Questions and Answers
WCR webinars always plan plenty of time for participants to interact with guest speakers. Find out what advises and recommendations were provided in response to questions and concerns from the audience.
The answers below reflect the opinions and points of view of speakers.
DH => Answer from Professor David Hunter
BWS => Answer from Professor Bernard W. Stewart
BLS => Answer from Dr Béatrice Lauby-Secretan
Diet, physical activity, and body weight
DH: The evidence is strongest when it comes to the role of red meat and processed meat as causes of colorectal cancer, and perhaps some other digestive cancers.* These are probably the best-established risk factors. There is also some evidence that high consumption of dairy foods, which is often interpreted as high calcium consumption, may reduce risk of colorectal cancer.
For most other cancer types, the role of diet is not well established. The recommendations related to diet and cancer stress that the healthiest diet is mainly a plant-based diet, and that red meat and processed meat should be eaten, if at all, in moderation. The recommendations for a plant-based diet are partly built around less-definitive evidence for some cancers, and partly because such a diet is associated with lower risk of heart disease, stroke, and other noncommunicable diseases.
The other role of diet is its role in obesity. Obesity is a risk factor for many cancer types. Sugar-sweetened beverages and fast foods are clearly associated with obesity, and reduction in anything that causes obesity is going to reduce risks of many types of cancer. Increased physical activity is also usually recommended as a means of limiting weight gain.
*Note from IARC: In October 2015, the IARC Monographs Programme evaluated the carcinogenicity of red meat and of processed meat. Consumption of red meat was classified as probably carcinogenic to humans (Group 2A), based on limited evidence that the consumption of red meat causes cancer in humans and strong mechanistic evidence supporting a carcinogenic effect. This association was observed mainly for colorectal cancer, but associations were also seen for pancreatic cancer and prostate cancer. Consumption of processed meat was classified as carcinogenic to humans (Group 1). Consumption of processed meat causes colorectal cancer, and positive associations have been observed for stomach cancer. Learn more: https://www.iarc.who.int/featured-news/media-centre-iarc-news-redmeat/
DH: It is not clear whether physical activity has a beneficial effect on reducing cancer, independent of obesity. It probably does a little bit. The clearest message is that prevention of obesity does prevent a wide variety of cancers, and physical activity is an important means of reducing obesity.
DH: Right now, this proportion is modest, particularly in countries where the most common cancers (stomach, liver, and cervical cancers) are associated with infections. But as the obesity epidemic is growing rapidly in almost all countries, the proportion of cancers due to obesity is almost guaranteed to increase in low- and middle-income countries over time.
Obesity-related cancers, which are observed in developed countries, will become more common in less-developed countries as these move up the Human Development Index.
Interventions and policy
BWS: In high-income countries, health promotion and education in schools can have an impact. Examples of this in Australia, for instance, include a reduction in sun exposure in infants and children due to increased wearing of hats, etc., and a reduction of tobacco smoking in the teenage population due to increased education on the harmful effects of smoking.
DH: There is indeed an example of a success story on controlling sun exposure in Australian schools. Shades were installed over playgrounds, all kids are wearing hats, and in some parts of Australia, the school-day schedule has been changed to limit kids’ exposure during peak periods of sunlight. These changes are impressive, considering that 50 years ago, the dogma for young people was that you were supposed to get a tan as early as possible in summer and retain it.
This 180-degree shift in the social attitude towards sun exposure shows that even behaviours that we think are completely entrenched, socially determined, and non-modifiable can be changed.
In India, one issue is children’s exposure to smokeless tobacco and betel quid at a young age. There is very good evidence that this exposure leads to oral cancer, and someone starting that behaviour at age 10 is much more susceptible than somebody starting that behaviour at age 25, not just because they’ll be using those products for a much longer period but because somebody who’s growing is more susceptible than somebody who’s completed their growth. Legislation is needed, and enforcement of that legislation is even more important.
