Nutrition 331: Nutrition for Health
Study Guide: Unit 13
Diet and Cancer
Cancer is a broad disease family covering over 170 types of cancer. Lifestyle factors contribute to many of them. This unit will focus on nutrition links to prominent types of cancer: lung, colon, and breast cancers. About 30% of all Canadian deaths are caused by cancer. The types of cancer that are the leading causes of death include lung, colon, prostate (men), pancreas, and breast (women). There is a perception that breast cancer is a leading cause of death for women, yet almost four times as many women die from heart disease as from breast cancer.
Investigating the cause-and-effect relationship between diet and cancer is a complex undertaking, largely because the various dietary factors are highly interrelated. For example, populations that consume a low-fat/high-fibre diet have a low incidence of cancers that are common in the Western world. But is this a cause-and-effect relationship? It might be. On the other hand, this dietary pattern could simply be associated with the true protective factor. Possible protective factors might be exercise, a diet rich in plant foods, not smoking, or many other things.
After completing this unit, you should be able to
- discuss the problems in interpreting the results of research studies on the relationship between diet and risk of cancer.
- discuss dietary factors that help prevent colon cancer, and describe the known mechanisms by which this occurs.
- discuss the relationship between diet, lifestyle, and breast cancer.
- discuss the relationship between diet, meat, and cancer
- discuss the role of fruit, vegetables, and whole grains in cancer prevention.
- discuss the role of selenium and vitamin D in cancer.
Section 1 Causes of Cancer
Read pages 491–497, “Nutrition and Cancer” of Chapter 11: “Diet and Health.” Table 11‑8 is optional.
Do you have a mental short-list of risk factors for cancer including items such as tobacco use or sun exposure? Is the list a long one? Often our perceptions of cancer-causing agents are inconsistent with reality. Many people blame environmental toxins or food additives as leading causes of cancer, whereas they account for only two and one percent, respectively, of all cancers. The food additive that contributes specifically to cancer is sodium. Diet and obesity however, contribute to 30% of all cancers (see Table 13.1).
|family history of cancer||5%|
|viruses/other biological agents||5%|
|perinatal growth factors/growth||5%|
|prescription drugs/medical procedures||1%|
|salt and other food additives/ contaminants||1%|
Source: Harvard report on cancer prevention, Volume 1: Causes of human cancer (1996). Harvard School of Public Health.
Determining causes of cancer requires patience and detective work. Epidemiological studies and animal research are the two main methods of identifying causes of cancer. As discussed in Unit 2, both methods have inherent flaws. Clinical research trials are seldom practical with cancer because of the long period (often decades) between exposure to the cause and the initial presence of cancer cells. Consequently, researchers must piece together small clues to see whether the bulk of the evidence supports a specific theory. Generally, health experts are looking for consistent results between different epidemiological research methods, asking whether a cancer can be attributed to specific causes (in the case of nutrition and diet, specific nutrients) and whether there are biologically plausible explanations for the cancer (among other considerations).
Newspapers, television, and radio often carry health-related stories. Regularly, we hear reports connecting a type of cancer to a specific nutrient or lifestyle facet. This contributes to the perception that everything causes cancer. News stories, however, rarely include a discussion of how the research fits into a complete picture regarding a specific cancer. Critical thinking skills are required to keep a perspective on how specific research studies fit into the overall recommendations to lower cancer risks. “Consumer Corner: Reading Nutrition News with an Educated Eye” (textbook, p. 18) serves as a guideline for interpreting news stories. Also, a key question to ask is whether the study demonstrates association or causation. Epidemiological studies, often used in cancer research, demonstrate association.
Section 2 Lung Cancer
The main lifestyle contribution to lung cancer is clearly tobacco. However, smokers with higher consumption of fruits and vegetables have demonstrated lower lung cancer rates than those who consumed fewer fruits and vegetables. This link has been further researched to determine which nutrients may be most beneficial. Beta-carotene seemed to be the key nutrient and quickly, supplement companies started selling beta-carotene supplements and included beta-carotene in one-a-day vitamin pills. However, clinical intervention trials with long-term smokers (followed for several years) showed that beta-carotene increased lung cancer rates. Anticarcinogenic benefits may come from other nutrients typically found in diets rich in vegetables and fruit or from the synergy between nutrients. Synergy is when two or more nutrients acting together create a greater benefit than would be expected from them individually.
Section 3 Colon Cancer
Fibre is one of the better understood food components that helps prevent colon cancer. As described in the textbook (pp. 495–496), there is still room for doubt about the specific role of fibre in preventing colon cancer. The uncertainty may be due to the different types of fibre and the incomplete data on these types of fibre in food composition databases (e.g., databases don’t differentiate between viscous and nonviscous fibre).
Many case-control and cohort studies have examined the relationship between fibre intake and risk of colon cancer. These studies have failed to provide convincing evidence that fibre does indeed prevent colon cancer. One finding that has emerged is that people whose diets are rich in fibre have a lowered risk of colon cancer (WCRF/AICR, 2007). But does this mean that it is fibre that prevents cancer or other substances associated with fibre?
Higher than average vegetable intake is related to lower colon cancer risks, in part due to the fibre and carotenoids found in the vegetables. Fruits do not seem to provide the same benefit. The specific relationships between cancer risk and dietary factors are difficult to identify with confidence because there are many confounding variables. This challenge is discussed in Unit 2.
Overall, the evidence suggesting that a diet rich in fat increases the risk of colon cancer is rather weak (Davis & Milner, 2012). However, meat consumption has a much more consistent association with colon cancer. Numerous studies have revealed that consumption of red meat and processed meat are each associated with a 25% higher risk of colon cancer (Wang et al., 2016).
