Coverage of the President’s Cancer Panel report this week draws attention to environmental contaminants as a potential cause of cancer (report). While this is an area of much public interest and certainly an important part of comprehensive health policy, it is a strange focus for a report that is meant to influence the nation’s approach to cancer control and prevention. The excitement and fear this report is likely to stir up could direct efforts away from combating known lifestyle factors that have a much larger effect on cancer risk than environmental contaminants (see figure). Current evidence shows that pollutants cause just 1 – 4 percent of all cancer, while obesity and tobacco each cause 20 percent and 30 percent, respectively. Even when occupational exposures are added in, lifestyle factors trump environmental factors by at least a factor of six.
At a time when we know we’re in the midst of an obesity epidemic and we know that we’re not as physically active as we should be and we know that 20 percent of the population still smokes, why focus so much effort exploring a topic we also know has such a modest impact on risk? At its worst, it plays into the hands of tobacco companies and food manufacturers by deflecting discussions of harsher regulations onto other fields. If we acted with passion to remove causes of cancer as is proposed in the report for plastics, then clearly tobacco products would have been banned decades ago.
It’s only natural for humans to search for a magic bullet, that one thing that can easily fix it all – easily erase our cancer risk. Better studying and controlling environmental pollutants is an easy projection for this. But, the evidence clearly shows this would be no magic bullet. More than half of all cancer can be prevented with what we know today, and pollutants make up only a small part of this.
Below we summarize the known causes of cancer.
1. Tobacco
Established as the primary cause of cancer-related deaths and considered the single largest preventable cause of cancer in the world (1), the impact of tobacco on international health is hugely detrimental. Tobacco smoking causes bladder, cervical, esophageal, kidney, laryngeal, lung, oral, pancreatic, and stomach cancers and acute myeloid leukaemia (AML) (2). In the United States alone, smoking causes at least 30 per cent of cancer deaths annually; globally tobacco will kill more than five million people. Risk increases with daily consumption as well as duration of smoking. Second hand smoke poses significant risk as well, which makes tobacco the only legal consumer product that can harm everyone exposed to it.
Stopping smoking reduces future risk of cancer. For example, the reduction in risk of lung cancer is rapid with 50 percent risk reduction in less than 10 years (3). After more than 20 years, risk drops to near that of a never smoker.
2. Alcohol consumption
Alcohol is estimated to cause 4 per cent of all cancers in high income countries (4), with a higher burden in men than women, reflecting overall intake. Health risks increase with heavier drinking leading to oral cavity, pharynx, larynx, oesophagus, liver, breast and colorectal cancers (5). Risks increase further when heavy alcohol use is combined with smoking.
3. Physical activity
Globally, inactivity causes close to 2 million deaths each year (6). Lack of activity is linked to most major chronic diseases, including type II diabetes, osteoporosis, stroke, cardiovascular disease, and cancer. Based on a well-designed systematic review of published evidence, the World Cancer Research Fund reports there is “convincing” evidence that physical activity decreases risk for colon cancer, “probable” evidence of a decrease in postmenopausal breast cancer and endometrial cancer, and “suggestive” evidence of an impact on lung, pancreas, and premenopausal breast cancer (5). Growing evidence also points to physical activity substantially lower the risk of premenopausal breast cancer (7, 8) as well as other chronic diseases.
4. Weight control
Obesity is increasing at epidemic rates around the world (9). United States data from 2003-2004 show that 66 percent of adults are overweight or obese (BMI ≥25) and 32 percent of adults are obese (BMI ≥ 30). Since 1988, these rates have been steadily increasing (10).
