BMI but much lower VO2max. They put all the subjects through a series of tests to measure metabolic health and risk factors: glucose tolerance, insulin levels, cholesterol levels, and body composition.
The general finding, as you can guess, was that the runners had better results for all the tests than the non-runners. What's interesting is that this was true even in the comparison between runners and non-runners with similar VO2max – so exercise has metabolic health benefits that aren't entirely explained or accounted for by cardiorespiratory fitness. The takeaway? If you're blessed with good fitness without having to work at it, don't rest on your laurels. And conversely, if your aerobic fitness levels seems stubbornly low despite consistent training, don't worry – you're still getting lots of health benefits.