Fighting The Big C
How to lower your risk of cancer

Alice Fisher

So you don’t plan on being one of the millions of Canadians who will be diagnosed with cancer in their lifetime? You probably already know enough not to smoke and to stay away from excessive sun exposure. But did you also know that to help keep cancer at bay, you should keep active, maintain a healthy weight and get appropriate screening tests? Taking such steps, say experts, can reduce the incidence of some cancers and cut cancer deaths by 50% — some believe by much more. In the field of cancer prevention, here’s what we know today.

Photo by Pali Rao/iStockphoto

Smoking

An estimated 30% of fatal cancers in Canada are caused by tobacco, and about 80% of lung cancers are smoking-related. But cancers of the bladder, tongue, esophagus and pancreas are linked to the evil weed, too. “The one lifestyle habit that we know the most about — and the one we could do the most with via the smallest change in population behaviour — is cigarette smoking,” says Dr. Richard Gallagher, leader at the Cancer Control Research Program of the BC Cancer Agency in Vancouver and a clinical professor of health care and epidemiology at the University of British Columbia. “Every percentage point that you reduce the smoking population by will have a major impact.”

Sharon Earle, 63, of Halifax, hopes the impact will be a positive one for her. When she battled ovarian and breast cancer in the 1980s and ’90s, she was a smoker. After getting breast cancer a second time, she quit the habit with the help of a nicotine patch. “I thought it would be foolhardy to keep smoking,” says Sharon, smoke-free now for seven years. “But it took a mastectomy to convince me that I couldn’t take any more chances.”

Physical Activity

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Recent studies have repeatedly indicated that regular exercise can reduce the risk of breast cancer. A 2006 study, sponsored by the National Cancer Institute in the U.S., found that vigorous exercise cut breast cancer risk by 12%, even taking into consideration other risk factors such as age, family history, number of pregnancies and use of hormone replacement therapy. And the Women’s Health Initiative in the U.S. concluded that those who participated in about two hours per week of brisk walking or other moderate exercise had an 18% lower  risk of breast malignancy.

And it’s a good idea to get active early on. An Edmonton literature review demonstrated that being physically active in childhood and adolescence may reduce the risk of breast cancer in adulthood.

The evidence is even stronger for colon cancer. Studies show that regular physical activity can reduce the risk of colon cancer by some 40% to 50%. Researchers speculate that physical activity accelerates the digestive process, which reduces the amount of time the colon is exposed to harmful substances. And being active may also reduce your chances of developing prostate cancer, according to a recent Harvard School of Public Health study.

Experts believe the main impact of exercise is related to maintaining a healthy body mass and healthy blood sugar levels to avoid insulin resistance, which is a risk factor for cancer.

Diet

Almost weekly, we learn more about the cancer-fighting capacities of plant chemicals. Some of these compounds interfere with cell cycles and stop the runaway replication of cancer cells. Others reduce oxidative stress, a process that generates rogue oxygen molecules called free radicals. These can damage cells and turn them malignant. Still others induce apoptosis, the programmed cell death that signals cancer cells to commit suicide.

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Diet may also affect how oncogenes, genes involved in cancer, respond when exposed to carcinogens. In one study, reported by the American Institute for Cancer Research, smokers with a gene tied to cancer risk who ate a diet high in fruits and vegetables and low in animal fats were as much as 84% less likely to develop lung cancer compared with smokers with the same form of the gene who had a less healthy diet.

As for fat, though the connection is not yet definite, evidence is emerging that a high-fat diet is linked to cancer. In one five-year study, less than 10% of women assigned to a low-fat dietary group had a recurrence of breast cancer compared with more than 12% of women who ate their regular diets. Although the exact cause-and-effect mechanism here is still being debated, some researchers speculate that saturated fat promotes the production of estrogen, which may fuel hormone-dependent cancer growth.

Other researchers are looking at the preventive role of various vitamins. University of Toronto (U of T) researchers are interested in vitamin B₁ (thiamine), found in vegetables and grains and routinely added to flour. One Toronto study found that patients with colonic polyps had low thiamine levels. A deficiency of this vitamin results in the formation of toxic glucose (sugar) compounds that leave your body susceptible to potential carcinogens, notes Dr. Peter O’Brien, a U of T professor emeritus in the Leslie Dan Faculty of Pharmacy. “When you deprive your body of vitamin B₁, you make yourself more sensitive to oxidative stress,” he says. O’Brien estimates that about 10% of Canada’s current population is B₁-deficient.

Dr. Robert Bruce, a professor emeritus in U of T’s nutritional sciences department, is convinced that many cancers are 100% preventable through diet. While some people blame external factors such as pollution, insecticides and exposure to sun and radiation, Bruce is convinced that what you choose to put inside your body — or not to — is a more likely culprit. “My intuition tells me that what’s driving cancer is endogenous compounds,” and these you can control, he says.

Other nutrients such as calcium, folate and vitamin D are being studied for their role in cancer prevention. A review by researchers at the University of California at San Diego recently suggested that increasing daily intake of vitamin D could cut breast cancer risk by half and colorectal cancer risk by as much as two thirds.

Gallagher cites studies that followed subjects for at least 10 years to find out who developed colon cancer. “Almost uniformly, the vitamin D levels in the serum of people who did not get colon cancer were higher than in those people who went on to develop colon cancer,” he says. It’s research like this that has prompted the Canadian Cancer Society to recommend that we take vitamin D supplements, especially during the dark fall and winter months.

As for calcium, several studies  have suggested suggested that high intakes from food and supplements helps reduce the risk of precancerous polyps and colorectal tumours.