Historical data from the past 25 years point to obesity as a cause of approximately 14 percent of cancer deaths in men and up to 20 percent of cancer deaths in women (11). These may be conservative estimates as the population has gained substantial weight over this time period, with the prevalence of overweight and obesity increasing from 15 percent in 1980 to 35 percent in 2005. The American Institute for Cancer Research (AICR) and World Cancer Research Fund (WCRF) reported there is convincing evidence for a relation between obesity and esophageal, pancreatic, colorectal, postmenopausal breast, endometrial and kidney cancers with probable evidence for cancer of the gallbladder. In addition, they found probable evidence that fat around the mid section (abdominal adiposity) in particular increases risk of pancreas, endometrial and postmenopausal breast cancer. Finally, emerging evidence suggests that obesity increases the risk of aggressive prostate cancer (12).
Overall, we estimate that overweight and obesity cause approximately 20 percent of all cancer. Previously, Doll and Peto (13) combined “overnutrition” (overweight) with diet and estimated that together they caused 35 percent of all cancer. We break out overweight and obesity from diet and provide updated estimates for the causes of cancer (figure).
The burden of obesity has increased so much that some now estimate that the total health burden of overweight and obesity may exceed that for cigarette smoking (14).
Public health recommendations call for adults to stay within the recommended BMI range (18.5-24.9) and avoid weight gain.
5. Diet and dietary supplementation
aflatoxin and liver cancer, and salt and stomach cancer, while non-starchy vegetables reduce risk of cancers of the mouth, oesophagus, and stomach (5). Because of benefits in preventing other major chronic diseases such as cardiovascular disease and diabetes, it is estimated that a global increase in fruit and vegetable consumption would save 2.7 million lives annually (6).
International recommendations prescribe a diet high in fruits and vegetables (at least 4-13 servings per day) with emphasis placed on nutrient-rich green leafy vegetables, orange vegetables, and legumes. Avoiding certain foods potentially decreases cancer risk, such as salt-preserved meats or other foods, red meat, and very hot food or drinks. The U.S. National Cancer Institute (NCI) recommends that only 20-35 percent of daily calories be from fat; comprised of primarily polyunsaturated or monounsaturated fats in fish, nuts, and vegetable oils, 10 per cent saturated fats, and as little trans fat as possible (15).
Substantial evidence supports a link between vitamin D and reduced incidence of colon cancer – the third most common cancer among both men and women in the United States. Studies show that people with higher circulating vitamin D levels can have as little as half the risk of developing colon cancer as those with lower vitamin D levels (16). This and other possible benefits were reviewed systematically by the International Agency for Research on Cancer (IARC) (16), and led to the recommendation that better understanding of possible adverse health effects of population supplementation, and the possible variation in benefits depending on the baseline serum 25-hydroxyvitamin D level, are necessary before recommending routine vitamin D supplementation for cancer prevention. Further research is needed to define the optimal dose or level of vitamin D, its efficacy in reducing cancer incidence, and the time course for change in risk of cancer after increasing levels.
6. Sun Exposure
The sun, as the primary source of ultraviolet radiation, poses a significant risk of skin cancer particularly in fair-skinned individuals. Internationally, nearly 60,000 deaths are attributed to over-exposure leading to malignant melanomas and skin cancer annually (17). Observing trends of increasing rates of skin cancer, the U.S. National Cancer Institute reports 60,000 new cases in 2007 in the United States alone (15). Prevention recommendations as simple as avoiding the sun in peak hours (approximately 10 am to 3 pm), covering skin whenever possible, protecting exposed skin with sunscreen, and avoiding tanning booths are effective in reducing skin cancer incidence if these lifestyle changes are adopted, particularly, at an early age.
7. Infections
Some 18 per cent of cancers worldwide can be linked to chronic infections due to agents such as Helicobacter pylori, human papillomaviruses (HPV), Hepatitis B, Hepatitis C, Epstein-Barr virus (EBV), human immunodeficiency virus (HIV), human herpes virus 8 (HHV-8), and Schistosoma haematobium (18), with the proportion of all cancer due to infections being much higher in developing countries (26 per cent, compared to 7.7 per cent in developed countries). The current burden of cancer in the developing world is dominated by infection, once smoking is accounted for. (See related post) .
