Preventing Cancer: Identifying Risk Factors
In this section, you will learn:
In the United States, 4 out of 10 cancer cases and almost half of all deaths from cancer are associated with preventable risk factors.
Not using tobacco is one of the most effective ways a person can prevent cancer from developing.
Nearly 20 percent of U.S. cancer diagnoses are related to excess body weight, alcohol intake, poor diet, and physical inactivity.
Many cases of skin cancer could be prevented by protecting the skin from ultraviolet radiation from the sun and indoor tanning devices.
Nearly all cases of cervical cancer could be prevented by HPV vaccination, but most U.S. adolescents have not received the recommended doses of the vaccine.
There are disparities in the burden of cancer attributable to preventable causes among certain segments of the U.S. population.
Decades of basic, epidemiologic, and clinical research have led to the identification of several factors that increase a person’s chance of developing cancer (see Figure 3). These factors, which are known as cancer risk factors, can alter the genetic or epigenetic information in a person’s cells. This may directly lead to cancer development or increase the person’s chance of developing cancer later in life. Many cancer risk factors, such as smoking, are also associated with worse outcomes after a cancer diagnosis (see Modifying Behaviors to Improve Outcomes).
Researchers estimate that more than 40 percent of the cancer cases diagnosed in the United States in 2014 and nearly half of all deaths from cancer were caused by potentially avoidable cancer risk factors, including tobacco use, poor diet, alcohol intake, physical inactivity, and obesity (21). In addition, vaccination against infection with the human papillomavirus (HPV) and decreasing exposure to ultraviolet (UV) radiation from the sun and indoor tanning devices can further reduce the burden of certain types of cancer (40).
Many cancer risk factors are also risk factors for other chronic diseases, such as cardiovascular disease, respiratory diseases, and diabetes. Thus, public education and policy initiatives to reduce or eliminate exposure to potentially modifiable cancer risk factors have the potential to reduce the burden of several other diseases in addition to cancer. In fact, a recent study showed that adherence to a low-risk lifestyle, such as never smoking, eating healthily, staying active, and limiting alcohol consumption can increase life expectancy by more than a decade (41).
A critical issue hindering improvements in public health is our inability to effectively communicate the current knowledge on avoidable cancer risk factors to the general population and implement interventions to minimize these risks. In fact, according to a recent report, most U.S. adults are still unaware of the significant cancer risks associated with obesity and alcohol use (42). This emphasizes the continued need for widespread dissemination of our current knowledge of these cancer risk factors, as well as the implementation of known preventive strategies to reduce risky behaviors in all population groups. In addition to health benefits, effective implementation of preventive measures may also lead to significant economic savings over time. Targeted efforts are also important since certain segments of the U.S. population, such as racial and ethnic minorities, and individuals of lower socioeconomic status, are disproportionately exposed to many of the potentially avoidable risk factors (see sidebar on Disparities in the Burden of Avoidable Cancer Risk Factors)
Eliminate Tobacco Use
Tobacco use is the leading preventable cause of cancer and deaths from cancer. It causes cancer because tobacco or secondhand smoke exposes individuals to many harmful chemicals that damage DNA, causing genetic and epigenetic alterations that lead to cancer development (48-50).
Smoking is linked to 17 different types of cancers in addition to lung cancer (see Figure 4), and in 2014, which is the most recent year for which data are available, it caused about 169,180 deaths from cancer (51)(21). Even individuals who smoke fewer than one cigarette per day over their lifetime have a higher risk of death than nonsmokers. Fortunately, cessation at any age can reduce the risk of cancer occurrence and cancer-related death (52)(53). Thus, one of the most effective ways a person can lower his or her risk of developing cancer and lower his or her risk of other smoking-related conditions such as cardiovascular, metabolic, and lung diseases, is to avoid or eliminate tobacco use.
