How exercise can protect us from cancer

One in three breast cancer cases in Western countries could be avoided if women ate less and exercised more, experts said today at a breast cancer conference in Barcelona. While better treatments, early diagnosis and mammogram screenings have dramatically slowed the disease, they said the focus should now shift to changing behaviors like diet and physical activity.

“What can be achieved with screening has been achieved. We can't do much more,” Carlo La Vecchia, head of epidemiology at the University of Milan, told The Associated Press. “It's time to move onto other things.” (Here’s the AP article.)

Let’s take a closer look at the link between exercise and cancer.

We live highly sedentary lives. Only a third of Americans undertake enough regular leisure-time physical activity to derive health benefits — that is, moderate exercise for 30 minutes five times a week or vigorous activity for 20 minutes three times a week. In the UK, only 40% of men and 28% of women reach the recommended level of physical activity.

To a large extent, this is due to the increasing mechanization of our daily lives. Household tasks are largely automated, most journeys are made by car or public transport, many formerly manual jobs in industry or agriculture are increasingly carried out by machines, and active outdoor recreation has largely been replaced by sedentary activities such as television-watching or computer games. Even a five-minute walk to the letter box has been replaced by instant messaging via email and Facebook.

Many of us confuse being “busy” with being “active”, as my own experiment in pedometry over the last week has highlighted. On typical work days, when I spend most of my time at the computer, in meetings or driving my car, I’m lucky if I clock up 8,000 steps. Days like these are busy and tiring, but involve very little physical activity.

The International Agency for Research on Cancer estimates that 25% of cancer cases worldwide are due to overweight, obesity and a sedentary lifestyle. According to the U.S. government’s Physical Activity Guidelines for Americans, individuals engaging in regular aerobic physical activity 3-4 hours a week at moderate or greater levels of intensity have on average a 30% reduction of colon cancer risk and a 20-40% reduction of breast cancer risk compared to those who are sedentary; better still, the more people exercise, the lower their cancer risk, the report states.

Meanwhile, the World Cancer Research Fund notes that exercise protects against colon cancer, probably helps reduce the risk of postmenopausal breast cancer and endometrial cancer and may also lower the risk of lung cancer, pancreatic cancer and pre-menopausal breast cancer.

Various mechanisms link sedentary lifestyles to increased cancer risk. Obesity is the most obvious connection: by moving less (and in some cases, by eating more, or making poor food choices) we risk gaining weight. Extra fat – especially around the waist – can act like a hormone “pump” and raise levels of estrogen and other substances in the blood which increase cancer risk. Being overweight can also cause inflammation, another risk factor for cancer.

Regular exercise meanwhile may boost the immune system, thus increasing our bodies’ ability to ward off enemy invaders. One as yet unproven hypothesis is that physical activity may boost the number or function of natural killer cells which may play a role in tumor suppression.

Lastly, by improving colon motility, exercise helps food move through the digestive tract more quickly. This reduces the exposure of the cells lining the digestive tract to potentially cancer-causing substances, thus potentially lowering colon cancer risk.

So the next time we put off physical activity, let’s remind ourselves that even relatively small physical efforts can translate into significant health gains.

(The bike helmet's for you, Silvia!)