8. Environmental and occupational exposures
Environmental exposures account for 1-4 percent of cancers. Occupational exposures such as asbestos, arsenic in drinking water, food contaminants such as aflatoxins and pesticides, and radiation exposure are classified as environmental carcinogens, but in countries with established market economies, exposure is now largely limited by regulation to reduce harm. International agencies have responded by identifying carcinogens (e.g., IARC classification of carcinogenic compounds) and regulating use, exposure, and protection for employees in the case of occupational hazards.
As better regulations of contaminants have been put into place in the most developed countries, production has been exported, in some cases, to countries with more lenient requirements for environmental exposure and contaminants thereby not eliminating, but shifting, the cancer risk from an international scope. Despite regulatory changes in many countries, exposure to asbestos, for example, continues through occupations such as construction, ship work, and asbestos mining. Given the long lag between exposure and lung and pleural cancers, mortality from asbestos-related disease is estimated to remain at 90,000 per year (19).
Successful enforcement of approaches to reduce exposure to known carcinogens in both the work place and the home is necessary to achieve successful cancer prevention.
9. Medications
Medication use is widespread in high income countries and limited in low and middle income countries. Strong evidence supports several medications as either causing cancer – for example, postmenopausal hormone therapy with estrogen plus progestin (20) – or reducing cancer – for example, oral contraceptives and ovarian cancer (21), and aspirin and colon cancer (22).
For combination estrogen plus progestin, the IARC has now classified this combination therapy as carcinogenic in humans (23) and estimates indicate that the reduction in use of hormones after the widespread publicity of the results of the Women’s Health Initiative (stopped early due to excess breast cancer) accounts for approximately a 10 percent decline in incidence among women 40 – 70 years of age (20). Thus for this combination therapy, evidence shows that risk rises with duration of use and that acting as a late promoter, removal of the drug leads to a rapid decline in incidence (20), though among women with longer durations of use risk may not return to that of women who have never used combination therapy (24). Other less widespread drugs may also contribute to cancer risk (such, DES), but the population impact will be substantially smaller because of their relatively low use in the population.
For some medications that reduce risk, the benefits have been limited to date to those who have had specific indications for use of the medication. Broader population strategies may be developed for more widespread protection, such as could be achieved if all women took oral contraceptives as a chemopreventive for a minimum of 5-years
Related CNiC posts:
Literature cited:
1. Peto, R., et al., Mortality from smoking worldwide. Br Med Bull, 1996. 52(1): p. 12-21.
2. U.S. Department of Health and Human Services, The health consequences of smoking: a report of the Surgeon General. 2004, Centers for Disease Control and Prevention: Washington, DC.
3. Kenfield, S.A., et al., Smoking and smoking cessation in relation to mortality in women. Jama, 2008. 299(17): p. 2037-47.
4. Danaei, G., et al., Causes of cancer in the world: comparative risk assessment of nine behavioural and environmental risk factors. Lancet, 2005. 366(9499): p. 1784-93.
5. World Cancer Research Fund, Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. 2007, Washington, DC: AICR.
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15. National Cancer Institute, Cancer Trends Progress Report – 2007 Update. 2007, NIH, DHHS: Bethesda, MD.
16. International Agency for Research on Cancer, Vitamin D and Cancer. 2008, International Agency for Research on Cancer: Lyon.
17. World Health Organization, The World Health Organization’s Fight Against Cancer: Strategies that prevent, cure, and care. 2007, World Health Organization: Geneva.
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19. World Health Organization, Elimination of Asbestos-related Disease, in Public Health and the Environment. 2006, World Health Organization: Geneva.
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24. Colditz, G.A. and B. Rosner, Cumulative risk of breast cancer to age 70 years according to risk factor status: data from the Nurses’ Health Study. Am J Epidemiol, 2000. 152(10): p. 950-64.