Vegetarianism and colon cancer

Colon cancer is strongly linked to an affluent western lifestyle and diet. As well-known epidemiologists have observed: (1)
" The incidence of colon cancer varies with geographic area and socio-economic level. It is higher in Northwest Europe, North America and other Anglo-Saxon areas, and low in South America, Africa and Asia. Epidemiologic data indicate that colon cancer incidence is associated mainly with environmental factors rather than genetic or racial factors. "

It is estimated that environmental and especially dietary factors are responsible for 85-90% of cases (2).

International comparisons

Several studies have investigated the relationship between death rates due to colon cancer and per capita consumption of various foods across different countries.

• Across 41 countries the highest correlation between colon cancer death rates and food intake was with meat, especially beef (3).

• Across 37 countries, colon cancer was linked to consumption of animal fat/animal protein, but intake of fibre had no effect (4).

• Across 30 countries, cancer rates were again linked to consumption of animal fat, especially meat, while intake of cereals had a protective effect (5).

• Across 20 industrialised countries, average cholesterol intake had the closest connection with colon cancer deaths, and cholesterol is found only in animal products (6).

Migration studies

As already pointed out, the differences in cancer rates between countries or ethnic groups are not due to genetic factors. This is demonstrated clearly when a particular ethnic group migrates from a low to a high cancer region.

Chinese living in mainland China have a 4-7 times lower rates of colorectal cancer than Chinese migrants to America. Their risk increases with number of years of living in America - as they become acculturated and change to a western diet, they develop colorectal cancer at the same rate as Americans. Relative to their diet in China, the migrants to America begin to eat more meat and fat, and less grains (7).

Mediterranean migrants to Australia in their home countries have half the rate of colon cancer of Australians. These migrants initially have a diet higher in cereals, pulses, nuts and vegetables, and lower in meat, eggs and total saturated fat. The longer they live in Australia and the more they change their traditional ways, the more their rate of colon cancer approaches that of the general population (8).

Group comparisons

Comparing groups that are known to have different diets can be very useful, because if certain aspects of the diet are related to cancer, these aspects should become evident in cases where the diets are very different.

In South Africa, black deaths due to colon cancer are only 1 per 100,000 compared to 17 per 100,000 white deaths. Whites eat much more meat and cheese, containing saturated fat and cholesterol, while blacks eat more maize and green vegetables (9). White volunteers had higher rectal cell proliferation than blacks, confirming higher colon cancer risk. Blacks had a relatively low intake of nutrients thought to be protective - fibre, antioxidant vitamins and calcium. Rather, there reduced colon cancer risk is due to low consumption of animal products.

Seventh Day Adventists (SDAs) are advised by their church to be vegetarian, and just over half follow this advice. In a 21 year study, the Californian SDAs had only about half the rate of colon cancer compared to the general population. Within the SDA group, colon cancer was associated with egg consumption, but not meat or dairy products (10).

However, in a 6 year study of this population, those who ate meat more than once a week were more likely to die of colon cancer than those who ate no meat. This link applied to both red and white meat. Those who ate legumes more than twice a week were less likely to die of colon cancer than those who ate legumes less than once a week (11).

In an 11 year study of vegetarians in Germany, the rate of colon cancer was also much lower than in the general population (12). The rate of colon cancer in men was less than half that expected.

Prospective studies

In prospective studies, large numbers of people are surveyed and then tracked over several years. Those who die of, or are diagnosed with colon cancer over this time are noted. Dietary (and other) information from those who have cancer is compared with information from those who don't develop cancer to see if there are any differences.

In a study of over 88,000 nurses in the USA, colon cancer was associated with animal fat. Those who ate beef, pork of lamb daily had a 2.5 times higher risk of developing this cancer than women who ate these meats less than once a month. Higher consumption of fruit had a slight protective effect (13).

Among over 48,000 male health professionals in the USA, those who ate red meat (beef, pork,, lamb) as a main dish 5 or more times per week had a 3 times higher risk of dying of colon cancer than men who ate these meats less than once a month. White meat was not linked to colon cancer, but there was a link with processed meats, especially hamburgers and bacon. Fibre, especially fruit and vegetables, had a protective effect (14).

In a Swedish study of 16,000 people over a 14 year period, risk of colon cancer again increased with meat (beef, lamb) consumption (15).

However, in a Dutch study of more than 120,000 people but over only 3.3 years, cases of colon cancer were not linked to fresh meat, but only to processed meats, especially sausages and ham (16).

Case-control studies

In case-control studies, people diagnosed with colon cancer (cases) are matched with people of similar demographic characteristics (age, sex, neighbourhood) who do not have cancer (controls). The two groups are compared to see if there are any differences in their lifestyle.

Several case control studies have found that colorectal cancer is associated with animal fat, especially from meat (7, 17-22).

• In a Greek study, the risk of colorectal cancer was 8 times higher among people who had a low vegetable/high meat diet than those who had a high vegetable/low meat diet (17).