A diet that appears to reduce the risk of colon cancer is high in whole grains, vegetables, and milk products, low in fat, and limited in red meat content. This diet is consistent with Eating Well with Canada’s Food Guide.
Section 4 Breast Cancer
The relationship between diet and breast cancer is complicated by the age of women when it is diagnosed. Postmenopausal breast cancer, which accounts for 80% of cases, has more contributing lifestyle causes than premenopausal breast cancer.
Obesity has one of the strongest lifestyle relationships to breast cancer. The textbook discusses the role of different types of fat in breast cancer (p. 494). As a whole, this evidence suggests—but is far from conclusive—that dietary fat increases the risk of breast cancer. For each study that reports a link between dietary fat and the risk of breast cancer, another shows no association.
As with several other types of cancer, alcohol intake is also related to breast cancer development. Two or more drinks per day appears to increase risk. The reason for this relationship is not really understood.
Section 5 Meat
Taking the evidence as a whole, findings from cohort studies have reported that people who eat relatively high amounts of red meat or processed meat have a raised risk of developing cancer in comparison to people who eat relatively little meat (Wang et al., 2016). This association is somewhat stronger for processed meat than for red meat. This evidence suggests that a relatively high intake of red meat and processed meat increases the risk of cancer, especially colon cancer.
Section 6 Fruit, Vegetables, Whole Grains, Phytochemicals, and Cancer Prevention
Much research has been conducted regarding whether specific vitamins or minerals are protective against cancer, but it has proven tremendously difficult to reach firm conclusions. There are several reasons for this difficulty. One problem is obtaining accurate information on nutrient intakes. Another is that micronutrients are often correlated with one another and with other food components. For example, a diet rich in vegetables will contain not only plenty of fibre, but also a lot of vitamin C, carotenoids, folate, and potassium. Also, in addition to nutrients, there are hundreds of other substances in food that might positively or negatively affect health. As a result, when case-control studies indicate that various nutrients are associated with a particular cancer, it is difficult to determine which associations are causal.
The textbook (pp. 496–497) discusses the protective role of fruit and vegetables against cancer. Hundreds of case-control and cohort studies over the past 30 years support this (WCRF/AICR, 2007; Wang et al., 2014). There are hundreds of phytochemicals in fruit and vegetables that might deserve some of the credit for protection against cancer. Perhaps one day researchers will be able to formulate a dietary supplement that contains the right substances in a purified form. For now, the best advice is to eat generous amounts of a variety of fruit and vegetables.
Whole Grain Cereals
Many cohort studies have been carried out in recent decades and these suggest that people who have a relatively high intake of whole-grain cereals have a modestly lower risk of cancer (Zong et al., 2016). Attempting to identify the reason for this is no easier than is the case with fruit and vegetables. These foods are an important source of fibre and folate. As with fruit and vegetables, phytochemicals may turn out to be the explanation.
Section 7 Micronutrients and Cancer Prevention
An especially important development in recent years has been reports that vitamin D may have a strong protective association with the risk of cancer (Yin et al., 2013; Keum & Giovannucci, 2014). This finding emerged when it was noticed that mortality rates for several major types of cancer in the United States are considerably higher in the cold northern states than in the sunny southern states. The obvious explanation for this is that sunshine boosts production of vitamin D and thereby helps prevent cancer. Further studies have revealed that people with higher blood levels of vitamin D do have lower risk of developing some forms of cancer. The strongest protective effect has been reported for colon cancer (Touvier et al., 2011).
While the evidence is not conclusive and research is ongoing, overall, vitamin D supplements appear to have the potential to lower colon cancer rates by around 12% for people who have poor exposure to the sun for much of the year; this clearly includes the whole of Canada. The ideal dose for prevention is significantly above the RDA (currently 15 micrograms or 600IU per day at ages up to 70). Most people in Canada receive much less than this amount. Thus, a supplemental dose of 25 micrograms (1000IU) per day is indicated for the prevention of cancer. (Note: μg is the abbreviation for micrograms.)
Population studies have pointed to an inverse relationship between selenium intake and several types of cancer. Cohort studies have generally demonstrated a protective effect by selenium. Dozens of animal studies have also generally demonstrated that selenium supplementation can reduce cancer incidence. This evidence suggests that selenium may be a valuable anticarcinogen. However, we are still well short of proof. For example, most animal studies have used a nearly toxic dose of selenium. Experiments with lower levels of the mineral, levels more akin to the normal human intake, have shown less of a protective effect. As stressed before, human studies can often generate misleading results. So far, the results of a randomized intervention study have been encouraging; fewer cancers were recorded in the supplemented group (Lee et al., 2011). However, more such studies are needed. If selenium proves to be effective as an anticarcinogen, it will have to be used carefully as a supplement because of its documented toxicity and its narrow range of safety.
Sodium appears to increase the rate of stomach cancers. Epidemiological studies show that Asian countries (like Japan) have much higher rates of stomach cancers than Canada does. People in Asian countries also consume much more sodium than Canadians do.
The various lines of evidence discussed in this unit point to the possibility that one day scientists may formulate some combination of micronutrients and phytochemicals that provides a cheap, safe, and effective means to prevent a substantial fraction of cancers. For now, the best advice is to eat a healthy diet, paying particular attention to consuming a generous amount of fruit, vegetables, and whole grain cereals, limiting the intake of red meat and processed meat, avoiding excessive weight gain, exercising often, and, of course, not smoking! If everyone in Canada followed such a lifestyle, there is little doubt that cancer rates would fall by more than half.
Assignment 1: Diet Analysis
If you have not already handed in Assignment 1: Diet Analysis, you should complete and submit it now through the appropriate assignment drop box. This will allow you to get feedback on the assignment prior to writing the final exam. If you have any questions, please contact your tutor for assistance.