If you met the guy I was chatting with last Friday, your first thought wouldn't be "cancer survivor." He's an athlete -- younger and taller than me, and probably more fit. That's why I was so surprised when he told me about his two close calls with melanoma. He and his doctors caught it both times before it got into his bloodstream, which would've been fatal.

Just about everyone I know who has gotten cancer was young, healthy, and extremely active at the time of diagnosis. Alwyn Cosgrove, my friend and coauthor, has survived stage IV cancer twice. Rob Duffield, a triathlete I know via the JP Fitness community, lost a nut to the Beast. And those are just two who happen to be friends and happen to have websites.

Granted, all these people have survived cancer, and their fitness levels almost certainly helped ensure that outcome. But I still wonder why all the people I know who've gotten the disease at relatively young ages were fit and active at the time they were diagnosed. Conversely, I can't think of any sedentary person I know who got cancer before retirement age.

I bring this up because of the news that cancer mortality rates in the U.S. are declining:

My first thought upon seeing the headlines was to wonder if exercise has anything to do with the decline. I know the percentage of people who exercise seriously isn't growing, but I wondered if enough of them have been exercising long enough to affect the statistics cited in the study. (The headline is a decline in mortality from cancer, but the abstract jumbles statistics on incidence and death rates. It seems that both declined, but not necessarily at the same rate.)

So, does exercise in fact prevent cancer? This roundup offers a broad endorsement of that idea:

Then there's this:

But when I did a quick and shallow search of research that might explain this link, I came up with a lot of conclusions like this one, from a 1997 review of the cancer-prevention literature:

More recent studies show that researchers are still trying to figure out the connection. Here's one from 2003:

This review, from 2007, suggests the mechanisms have been narrowed down:

Now we're getting somewhere. Or, at least, I thought we were. I tried to figure out if exercise increases a hormone called insulin-like growth factor (IGF-1) and the binding proteins associated with it. After an hour of online searching, I couldn't come up with a satisfyingly definitive answer. (They don't teach endocrinology in journalism school, for some reason.) I think strength training increases IGF-1 and one or more of its binding proteins. But it's complicated.

One of the complications is that IGF-1 decreases when the person exercising is in what's called "negative energy balance." That is, he or she is eating less than needed to maintain weight. But one of the IGF-1 binding proteins increases when someone is undereating. And then it all returns to normal a few days after energy balance is restored.

And I don't even want to think about prostaglandins; that could take a couple days to figure out, given my remedial research skills.

But maybe there's a simpler answer. Maybe it's not exercise itself that reduces cancer risk, but all the lifestyle changes that go along with consistent, systematic exercise: a better diet, better weight control, better body composition (more muscle, less fat), more sleep, less stress.

Or maybe the explanation is even farther off the deep end. People who exercise probably take more showers than people who don't. Is better hygiene linked to cancer rates? Are we bored less often? Do we have more frequent bowel movements?

As long as the responsible answer is "nobody really knows," who can say it isn't something off the wall?

Tuesday blog polyps

• Then again, maybe the answer will be found in the emerging science of epigenetics. The short explanation: We all have potentially cancerous cells lurking throughout our bodies. Aging and smoking switch these cells on, or at least break down the mechanisms that keep them inactive. So if exercise somehow delays the aging of cells, then that could explain the lower risk of cancer for fit people.
• I gave Dr. Bryan Chung a hard time about not posting to his blog, Evidence-Based Fitness. He had that excuse you always hear from doctors: "Saving people's lives in the cardiovascular ICU is more important than blogging." Haven't been there, haven't done that, but still, how hard can it be? You walk around with a clipboard, use a bunch of words nobody understands, and then hook somebody up to a bunch of machines that look like old Atari games. Right? So, out of guilt, Bryan left some patients to hook themselves up to the Super Pong machine and wrote a really interesting post about the case for and against static stretching.