Sun Exposure

So is the answer to preventing some of these cancers simply to get more sun exposure to prompt your body to produce more D, the sunshine vitamin? Most experts agree that Canadians can’t make enough vitamin D from sun exposure alone year-round. And virtually everyone agrees that the sun is the main cause of skin cancer. So no one recommends getting excessive exposure to solar rays, but advocacy groups such as the Canadian Cancer Society are now recommending that you get a few minutes of daily sun exposure on unprotected skin to boost your body’s production of vitamin D. The body makes this nutrient from a cholesterol-like substance in the skin when it’s exposed to UVB rays.

Increasing your daily intake of vitamin D could cut breast cancer risk by half and colorectal cancer risk by as much as two thirds, U.S. researchers say. Maintaining healthy blood sugar levels to avoid insulin resistance may reduce your chances of developing colon, breast and endometrial cancers.

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Preventing Infections

There’s growing acceptance of the principle that preventing certain viral and bacterial infections will reduce cancer rates. “Altogether, infections are responsible for up to 25% of cancers in the developing world and just short of 10% in the developed world,” says Dr. Franco Eduardo, director of the Division of Cancer Epidemiology at McGill University in Montreal.

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We’re already on the road to preventing some of these. Two vaccines have been introduced in Canada to target the main strains of human papilloma virus (HPV), which causes cervical cancer. These vaccines could prevent 75% to 80% of cervical cancers, says Eduardo. The HPV vaccine can also prevent other cancers, including those of the vulva and vagina in women, the penis in men, and the mouth, throat and anus in both sexes, he says.

HPV infections may also be involved in certain skin cancers, although there is currently no vaccine available against strains that affect the skin. And a notable proportion of stomach cancers are triggered by Helicobacter pylori, a bacterium that infects the stomach. “People are looking at whether infectious agents might also be involved in prostate cancer and breast cancer,” he says. “In the future, we may look for a new cancer paradigm, in which an infectious agent — in conjunction with lifestyle factors such as lack of physical activity and poor diet — is going to be the key.”

Environmental Hazards

Pat Anderson, 56, of Toronto, has tossed out her harsh commercial cleaning products and replaced them with homemade ones. She’s not taking any chances her cleaning addiction will translate into an increased cancer risk — even if the link is far from proven. “Who knows the impact of these weird-sounding ingredients in expensive cleaning products?” she asks. Pat’s also buying organic produce and avoiding plastic bottles, having read that some may contain a toxin linked to prostate and breast cancers. Her efforts may help her breathe easier but in terms of cancer prevention they might be in vain. Other than cigarette smoke, the sun, radioactive radon gas and asbestos, we don’t precisely know what agents in the environment cause cancer, says Gallagher. “It’s very difficult to do studies big enough to identify an environmental carcinogen.” But the evidence we do have suggests that we shouldn’t worry too much about household chemicals. “You usually need prolonged exposure at a fair concentration to develop a cancer. Even if we eliminate all household cleaners, we’re probably not going to see any measurable difference in cancer rates 10 years from now.”

What about pesticides? Though some reports have suggested a link, “I don’t feel we’ll attack the cancer problem much by eating organic produce grown without pesticides,” says Gallagher. “The evidence is pretty thin. But we do need to look closely at more of the potential causes in the environment.”

NSAIDs

Studies show that regular users of acetylsalicylic acid and other non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen have a 20% to 50% lower risk of colorectal cancer. Some studies have suggested a reduced risk of breast cancer with NSAID use, too. Inflammation can be an early step on a cell’s path to malignancy.

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Insulin Resistence

New research points to a possible link between pre-diabetes and colon cancer. This suggests that avoiding the condition that leads to type 2 diabetes — insulin resistance — will become a major prevention goal. Diabetes begins with the body’s inability to produce enough or effectively use insulin after it becomes resistant to this energy-storing hormone. “The hypothesis is that the same factors that give you cancer are the ones that also give you diabetes,” says O’Brien, referring to a large European study showing that patients with colon cancer had higher levels of insulin resistance. “We think that some people with insulin resistance develop diabetes and some develop colon cancer.”

A Swedish study recently shed more light on this connection. After following about 65,000 people for eight years, it found that women whose blood sugar readings registered in the top 25% range after fasting had a 26% higher chance of developing breast or endometrial cancer than those in the bottom quarter.

“We think insulin resistance may be associated with an elevated risk of a several cancers, including pancreatic,” says Gallagher. Since insulin resistance is modifiable through diet and exercise, perhaps we could prevent many cancers simply by making sure we don’t develop this pre-diabetic condition.

Alison Proctor, 59, is one woman who wishes she had maintained healthy blood sugar levels. The Toronto resident was diagnosed with type 2 diabetes at age 51 and then at age 58 with cancer of the uterus. “I  strongly suspect that years of high blood sugar levels played a role in my cancer,” she says.

Screening

Earlier detection of precancerous lesions and small treatable cancers is now possible through screening tests. The ones that are known to be effective are: mammography for breast cancer, fecal occult blood testing for precancerous colon polyps and early colorectal cancers, and Pap smears for cervical cancer. The effectiveness of widespread screening for prostate cancer with the prostate-specific antigen blood test is still being debated. Ask your physician which tests you should undergo and how often. For more on both established and controversial screening tests, watch for Canadian Health’s January/February 2008 issue available in early January in your doctor’s office or online at www.canadian-health.ca.

Hormone Replacement Therapy

The U.S.’s Women’s Health Initiative study reported a small but significant 26% increase in the risk of breast cancer in older women on longer-term hormone replacement therapy (HRT). The U.K.’s Million Women Study found a 20% increase in the risk of developing ovarian cancer in HRT users. Current Canadian guidelines recommend using HRT to manage menopausal symptoms for up to five years only and at the lowest effective dose.

For more on cancer prevention, contact your provincial office of the Canadian Cancer Society. For telephone numbers by province, go to www.cancer.ca.