BLS: The IARC Handbooks Programme has just reviewed the evidence on primary and secondary prevention of oral cancer. We have evaluated 1) whether reducing exposure to oral cancer risk factors reduces incidence or mortality of cancer of the oral cavity; and 2) whether interventions to quit consumption of smokeless tobacco and areca nut are effective. Behavioural interventions in adults were considered effective, and those in adolescents, including in school, may be effective.
BWS: Communication with decision-makers (e.g. governments, regulators) is challenging and in many societies is in competition with a wealth of matters that are being brought to their attention as desirable aspects for action. It is up to the medical and scientific community to advocate as clearly as possible in respect of proven measures to reduce cancer incidence. Some measures have been communicated with great success (e.g. the law on cigarette packaging in Australia). For others, such as discouraging the consumption of sugar-sweetened beverages, there is still a great challenge. Much of the response is to say that regulatory intervention on this matter involves some sort of “nanny-state” attitude, and it should be up to the individuals to decide for themselves what the evidence indicates and what actions they should take.
BWS: In terms of recommendations, there is not an immediate priority action list. The challenge is very context- and region-specific; general principles cannot be applied worldwide. Reducing inequalities in cancer prevention services in a country depends on the resources available and the priorities adopted by governments and on the extent to which actions can be taken that will make an impact. Bringing cancer prevention programmes to disadvantaged communities is often achievable, but removing the causes of the inequalities is a much bigger challenge.
OTHER QUESTIONS
DH: There are clear socioeconomic disparities in cancer risks. Those disparities may relate to 1) causes of cancer; 2) higher prevalence of risk factors and exposure to those cancer causes; or 3) lack of access to services, such as cancer screening and optimal treatments, which results in higher death rates. To the extent that there is more stress among people with lower socioeconomic status, there is that sort of association, but there is no solid evidence that it is the stress itself that is causing the cancer; it could just be a correlated factor. Most of the studies that have assessed people’s stress and level of happiness have not seen a relationship with any cancer type.
There is another body of evidence in people with cancer, about the association between mindfulness and relaxation for stress reduction and risk of cancer death. There are positive studies and negative studies, which should be looked at with caution. However, people in these studies report a higher quality of life if they can participate in group therapy, individual therapy, and other therapies that make the cancer treatment process less stressful.
In summary, there is no great evidence for stress and psychosocial attitudes, independent of socioeconomic status, on causes of cancers. Stress reduction as a means to treat cancer is mainly about improving quality of life, not necessarily extending life.
BWS: In a study by Willcox et al.,* 4000 cancer patients were surveyed and asked to state what they thought caused their cancer. Roughly 2000 said they had no idea, and an overwhelming majority of the other 2000 stated that they thought cancer was caused by stress. Therefore, there is a community perception related to stress as a risk factor for cancer that does not match the available scientific data.
*Willcox SJ, Stewart BW, Sitas F (2011). What factors do cancer patients believe contribute to the development of their cancer? (New South Wales, Australia). Cancer Causes Control. 22(11):1503–1511. https://doi.org/10.1007/s10552-011-9824-6
DH: It was thought that liver cancer was due, almost exclusively, to hepatitis B, hepatitis C, and aflatoxins. That was – and still is – largely the case in areas of the world where liver cancer incidence is higher. However, as the obesity epidemic accelerates in developed countries, we are seeing more liver cancer than we should, considering the vaccination coverage against hepatitis B and the potential elimination of hepatitis C. This is because a fatty liver is a liver that is being injured and is more likely to develop liver cancer (in the absence of high alcohol intake or infectious causes). This is an obesity-attributable component of liver cancer largely in developed countries, and another reason why avoiding obesity is critical to the reduction of the cancer burden.
BWS: Susceptibility to sunlight-induced skin cancer varies markedly with racial origin (i.e. skin type) and genetic makeup. Sunscreen use is the most effective method of skin cancer prevention for the most vulnerable population (e.g. pale skin, red hair, freckles). It is less important for people with olive or darker skin types, who are more resistant and less susceptible to ultraviolet-induced carcinogenesis.