Cancer Risk Factors
In total, 1.3 billion people smoke globally, making tobacco a major avoidable cause of disease and mortality. The link between cancer and tobacco smoking was established in the 1950s, and was identified as the primary cause of lung cancer in the US Surgeon General's Advisory Commission Report and, ever since, efforts have been ongoing to reduce tobacco use. According to the WHO, tobacco is the single largest preventable cause of cancer in the world today, causing 80–90% of all lung cancer deaths. Additionally, over US$96 billion in health expenses and over $97 billion in loss of productivity are attributed to smoking each year in the USA alone. Smoking is linked to developing cancers of lung, oral cavity, nasal cavity, pharynx, larynx, esophageal, stomach, pancreas, liver, bladder, kidney, uterine cervix and myeloid leukemia.
There is convincing evidence that the risk of cancer follows an exposure–response relationship, with increases in cancer rates associated with the number of cigarettes smoked, deepness of inhalation and duration of smoking. While cigarette smoking is the most common form of tobacco use, exposure to tobacco and tobacco smoke in the environment, cigars, pipes and chewing tobacco also increase cancer risks.
Smoking rates in adults have been decreasing for both men and women for decades, particularly in men in North America and several European countries, as seen in Figure 1. Lung cancer rates in men are also starting to decrease in the USA and Europe. In 1965, 42.4% of adults in the USA were current cigarette smokers, compared with 19.8% in 2007. Given its success in terms of sustained, population-wide change, tobacco control has been described as one of the "ten greatest public health successes of the 20th Century". The success of tobacco control is owing to a multiplicity of influences, including a focus on both cessation and prevention, the combination of pharmacologic and behavioral techniques, and the use of effective communicators (e.g., physicians) in stop-smoking efforts. As the US CDC note, effective tobacco control has been driven by changes in the social norms surrounding smoking, including changes in public policy about where smoking is allowed and not allowed (e.g., workplaces, airplanes and restaurants), counter advertising media messages and tobacco advertisement bans. One of the most effective means of reducing the prevalence of tobacco use is by increasing tax rates. A 10% increase in the price of cigarettes leads to an approximate 4% reduction in the demand for cigarettes. Further data indicate that the more comprehensive, varied and sustained a program, the more successful, and that higher tobacco product prices plus mass media and public education are synergistic.
Despite these positive accomplishments, the current US smoking rate of approximately 20% is still higher than the 2010 Healthy People Goal of 12%. In addition, there are subgroups and regions where smoking rates are increasing, as seen in Figure 1, particularly in the Western Pacific (including China) and the Eastern Mediterranean, where increased smoking rates are seen, particularly for women in eastern, central and southern Europe.[20,21] Almost two-thirds of the world's current smokers are from ten countries, seven of which are developing countries. China alone accounts for approximately 30% of smokers worldwide. The challenge for future cancer prevention and tobacco control strategies is to target smoking in women and in developing countries,[1,21] and initiatives in this area are discussed in more detail later in this review.
Exposure to Infectious Agents
Infectious agents – primarily viruses – account for approximately 15–20% of cancers worldwide. The proportion is higher in developing countries (26%) than in high-income countries (8%). The International Union Against Cancer (UICC) made cancer prevention and, in particular, cancer prevention related to infectious agents, its focus for the 2010 World Cancer Day campaign and released a summary report in this area.
Table 2 lists the leading infectious agents that have been linked with various cancers, as well as a summary of the preventive and early detection strategies associated with each. Table 3 lists the number of cancers associated with infections in both developed and developing countries and the number of cancers that could be prevented if effective preventive strategies were widely implemented. Several conclusions are clear: infectious agents are linked with a diversity of cancers; a variety of preventive strategies has already been identified, and the burden of infection-linked cancers is disproportionately high in developing countries. This is particularly true for liver, cervical and stomach cancers, which are challenging to treat effectively, even in developed countries, and hence, account for many deaths. A corollary to these summary statements is that implementing infection prevention and control strategies is one of the highest current priorities for global cancer control, and recent experience with vaccination programs is discussed later.
