5. Population nutrient intake goals for preventing diet-related chronic diseases:
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5.5 Recommendations for preventing cancer
Cancer is caused by a variety of identified and unidentified factors. The most important established cause of cancer is tobacco smoking. Other important determinants of cancer risk include diet, alcohol and physical activity, infections, hormonal factors and radiation. The relative importance of cancers as a cause of death is increasing, mostly because of the increasing proportion of people who are old, and also in part because of reductions in mortality from some other causes, especially infectious diseases. The incidence of cancers of the lung, colon and rectum, breast and prostate generally increases in parallel with economic development, while the incidence of stomach cancer usually declines with development.
Cancer is now a major cause of mortality throughout the world and, in the developed world, is generally exceeded only by cardiovascular diseases. An estimated 10 million new cases and over 6 million deaths from cancer occurred in 2000 (1). As developing countries become urbanized, patterns of cancer, including those most strongly associated with diet, tend to shift towards those of economically developed countries. Between 2000 and 2020, the total number of cases of cancer in the developing world is predicted to increase by 73% and, in the developed world, to increase by 29%, largely as a result of an increase in the number of old people (1).
5.5.3 Diet, physical activity and cancer
Dietary factors are estimated to account for approximately 30% of cancers in industrialized countries (2), making diet second only to tobacco as a theoretically preventable cause of cancer. This proportion is thought to be about 20% in developing countries (3), but may grow with dietary change, particularly if the importance of other causes, especially infections, declines. Cancer rates change as populations move between countries and adopt different dietary (and other) behaviours, further implicating dietary factors in the etiology of cancer.
Body weight and physical inactivity together are estimated to account for approximately one-fifth to one-third of several of the most common cancers, specifically cancers of the breast (postmenopausal), colon, endometrium, kidney and oesophagus (adenocarcinoma) (4).
5.5.4 Strength of evidence
Research to date has uncovered few definite relationships between diet and cancer risk. Dietary factors for which there is convincing evidence for an increase in risk are overweight and obesity, and a high consumption of alcoholic beverages, aflatoxins, and some forms of salting and fermenting fish. There is also convincing evidence to indicate that physical activity decreases the risk of colon cancer. Factors which probably increase risk include high dietary intake of preserved meats, salt-preserved foods and salt, and very hot (thermally) drinks and food. Probable protective factors are consumption of fruits and vegetables, and physical activity (for breast cancer). After tobacco, overweight and obesity appear to be the most important known avoidable causes of cancer.
The role of diet in the etiology of the major cancers
Cancers of the oral cavity, pharynx and oesophagus. In developed countries the main risk factors for cancers of the oral cavity, pharynx and oesophagus are alcohol and tobacco, and up to 75% of such cancers are attributable to these two lifestyle factors (5). Overweight and obesity are established risk factors specifically for adenocarcinoma (but not squamous cell carcinoma) of the oesophagus (6-8). In developing countries, around 60% of cancers of the oral cavity, pharynx and oesophagus are thought to be a result of micronutrient deficiencies related to a restricted diet that is low in fruits and vegetables and animal products (5, 9). The relative roles of various micronutrients are not yet clear (5, 9). There is also consistent evidence that consuming drinks and foods at a very high temperature increases the risk for these cancers (10). Nasopharyngeal cancer is particularly common in South-East Asia (11), and has been clearly associated with a high intake of Chinese-style salted fish, especially during early childhood (12, 13), as well as with infection with the Epstein-Barr virus (2).
Stomach cancer. Until about 20 years ago stomach cancer was the most common cancer in the world, but mortality rates have been falling in all industrialized countries (14) and stomach cancer is currently much more common in Asia than in North America or Europe (11). Infection with the bacterium Helicobacter pylori is an established risk factor, but not a sufficient cause, for the development of stomach cancer (15). Diet is thought to be important in the etiology of this disease; substantial evidence suggests that risk is increased by high intakes of some traditionally preserved salted foods, especially meats and pickles, and with salt per se, and that risk is decreased by high intakes of fruits and vegetables (16), perhaps because of their vitamin C content. Further prospective data are needed, in particular to examine whether some of the dietary associations may be partly confounded by Helicobacter pylori infection and whether dietary factors may modify the association of Helicobacter pylori with risk.
Colorectal cancer. Colorectal cancer incidence rates are approximately ten-fold higher in developed than in developing countries (11), and it has been suggested that diet-related factors mayaccount for up to80%of the differences in rates between countries (17). The best established diet-related risk factor is overweight/obesity (8) and physical activity has been consistently associated with a reduced risk of colon cancer (but not of rectal cancer) (8, 18). These factors together, however, do not explain the large variation between populations in colorectal cancer rates. There is almost universal agreement that some aspects of the “westernized” diet are a major determinant of risk; for instance, there is some evidence that risk is increased by high intakes of meat and fat, and that risk is decreased by high intakes of fruits and vegetables, dietary fibre, folate and calcium, but none of these hypotheses has been firmly established.
