Breast cancer is now the most common cancer on the planet, according to the World Cancer Research Fund – and physical inactivity can take some of the blame.
“We can now say definitively that physical activity reduces breast cancer risk,” says epidemiologist Associate Professor Brigid Lynch, whose research has found a clear biological link between too little physical activity and too-high levels of sex hormones like oestrogens and androgens that drive around 70 per cent of breast cancers.
Exercising can help reduce the risk of cancer.Credit: Getty
But here’s the good news: physical activity can lower high levels of these hormones in two separate ways.
“One is by reducing levels of these circulating hormones, and the other is by helping people maintain a healthy body size – we know that these sex hormones are produced by fat tissue, especially after menopause,” says Lynch, the deputy director of the Cancer Epidemiology Division at Cancer Council Victoria.
“It’s not just breast cancer – prostate, endometrial and ovarian cancers are also linked to high levels of sex hormones,” she adds.
Exercise shrinks the risk of around a dozen cancers, with the strongest evidence for bowel, breast, bladder, gastric, oesophageal, prostate and endometrial cancers, she says. Now scientists are trying to work out what it is about moving muscles that helps halt or slow down cancer.
Keeping the kilos off is part of the story. Extra weight and obesity cause almost 5300 cases of cancer in Australia each year, according to the Cancer Council. Too much fat around the middle can make it harder for insulin to control blood sugar, forcing the pancreas to pump out more insulin. This leads to high levels of bioavailable IGF-1, another hormone that at high levels can promote cancer.
But it’s not all about weight.
In the UK, a small study of older men with risk factors for bowel cancer like being overweight or physically inactive found that after 30 minutes pedalling an exercise bike, blood tests showed they produced a cancer-fighting substance called IL-6, capable of repairing damaged cells.
When the blood samples were added to bowel cancer cells in the lab, the samples taken immediately after exercise reduced DNA damage and slowed the growth of these cells.
“We did this study to explore how moderate-intensity exercise might affect the growth of cancer cells in ways that had nothing to do with weight,” says Dr Sam Orange, a lecturer in exercise physiology at the University of Newcastle in the UK. “The benefits of exercise for reducing bowel cancer risk are usually attributed to weight loss or avoiding weight gain, but emerging evidence suggests exercise lowers the risk, regardless of your weight.
“When we exercise, muscles release thousands of molecules into the bloodstream that benefit other parts of the body,” he explains. “We thought some of these molecules, especially IL-6, might interact with abnormal or cancerous cells to reduce their growth – and this research suggests it does.”
Meanwhile, a new international study of endurance exercisers in their 50s and 60s offers another clue, and another reason to jump on your bike. These men and women – all regular distance runners, bike riders or swimmers – had lower numbers of a type of cell in the bowel that can contribute to cancer.
“These senescent cells, as they’re known, are unique in that they eventually stop multiplying but don’t die off when they should. Instead, they accumulate in tissues where they release chemicals that trigger inflammation, and promote cancer and accelerated ageing,” says Professor Luigi Fontana, scientific director of the Charles Perkins Centre RPA Clinic and Healthy Longevity Program of the University of Sydney, who led this study.
“We don’t know exactly how intensive endurance exercise might have this effect, but it’s likely that it reduces the accumulation of DNA damage that is the major driver of cell senescence.”
Identifying the different ways that exercise might protect against cancer could lead to better guidelines for prevention, says Orange, but so far the evidence suggests that more movement is better.
“Two and half hours of exercise each week reduces bowel cancer risk by around 10 per cent, but doubling that amount to five hours a week reduces the risk by around 20 per cent,” he points out.
That’s more than the 30 minutes a day five days a week, plus two muscle-strengthening sessions recommended in Australia’s physical activity guidelines for general health – but it’s the best approach, agrees Lynch.
”For cancer prevention, aim for an hour of moderate-intensity exercise (half an hour if it’s vigorous) five times a week, along with two muscle-strengthening sessions,” she says. “But even smaller amounts of physical activity will have a benefit.”
This doesn’t mean exercise is a silver bullet against cancer – but it does lower the odds.
“It can also help improve the prognosis of breast, colon, and prostate cancer,” adds Fontana. “Studies have found that people with colon or breast cancer who exercise regularly, die less of recurrences or metastasis. Breast cancer research suggests that 30 to 40 minutes of brisk walking each day can contribute to markedly reduce mortality, especially when associated with a healthy plant-based diet.”
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