The interest in diet and cancer etiology was initially sparked by international differences in cancer rates and their covariation with traditional diets. Migrant studies have shown that individuals who relocate and adopt the diet and lifestyle of their new home are more likely to experience cancer rates of the new country rather than their country of origin. It has been estimated that 32% of cancer deaths are linked to diet (including but not limited to the dietary components discussed in more detail here). The extent to which cancer deaths are attributable to diet varies greatly according to the type of cancer. Some specific aspects of dietary components that have been strongly linked with cancer are now discussed; however, the reader is encouraged to consult the WCRF/AICR Report I for much more information regarding data linking nutrition to cancer, including dietary components, supplements, dietary patterns (such as the Mediterranean diet) and food-preparation techniques.
Alcohol Alcohol is responsible for nearly 4% of the global cancer burden. There is convincing evidence that alcohol consumption increases the risk of oral cavity, pharynx, larynx, esophagus, colorectal (in men) and breast cancer. There is probable evidence that alcohol consumption increases the risk of liver and colorectal cancers in women. The WCRF/AICR Report I recommends that alcoholic drinks, if consumed, should be limited to two drinks per day for men, and one drink per day for women. A survey of alcohol consumption in 63 countries across the world indicated considerable variability, but many levels were high, unhealthy and increasing in many regions. The greatest alcohol consumption was in the WHO region that includes the Russian Federation and the Ukraine, where 19% of the population is characterized as 'heavy drinkers' (males 640 g and females 620 g). Extrapolating from the trends they observe, the authors conclude that "the role of alcohol as a major factor in the burden of disease will even be increasing in the future".
Room et al. identified a range of strategies that have been shown to be effective in reducing alcohol consumption; these include regulations (e.g., controlling distribution and sales), taxation, enforcement (e.g., penalties for drinking and driving), and education and counselling. There is need to incorporate such interventions in a comprehensive program and, in January 2010, the WHO Executive Board adopted a resolution for a draft global strategy on the harmful use of alcohol. This resolution includes recommendations for public policy and interventions, with support of both national plans and international collaboration. A number of countries have developed comprehensive alcohol control programs (e.g., Canada, Australia and the UK) and the WHO initiative may enhance coordination across regions. Clearly, the impact of alcohol use goes beyond its effects on cancer and health conditions, and includes social outcomes as well.
Fruits & Vegetables The WCRF/AICR Report I states that consumption of fruits and vegetables have been associated with a probable decreased risk of mouth, pharynx, larynx, esophagus, stomach and lung cancers, and evidence is presented suggesting that eating certain fruits decreases risks for nasopharyngeal, colorectal, ovarian, endometrial, cervical, liver and prostate cancers. It should be noted that the report concludes that evidence supporting the efficacy of fruits and vegetables for cancer prevention is currently weaker than estimated in previous reports. Nonetheless, the WHO estimates that 2.7 million deaths (from both cancer and other diseases) are attributable to low fruit and vegetable intake, and most dietary guidelines stress the importance of fruit and vegetable consumption in the diet. The WCRF/AICR Report I recommends that individuals eat at least five daily portions (at least 400 g) of nonstarchy vegetables and of fruits every day as does the WHO panel on diet, nutrition and prevention of chronic diseases.
An international survey of fruit and vegetable consumption in approximately 200,000 people from 52 countries (most developing countries) found that 78% of respondents ate less than the recommended five servings. This figure is comparable with 75% in the USA, and 74 and 78% (for women and men, respectively) in England (UK). In recognition of the worldwide deficiency in fruit and vegetable consumption, the WHO has developed a global strategy in this area. Thus far, workshops have been held in Portugal, Japan, New Zealand and Switzerland, educational materials have been developed, and liaisons have been established with 17 national organizations that advocate fruit and vegetable consumption.
Red Meat High consumption of red meat is a risk factor for colorectal cancer, and there is suggestive evidence that it may be linked to esophageal, lung, pancreatic and endometrial cancers. A recent study in over 0.5 million people in the USA found consistent increases in cancer rates with increases in red meat consumption and concluded that 11% of cancer deaths in men and 16% in women could have been prevented with decreased red meat consumption at the level of the lowest quintile in the cohort. Similar decreases in deaths from heart disease were also estimated.