International correlation studies have shown a strong association between per capita consumption of meat and colorectal cancer mortality (19), and a recent systematic review concluded that preserved meat is associated with an increased risk for colorectal cancer but that fresh meat is not (20). However, most studies have not observed positive associations with poultry or fish (9). Overall, the evidence suggests that high consumption of preserved and red meat probably increases the risk for colorectal cancer.
As with meat, international correlation studies show a strong association between per capita consumption of fat and colorectal cancer mortality (19). However, the results of observational studies of fat and colorectal cancer have, overall, not been supportive of an association with fat intake (9, 21).
Many case-control studies have observed a weak association between the risk of colorectal cancer and high consumption of fruits and vegetables and/or dietary fibre (22, 23), but the results of recent large prospective studies have been inconsistent (24-26). Furthermore, results from randomized controlled trials have not shown that intervention over a 3-4 year period with supplemental fibre or a diet low in fat and high in fibre and fruits and vegetables can reduce the recurrence of colorectal adenomas (27-29). It is possible that some of the inconsistencies are a result of differences between studies in the types of fibre eaten and in the methods for classifying fibre in food tables, or that the association with fruits and vegetables arises principally from an increase in risk at very low levels of consumption (30). On balance, the evidence that is currently available suggests that intake of fruits and vegetables probably reduces the risk for colorectal cancer.
Recent studies have suggested that vitamins and minerals might influence the risk for colorectal cancer. Some prospective studies have suggested that a high intake of folate from diet or vitamin supplements is associated with a reduced risk for colon cancer (31-33). Another promising hypothesis is that relatively high intakes of calcium may reduce the risk for colorectal cancer; several observational studies have supported this hypothesis (9, 34), and two trials have indicated that supplemental calcium may have a modest protective effect on the recurrence of colorectal adenomas (29, 35).
Liver cancer. Approximately 75% of cases of liver cancer occur in developing countries, and liver cancer rates vary over 20-fold between countries, being much higher in sub-Saharan Africa and South-East Asia than in North America and Europe (11). The major risk factor for hepatocellular carcinoma, the main type of liver cancer, is chronic infection with hepatitis B, and to a lesser extent, hepatitis C virus (36). Ingestion of foods contaminated with the mycotoxin, aflatoxin is an important risk factor among people in developing countries, together with active hepatitis virus infection (13, 37). Excessive alcohol consumption is the main diet-related risk factor for liver cancer in industrialized countries, probably via the development of cirrhosis and alcoholic hepatitis (5).
Pancreatic cancer. Cancer of the pancreas is more common in industrialized countries than in developing countries (11, 38). Overweight and obesity possibly increase the risk (9, 39). Some studies have suggested that risk is increased by high intakes of meat, and reduced by high intakes of vegetables, but these data are not consistent (9).
Lung cancer. Lung cancer is the most common cancer in the world (11). Heavy smoking increases the risk by around 30-fold, and smoking causes over 80% of lung cancers in developed countries (5). Numerous observational studies have found that lung cancer patients typically report a lower intake of fruits, vegetables and related nutrients (such as b-carotene) than controls (9, 34). The only one of these factors to have been tested in controlled trials, namely b-carotene, has, however, failed to produce any benefit when given as a supplement for up to 12 years (40-42). The possible effect of diet on lung cancer risk remains controversial, and the apparent protective effect of fruits and vegetables may be largely the result of residual confounding by smoking, since smokers generally consume less fruit and vegetables than non-smokers. In public health terms, the overriding priority for preventing lung cancer is to reduce the prevalence of smoking.
Breast cancer. Breast cancer is the second most common cancer in the world and the most common cancer among women. Incidence rates are about five times higher in industrialized countries than in less developed countries and Japan (11). Much of this international variation is a result of differences in established reproductive risk factors such as age at menarche, parity and age at births, and breastfeeding (43, 44), but differences in dietary habits and physical activity may also contribute. In fact, age at menarche is partly determined by dietary factors, in that restricted dietary intake during childhood and adolescence leads to delayed menarche. Adult height, also, is weakly positively associated with risk, and is partly determined by dietary factors during childhood and adolescence (43). Estradiol and perhaps other hormones play a key role in the etiology of breast cancer (43), and it is possible that any further dietary effects on risk are mediated by hormonal mechanisms.
5. Population nutrient intake goals for preventing diet-related chronic diseases:
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