Implementation of major public education and policy initiatives has significantly lowered cigarette smoking rates in the United States; the number of adult cigarette smokers dropped from 21 percent in 2005 to 14 percent in 2017 (20). The proportion of U.S. adult ever-smokers who quit smoking also significantly increased during the same period (54). Despite these trends, more than 37 million adults were still smoking in 2016 (54). There are striking sociodemographic disparities in smoking behavior (see sidebar on Disparities in the Burden of Avoidable Cancer Risk Factors). Thus, it is imperative that researchers, advocates, and policy makers continue to work together to develop and implement population-based interventions that have been shown to decrease smoking and lung cancer rates such as tobacco price increases, public campaigns, and smoke-free laws to reduce smoking and related cancer burden in the United States (see sidebar on Highlighting the New Corrective Advertisements) (55). Moreover, we need to use current tobacco-cessation strategies more widely because nicotine replacement therapy, use of prescription medications (e.g., buproprion and varenicline), and counseling have all been shown to be effective in enhancing the chances of long-term abstinence from smoking (56).
The use of other combustible tobacco products, such as cigars, smokeless tobacco products (e.g., chewing tobacco and snuff), and waterpipes are also associated with cancer (57). Electronic cigarettes (e-cigarettes) are a rapidly emerging tobacco product. With more than 2 million middle- and high-school students reporting using e-cigarettes in 2017 (58), their use among U.S. youth is a major public health concern (see sidebar on E-Cigarettes: What Have We Learned and What Do We Need to Know?) (59-60). A major appeal of e-cigarettes is the flavorings used in the e-liquids, which are often labelled as kid-friendly food products, such as juice, candy, or cookies (61). E-cigarette advertising is associated with higher use among youth (62). Therefore, it is concerning that nearly 80 percent of middle and high school students in the U.S. were exposed to e-cigarette advertisements in 2016 (63). Effective strategies to curb youth access to e-cigarettes and their exposure to e-cigarette advertising needs to be developed and implemented.
More research is needed to understand the long-term health risks of e-cigarettes as well as their effectiveness in smoking cessation (59). Based on current evidence the value of e-cigarettes in smoking cessation is equivocal (65-67). However, there is strong evidence that the use of e-cigarettes may act as a gateway to smoking in youth (61)(68). Therefore, researchers, clinicians, advocates, and policy makers must continue to work together to educate the public about the health risks and identify best strategies to implement population-level regulations to reduce e-cigarette use among youth and young adults.
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Maintain a Healthy Weight, Eat a Healthy Diet, and Stay Active
Researchers estimate that 15 percent of all cancer cases diagnosed in the United States are related to people being overweight or obese, inactive, and/or eating a poor diet (21). Therefore, maintaining a healthy weight, being physically active, and consuming a balanced diet are effective ways a person can lower his or her risk of developing or dying from cancer (see sidebar on Reduce Your Risk for Cancer Linked to Being Overweight or Obese, Being Inactive, and/or Consuming a Poor Diet). Exactly how obesity increases a person’s risk for cancer is not well understood, but accumulating evidence indicates a role for inflammatory immune cells in fat tissue (69).
Being overweight or obese as an adult increases a person’s risk for 15 types of cancer (see Figure 5) (72-73). In 2014, overweight- and obesity-related cancers accounted for 40 percent of all cancer cases in the United States (45). Therefore, it is concerning that in the U.S., adult obesity rates nearly doubled over the past two decades, and only half of U.S. adults meet the recommended federal physical activity guidelines (see sidebar on Physical Activity Guidelines) (20). These findings underscore the need for nationwide efforts to prevent and treat overweight and obesity.
The epidemic of obesity is directly related to an unbalanced energy intake. “Energy balance” refers to the difference between the number of calories consumed and the number burned. Tipping of this balance so that a person accumulates excess energy, which is stored in the body as fat, plays a crucial role in promoting obesity. While calories are consumed only through eating and drinking, they are burned in many ways. Simply existing, breathing, digesting food, and pumping blood around the body use some calories. Added to these expenditures are the calories burned through a person’s daily routine; the more physical activity in a routine, the more calories are burned.
Intensive efforts by all stakeholders are needed if we are to increase the number of people who consume a balanced diet, such as that recommended by the U.S. Department of Health and Human Services and the U.S. Department of Agriculture in the 2015—2020 Dietary Guidelines for Americans (74). One recent policy initiative that is aimed at reducing obesity is the introduction of taxes on sugar-sweetened beverages, which is a major contributor of caloric intake among U.S. youth and adults, in seven local jurisdictions within the U.S. (75-77). Ongoing research is needed to evaluate the long-term effects of such policies on consumption, obesity, and obesity-related health outcomes.