• Among the Japanese in Hawaii, colorectal cancer was associated with a change to western-style meals, especially consumption of beef (18).

• In Australia, people with a high intake of protein had a 2-3 times higher risk of colorectal cancer than those with a low intake (19), and in western societies animal products are the main source of protein.

• Among the Chinese in both China and America, colorectal cancer risk was associated with saturated fat, especially from meat and dairy products, while vegetables had a protective effect. Chinese Americans with a high intake of saturated fat had a 4 times higher risk of developing colorectal cancer than those with a low intake (7).

• In New York state, the risk of colon cancer was associated with total energy intake (overeating) and total fat intake. Most of this fat was from animal sources, so much so that it was not possible to analyse animal and vegetable fat separately. Total fibre intake had no effect (23).

• Similarly, in a Utah study, colon cancer cases consumed more total energy, more total fat, less crude fibre, as well as less vegetables from the cabbage family (24).

Contrary findings

Not all studies have found a link between colon cancer and animal fat. For example, increases in beef consumption in the USA since World War 2 are not reflected in increased deaths due to colon cancer (25). In the UK, per capita consumption of fat, animal protein and fibre in different regions was not related to rates of colorectal cancer in those areas (26). However, the authors commented:

" International studies on diet and colon cancer have suggested that meat, protein and fat intakes promote the development of colon cancer. Our studies do not exclude these components of the diet as risk factors, since it could be argued that the intakes in Britain are so high that the small variations observed are unlikely to affect the risk appreciably. If fat is an important dietary factor... our studies suggest that appreciable changes in intake would be needed to affect the risk ".

This is a very important point, which may apply to the American study mentioned previously. If a group as a whole eats, for example, a high fat diet, then it is very unlikely that any differences in disease rate will be found between the highest and lowest fat intake groups. However, comparison with a genuinely low fat intake group may show such differences.

No association was found between fat or meat and colorectal cancer in an American case-control study (27), among the Japanese in Hawaii (28), or among the Japanese in Japan (29). Nuns in a strict religious order who didn't eat meat did not have a lower rate of colorectal cancer than the general population (30).

In a case-control study in Marseilles, meat and total fat did not increase the risk of colon cancer, but vegetables reduced the risk (31). In discussing their results, the researchers note that in the Greek study which found colon cancer associated with meat consumption (17), very few people ate meat daily, and many ate meat one a week or less. In the Marseilles study, even "low" meat eaters ate 100gr daily, so it may not be possible to find differences between cases and controls when everyone's meat intake is relatively high.

In a prospective study of 6 years involving 760,000 people in the US, deaths due to colon cancer were not associated with red meat, total or saturated fat, but vegetables and high fibre grain has a a protective effect (32). However, the researchers note that the way their diet questionnaire was structured excluded 40% of the fat sources in the US diet.

Development of colon cancer

Even though not all studies show animal products, and meat in particular, as a risk factor for colon cancer, and vegetables and fibre as protective, many studies do show these factors. There are good reasons to believe that animal fat is a risk factor for colorectal cancer, while fibrous foods such as vegetables, fruit and grains have a protective effect. Bile acids and cholesterol are broken down by bacteria in the gut to produce compounds such as deoxycholic acid and lithocholic acid, which have been shown to be tumour promoters. A high fat/high meat diet has 2 undesirable effects: it increases the amount to bile acids and cholesterol available to be broken down, and it increases the activity of the bacteria responsible for the break-down (33). Fibre may decrease the risk of colon cancer by increasing the bulk of the stool, thereby decreasing the concentration of bile acids in contact with the intestinal wall.

The risk of colon cancer is much lower among South Asian immigrants to Britain than among the general English population. These immigrants, mostly vegetarians in this study, had lower concentrations of total bile acids in their faeces, including less of the potentially damaging lithocholic and deoxycholic acids (34). The bacteria in vegans were found to be less active in breaking down bile acids, and the concentration of bile acids in the faeces of vegans were lower than in meat eaters (35).

The same difference is found when meat eaters change to a vegetarian or vegan diet; the concentration of bile acids in the faeces, including deoxycholic acid, decreases on a vegetarian and especially a vegan diet (36-37).

Another possible risk factor are the increased N-nitroso compounds (NOCs) produced in the gut. When volunteers were fed high levels of red meat, they produced high levels of NOCs in the faeces. This did not occur with high levels of white meat. NOCs are known to be a risk factor for colon cancer (38).

In summary, there are known biological mechanisms whereby a diet high in animal fat/meat can promote tumours in the large bowel. There is also substantial, although not unequivocal evidence that people consuming such a high fat/meat diet have higher rates of colon cancer. However, it is unlikely that small reductions in animal fat will reduce the risk of this cancer - more substantial changes are required. A vegan diet which contains no animal fat and is high in fibre has the greatest chance of reducing the risk of colon cancer. In addition, several studies suggest that regular exercise also reduces the risk, possibly by increasing transit time through the gut (7,15,22)

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