Cancer sucks. No, wait: cancer REALLY sucks. Most of us perceive cancer as a gruesome condition that slowly degrades health and dignity. Though enormous advances in cancer therapy have been made in the last 30 years, the best option remains: just don’t get it.
Cancer can be difficult to conceptualize: many common forms (in first-world countries, save lung cancer and cervical cancer) have no clear pathogen. To add confusion, the ones that do, like lung cancer, can still affect non-smokers who make it a point to avoid second-hand smoke. Since you can't really be "exposed" to most cancer, how do you prevent it? (or at least use tools within your control to reduce risk)
A few months ago Jordan Rapp asked me to pen an article or two. I've always had a pet interest in exercise physiology, though my training as a scientific apprentice (sounds a lot nicer than "PhD Candidate") is centered on cancer research. So when I was asked to combine the two I started mining the scientific literature. And this is what I found:
1) People who do aerobic exercise tend to develop less cancer than sedentary individuals
2) The best mechanism to explain #1 has implications for slowing aging as well
In 2009 a group in Great Britain published a meta-analysis of 40 case-controlled studies in peer-reviewed scientific journals (http://www.nature.com/bjc/journal/v100/n4/abs/6604917a.html) on exercise and cancer. When comparing the most active to the most sedentary individuals across studies, the authors found a 24% overall risk reduction for colon cancer - one of the most common cancer types!
Of particular interest was a study in 2009 that followed 2,560 men from eastern Finland and required them to record their daily activities and diet for 17 years! (http://bjsm.bmj.com/content/early/2009/07/28/bjsm.2008.056713.abstract) The authors discovered a "dose dependent effect" whereby greater amounts of exercise associated with a greater risk reduction to dying of cancer. The minimum exercise required to yield a significant reduction? 30 minutes of aerobic exercise daily.
I would like to point out that the aforementioned reports are correlation studies. They indicate that people who have at least 4-6 hours of aerobic exercise per week develop less cancer than their couch potato buddies. Exercise could inhibit cancer development, or some other factor (genetic or environmental) could influence both one’s propensity to exercise as well as other habits that might prevent cancer.
Why does this matter? Unless there is a mechanism linking the two and establishing causation, exercising more would NOT lessen one's risk for developing cancer. Just because the average ocean temperature and the number of pirate attacks worldwide have both increased in the last 30 years does not mean that the pirates are causing the oceans to warm, and it does not mean that fighting piracy is the same as fighting global warming.
So, a correlation has no teeth without causation. If exercise does prevent cancer, there must be a mechanism. I've seen several theories suggesting that exercise modulates cancer development by lowering hormone levels that promote gender-specific cancers (men: testosterone + prostate cancer, women: estrogen + breast cancer). But, that does not account for correlations of decreased risk of gender-neutral cancers like colon cancer (http://www.nature.com/bjc/journal/v100/n4/abs/6604917a.html).
In each body cell (minus red blood cells) our DNA is housed in structures called chromosomes. On the molecular scale, chromosomes look a bit like short pieces of rope that are capped on each end like a dog's chew toy. Those caps are called telomeres, and they tend to get shorter as we age. (http://www.ncbi.nlm.nih.gov/pubmed/21270748) Once they get short enough the rope begins to fray and tangle with other unraveling chromosomes (pieces of DNA), causing random recombination and genetic chaos inside a cell. Many cancers also show evidence of these events, and cancer can be induced in the lab by artificially stimulating these events. So, it has been proposed that cancer development and aging are intrinsically linked.
Correlation studies echo this a thousand times over, as I have stressed time and time again in my cancer Q&A blog: Cancer is an age-related disease! (http://ryongraf.com/ask-a-scientist-cancer-for-dummies-public-education-blog/why-are-cancer-rates-rising/)
Telomere length is quantifiable. In many scientific circles the telomere length of white blood cells is used as a molecular measure of age (as opposed to a time-based measure of age). It's very interesting that active people tend to lose telomeres more slowly than sedentary individuals. (http://www.ncbi.nlm.nih.gov/pubmed/21102320). Taken literally, endurance athletes age more slowly than the rest of humanity.
But could this be due to some gene affecting telomere length and propensity to exercise? In a study in 2008, a group of scientists at King's College in London investigated the telomeres of genetically identical twins that were NOT identical in their athletic habits. What really intrigued me is that the athletic twins consistently had longer telomeres than their sedentary siblings! Clearly, the observed difference cannot be genetic in origin. (http://www.ncbi.nlm.nih.gov/pubmed/18227361)
It has been anecdotally said by many that sport is the fountain of youth. On a molecular scale, it might actually be so! Chronologically, identical twins Adam and Brian might both be 30. On the inside, Adam's running habit might keep him younger than Brian's couch habit.
It's well established that aerobic exercise greatly reduces one's risk for cardiovascular disease. (http://circ.ahajournals.org/content/107/1/e2.full). Aerobic exercise also appears to have a cancer-suppressing effect as well. The vast majority of Americans will either die of cardiovascular disease or cancer in their lifetimes. Knowing this, it is essential that we bring this preventive therapy to our friends and family that have yet to embrace it. In a purely monetary equation, many groups have postulated that it would be MUCH less expensive to prevent chronic diseases like cancer than to treat it. As a society, would it not make sense to allocate our resources toward measures that promote athletic habits? But I digress…
Before I sign off, I'd like to stress that outdoor endurance athletes ARE at greater risk of skin cancers, so keep that sunscreen close! And no, sunscreen does not actually promote cancer development; that's all a big scare based on shaky data and internet fear mongerers who don't actually read scientific literature or understand statistics (http://ryongraf.com/ask-a-scientist-cancer-for-dummies-public-education-blog/can-sunscreen-cause-cancer/). In case it needed any re-iterating, an experimental study in Australia published in January 2011 determined that UV-A protecting sunscreen DOES significantly reduce risk for the nastiest of skin cancers: melanoma (http://ryongraf.com/2011/07/not-taking-it-for-granted-sunscreen-does-prevent-melanoma/).
So there you have it: aerobic exercise prevents cancer. And by "prevent" I mean "reduces the risk of developing cancer, or delays the age of disease onset." I realize that I'm preaching to the choir here, so please help me spread cancer awareness (the real kind) and get your friends and family off the couch! Or even better yet, if you hold a position of influence to shape physical education policy in your region, get involved!
See you on the road! (or the pool! Or the Slowtwitch Forums!)
Ryon is a PhD Candidate in Molecular Medicine and enjoys writing and cycling in his spare time. He operates a cancer education blog aimed at directly answering "dumb" questions about cancer: Ask a Scientist: Cancer for Dummies (Ryongraf.com) and enjoys mining the scientific literature for new and challenging questions.
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