What does the evidence show?
It’s not a miracle, and it’s not a secret, but the link between fitness levels and the risk of lung and colorectal cancers is impressively strong
One of the first things they would teach you in health journalism school, if that were an actual thing, would be to treat purported anti-cancer news with extreme caution. People are desperate for hope, so hyping up the latest mouse breakthrough or Amazonian tree fungus is somewhere between unethical and cruel.
Still, a recent cancer-related tweet from University of Alabama at Birmingham exercise scientist Marcas Bamman caught my eye, and I think it’s worth sharing. There’s no miracle to report, and there’s not even a big surprise: the upshot is simply that high physical fitness appears to be protective against certain types of cancer. What’s noteworthy is the data—lots and lots of data that quantifies the amazing difference fitness can make.
Bamman’s tweet highlighted a new study in the journal Cancer from a research team at Johns Hopkins University and the Henry Ford Health System in Michigan. The Henry Ford group has a remarkable trove of data from nearly 70,000 people who were referred by their doctors for treadmill testing between 1991 and 2009, giving them a fairly accurate measure of cardiorespiratory fitness. (A methodological note: these researchers report fitness in peak “metabolic equivalents of task,” or METs, which is basically how many multiples of their basal metabolism they can reach on the treadmill before exhaustion. While the number is different, what they’re measuring is essentially equivalent to VO2max.)
By tracking subsequent health outcomes, both within the Henry Ford system and using external databases like the Social Security Death Master File (yes, that’s its real name), the researchers can figure out what effect your fitness level has on your future health. A simple example that I wrote about a few years ago is the link between fitness and longevity: their analysis found that the fitter a subject was, the longer he or she was likely to live. That seems obvious, but it came amid a big debate about the supposed risks of too much exercise—and, significantly, the Henry Ford data showed no hint of any negative effects at even the highest levels of objectively measured fitness.
The new cancer study analyzes data from 49,143 patients between the ages of 40 and 70, looking for diagnoses of lung cancer and colorectal cancer. They found 388 cases of lung cancer, and 220 cases of colorectal cancer during the follow-up period. They divided the subjects into four groups based on fitness level; after adjusting for other risk factors like age and sex, those in the fittest group were 77 percent less likely to be diagnosed with lung cancer and 61 percent less likely to be diagnosed with colorectal cancer.
It was a clear and unambiguous trend, with each increase in fitness adding further benefit. Here’s the relative risk for the four fitness groups for lung cancer (0.23 in the fittest group corresponds to a 77 percent reduction in risk, compared to the least fit group):
The four fitness groups corresponded to levels of less than 6 METs, 6 to 9 METs, 10 to 11 METs, and 12 or more METs. As a rough conversion to VO2max, you can multiply by 3.5, so 12 METs corresponds to about 42 ml/kg/min. That’s very good for someone over 40, but not impossibly good. Just under 10,000 of the subjects in the study achieved that benchmark.
There was a second bit of good news. For those who were diagnosed with one of the cancers, higher fitness seemed to have a further protective effect, making them less likely to die during the follow-up period. Those in the fittest group who were diagnosed with lung cancer or colorectal cancer were, respectively, 44 percent and 89 percent less likely to die during the study period compared to those in the least fit group.
There are two key questions arising from this data. The first is why fitness has such a protective effect against cancer. In short, the researchers don’t know. Their list of possibilities includes “improved respiratory function, decreased bowel transit time, better immune function, or reductions in systemic inflammation.”
The second question, closely related, is how much you can change it. “Fitness lowers your cancer risk” isn’t the same thing as “Exercise lowers your cancer risk.” By some estimates, your VO2max is more than 50 percent determined by your genes rather than your exercise habits. That leaves some ambiguity in the results. Is the link between fitness—i.e. VO2max—and cancer risk a direct consequence of your body’s ability to deliver oxygen to your muscles at a rapid rate? If that’s the case, and you happen to have a relatively high VO2max thanks to your genetics, you’re in luck. Or is the link between fitness and cancer risk simply a byproduct of the fact that most people with high VO2max exercise a lot? In that case, simply having a high VO2max isn’t helpful unless you’re also exercising and getting all the other benefits that exercise provides.
Along those lines, it’s worth noting that other studies like the famous National Runners’ Health Study have previously found links between exercise levels and cancer. In a multi-decade analysis of 92,000 runners and walkers, those running more than about 15 miles per week were 76 percent less likely to be diagnosed with kidney cancer compared to those doing less than 5 miles per week; a similar analysis found a 40 percent reduction in brain cancer risk. (Another methodological note: in these studies and the Henry Ford study, they did sensitivity analyses, excluding anyone who was diagnosed within a few years of their enrollment in the study to avoid the reverse causation problem of as-yet-undiagnosed sick people exercising less or scoring lower on the fitness test.)
Ambiguities aside, the bottom line is that physical fitness lowers your cancer risk, and the best way to improve your fitness is by exercising. You probably already knew that; but maybe, like me, you didn’t really have a sense of quite how stark the cancer link is. At this very moment, scientists around the world are racing to decode precisely what molecular pathway or genetic switch is responsible, with the ultimate goal of bottling it. Each forward step they take will produce optimistic headlines. I really hope they succeed, needless to say. But until they do, the best non-miracle-cure advice I’ve got is to tune out the noise and get as fit as you can.
My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performancef, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.