Unfortunately, the burden of diet-related diseases, including cancer, is disparately higher in low-income communities (70). Low-income populations frequently live in “food deserts,” which are neighborhoods lacking access to healthy food retail such as supermarkets, while having an overabundance of unhealthy and fast food options (70). Food deserts are associated with chronic conditions including obesity (78). The U.S. Department of Agriculture estimates that more than 23 million people live in low-income areas that are more than a mile (in the case of urban areas) or 10 miles (for rural areas) away from the nearest supermarket (70). These findings underscore the need for evidence-based health improvement strategies to increase access to affordable and nutritious food for all populations. Furthermore, educational interventions are essential to improve nutritional knowledge among low-income residents of food deserts, considering recent observations that the variation in access to supermarkets accounts for only a part of the difference in healthy eating between high- and low-income households (79). Also contributing to disparities in the burden of obesity-related diseases in low-income communities in urban areas is a lack of safe and affordable options for physical exercise, such as gyms, bike trails, and walking paths (70).
Limit Alcohol Consumption
Researchers estimate that alcohol consumption will be responsible for 5.6 percent of the new cancer cases diagnosed in the United States in 2018 and for 4 percent of the cancer deaths (21). Even modest use of alcohol may increase cancer risk, but the greatest risks are associated with excessive and/or long-term consumption (80-82) (see sidebar on Guidelines for Alcohol Consumption). Thus, it is concerning that in the United States, there has been a dramatic rise in high-risk alcohol consumption, with nearly 37 million adults reporting binge drinking, once a week, in 2015 (83-84).
Alcohol consumption has been causally linked with six different types of cancers (see Figure 6) (85). Researchers have identified multiple ways in which alcohol may increase the risk of cancer, including directly damaging cellular DNA and proteins through the production of toxic chemicals, once alcohol is metabolized after drinking (86-87).
Beyond the United States, alcohol poses a significant public health challenge globally, with a toll of more than 3 million deaths worldwide each year due to excessive use (86). These data underscore the importance of adherence to comprehensive guidelines, thus limiting alcohol intake (for those who drink) to minimize the risk of developing a disease or dying due to alcohol. Future efforts focusing on public education and evidence-based policy interventions, such as regulating alcohol retail density, taxes, and prices, need to be implemented along with effective clinical strategies to reduce the burden of cancer related to alcohol abuse.
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Protect Skin from UV Exposure
Exposure to ultraviolet (UV) radiation from the sun or indoor tanning devices can cause genetic mutations and poses a serious threat for the development of all three main types of skin cancer—basal cell carcinoma, squamous cell carcinoma, and melanoma, which is the deadliest form of skin cancer. Thus, one of the most effective ways a person can reduce his or her risk of skin cancer is by practicing sun-safe habits and not using UV indoor tanning devices (see sidebar on Ways to Protect Your Skin).
In the U.S., melanoma incidence among non-Hispanic whites has increased in the past decade, particularly in individuals older than 55 years (89). To break the current trend by establishing skin cancer prevention as a national priority, the U.S. Surgeon General released A Call to Action report in 2014 (90). Since its release, multiple sectors including health care, government, business, advocacy, and communities have coordinated efforts and made major strides toward reducing risk exposure. As a result, indoor tanning among U.S. youth and adults has decreased significantly (91-92). However, even in 2015, an estimated 7.8 adults and 1.2 million high school students engaged in indoor tanning and many reported experiencing sunburns (91-92). Continued efforts from public health and medical communities are needed to further reduce the prevalence of indoor tanning and sunburn. Reducing indoor tanning has the potential to reduce melanoma incidence and mortality, as well as the economic costs related to skin cancers (93).