Walker et al. report that Americans consume, on average, almost 40% more meat (100.7 kg) than the upper limit recommended by the American Heart Association (62.6 kg). Historically, consumption of meat in developing countries has been significantly lower than that in developed countries. However, rapid increases in meat consumption in developing countries occurred during the 1980s and 1990s, with consumption predicted to continue to increase over the next decade but at a slower rate.[29,33]
The WCRF/AICR Report I recommends limiting weekly meat consumption to 500 g, including very little processed meat, in people who eat meat. Global policy regarding meat consumption is complex, given that meat is a dense source of nutrition that can be of significant value in undernourished populations. At the same time, a broader public health perspective on meat consumption needs to consider how meat production affects and is affected by global concerns, such as the availability of water, food subsidies, transportation costs and other issues, as Popkin eloquently describes.
Obesity is generally attributed to an energy imbalance between calories consumed and calories expended. Overweight is defined as a BMI (which reflects weight relative to height) of 25 or more, and obesity is defined as a BMI of 30 or more. Obesity (as assessed through body or abdominal fatness) has been demonstrated to confer a convincing increased risk for esophagus, pancreas, colorectal, breast (postmenopausal), endometrial and kidney cancer and a probable increase in cancers of the gallbladder, pancreas and liver. Of all cancer deaths in the USA, 10–20% are attributable to excess weight or obesity.[5,35] European figures are similar, with approximately 8% of cancer deaths attributable to overweight and obesity.
Obesity has more than doubled in many developed countries in the last 25 years and is attributed to changes in diet and physical activity that have resulted in an energy imbalance. The WHO stated that, in 2005, approximately 1.6 billion adults were overweight and at least 400 million adults worldwide were obese. Additionally, 20 million children under the age of 5 years were overweight. It is estimated that, by 2015, approximately 2.5 billion adults will be overweight, and more than 700 million adults obese. Recommendations for reducing obesity include limiting energy intake from saturated fats and sugars, increasing consumption of fruit and vegetables, legumes, whole grains and nuts, and increasing physical activity.
Despite the importance of obesity prevention and control, only limited literature regarding various interventions is currently available, with mixed results on the efficacy of different approaches. One ambitious population-based approach to reduce obesity entitled 'Healthy Eating – Healthy Action' is currently being implemented in New Zealand. This national program includes multiple approaches – individual and community change strategies, development of public policy, and creation of a broadly-based supportive environment – with special attention focused on the aboriginal population in which obesity levels are very high. Evaluation is an important part of this innovative program, and findings will be very useful in providing a foundation for future programs.
Being physically active has been shown to result in a convincing decreased risk of colon cancer and probable decreased risk for breast (postmenopausal) and endometrial cancer. There is suggestive evidence that physical inactivity is linked with lung, pancreas and premenopausal breast cancer.
The WCRF/AICR Report I (consistent with recommendations of other bodies, such as the CDC and the Canadian Cancer Society) recommends that individuals should be moderately physically active for at least 30 min each day. As fitness improves, 60 min or more of moderate, or 30 min or more of vigorous, physical activity every day are recommended, as well as limiting sedentary habits, such as watching television.
Owing to factors such as sedentary occupations and passive means of transportation, and a lack of participation in recreational physical activity, approximately 60% of the world population does not meet the recommended levels of physical activity, and the WHO has described physical inactivity as a "global public health problem". With the growth in urbanization worldwide, individuals in the developing world are becoming increasingly inactive, similar to their counterparts in developed regions. To reach these goals, the WHO has proposed attention to the environmental factors that contribute to physical inactivity – including population overcrowding, poverty, crime, traffic, air quality and a lack of parks, sidewalks and recreation facilities. This ecological approach, which emphasizes the need to address the environmental and social influences on physical inactivity, has been adopted in a number of different regions across the world, including Europe, Canada and Australia.
Expert Rev Pharmacoeconomics Outcomes Res. 2010;10(2):143-154. © 2010 Expert Reviews Ltd.
Cite this: Cancer Prevention: Major Initiatives and Looking into the Future - Medscape - Apr 01, 2010.