Prevent Infection with Cancer-causing Pathogens
Persistent infection with several pathogens—bacteria, viruses, and parasites that cause disease—increases a person’s risk for several types of cancer (see Table 4). The most recent estimate is that 15 percent of all new cancer cases diagnosed worldwide in 2012 were attributable to persistent infection with pathogens, the most common of which were Helicobacter pylori, human papillomavirus (HPV), hepatitis B virus (HBV), and hepatitis C virus (HCV) (95). Thus, individuals can significantly lower their risks for certain types of cancer by protecting themselves from infection with cancer-associated pathogens or by obtaining treatment, if available, to eliminate an infection (see sidebar on Preventing or Eliminating Infection with the Four Main Cancer-causing Pathogens).
Although there are strategies available to eliminate, treat, or prevent infection with Helicobacter pylori, HBV, HCV, and HPV that can significantly lower an individual’s risks for developing an infection-related cancer, it is important to note that these strategies are not effective at treating infection-related cancers once they develop. It is also clear that these strategies are not being used optimally. For example, even though the U.S. Preventive Services Task Force (USPSTF) recommended one-time HCV testing for baby boomers in 2013, data from a 2015 national survey showed that only 10.5 out of 76.2 million adults eligible for testing reported getting tested (96). Given that infection with HCV is estimated to be responsible for six out of 10 liver cancer cases diagnosed in the United States since 2000, the burden of this disease could be significantly reduced through more effective implementation of HCV screening and treatment (95) (75).
In the United States, an average of 38,793 HPV-associated cancers were diagnosed annually, from 2008 to 2012 (97). Research suggests that HPV vaccination could prevent nearly all cases of cervical cancer and many cases of oral and anal cancer. However, less than 50 percent of adolescents aged 13 – 17 years were up to date with the recommended HPV vaccination series in 2016 (47). This rate of uptake is much lower than occurs for other vaccinations received in adolescence (47). Thus, development of effective strategies to increase the uptake of HPV vaccines could have an immense impact on cancer prevention (see sidebar on HPV Vaccination Recommendations). In this regard, one recent clinical trial showed an increase in vaccination rates when health care providers used a multicomponent intervention to facilitate communication with patients and their parents about the benefits of the HPV vaccine (98).
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Limit Exposure to Environmental Risk Factors
Environmental exposures to pollutants and occupational agents can increase a person’s risk of cancer. For example, radon, a naturally occurring radioactive gas that comes from the breakdown of uranium in soil, rock, and water, is the second leading cause of lung cancer in the United States (102). Other examples of environmental cancer risk factors include asbestos, lead, radiation, and benzene. According to the World Health Organization, environmental risk factors account for nearly 20 percent of all cancers, globally, most of which occur in low- and middle-income countries.
It is often difficult for people to avoid or reduce their exposure to many environmental cancer risk factors, and not every exposure will lead to cancer. The intensity and duration of exposure, combined with an individual’s biological characteristics, including genetic makeup, determine the individual’s chances of developing cancer over his or her lifetime. In addition, when studying environmental cancer risk factors, it is important to consider that exposure to several environmental cancer risk factors may occur simultaneously. A recent study from the U.S. Environmental Protection Agency (EPA) evaluated the environmental quality index, a measure of overall environmental exposures, and found a potential increase in overall cancer incidence with decreasing environmental quality (103).
Growing knowledge of the environmental pollutants to which different segments of the U.S. population are exposed highlights new opportunities for education and policy initiatives to improve public health. For example, arsenic exposure is a well-established cause of bladder and skin cancer, which is why, in 2006, the EPA lowered the maximum contaminant level for arsenic in public water systems from 50 to 10 micrograms per liter. As a result, the urinary arsenic content in individuals who used public water was significantly reduced between 2003 and 2014 (104). These reductions in exposure translated to an estimated 900 fewer cases of lung and bladder cancers or 50 fewer cases of skin cancer per year (104).
Involuntary exposures to environmental pollutants usually occur in subgroups of the population, such as workers in certain industries who may be exposed to carcinogens on the job or individuals living in low-income neighborhoods. Similarly, there are disparities in the burden of cancers caused by environmental exposures based on geographic locations and socioeconomic status (14) (16). As we learn more about environmental and occupational cancer risk factors and identify those segments of the U.S. population who are exposed to these factors, we need to develop and implement new and/or more effective policies that benefit everyone, including the most vulnerable and underserved populations.
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