Grains and pulses and cancer

Dietary factors can affect the development of cancer in any one of the three stages of its development:

• Initiation - involves alteration or damage to the chemical composition or to the structure of DNA.
• Promotion - the malignant transformation of the damaged cells which leads to carcinogenesis.
• Progression - growth of the tumour occurs.

There are several components in grains and pulses which may prevent the development of cancer:

Dietary fibre

• Formation of butyrate from fermentation of dietary fibre and resistant starch keeps the bowel healthy and has anti-carcinogenic properties.
• Decreased intestinal transit time and increased bulk limits absorption and contact of carcinogens and toxicants with the wall of the gastrointestinal tract.
• Decreased pH of the gut inhibits formation of secondary bile acids which are potentially carcinogenic.

Folate
May prevent damage to DNA.

Antioxidants

• Prevent production of free radicals or scavenging free radicals (Vitamin E, phytic acid).
• Enhance the activity of enzymes (Selenium).
• Prevent production of carcinogens such as nitrosamines and secondary bile acids (vitamin E, phenolic acids, lignans).
• Bind carcinogens and prevent them from attacking the DNA (phenolic acids).
• Enhance immune function (vitamins E, C and beta-carotene).
• Studies have shown that there is an association between blood levels of antioxidants (vitamin C, E and beta carotene) and protection from cancer of the lung, breast, cervix, pancreas, colon, stomach, oral, and skin. Low intake of selenium has been associated with increased risk of respiratory and gastrointestinal cancer.

Lignans

• Due to the similarity in structure of lignans to estrogens, they may play a role in modulating hormone-related cancers. High levels of biologically active androgens or estrogens are associated with increased risk of prostate cancer in men, and ovary, breast and endometrial cancers in women. Diets that lower these androgens or estrogens are associated with low hormone-related cancer risk.
• Lignans increase synthesis of sex hormone-binding globulin (SHBG) which binds estrogen and hence reduces the levels of circulating estrogens.
  In the gut, lignans may prevent the reabsorption of estrogen into the body.
• In vitro studies have shown that lignans can decrease cell proliferation and reduce tumour size and number.

Dietary fibre and cancer - the evidence

Epidemiological studies
The evidence to date is mostly based on case control studies where people diagnosed with cancer are identified and their past dietary intake is compared to that of similar people without cancer. This study design relies on people accurately reporting their past dietary intakes which may be biased and hence affect the quality of the evidence.

Most studies have focused on the relationship between the intake of nutrients, particularly dietary fibre, and cancer risk. The majority of case control studies conducted in countries from around the world have reported a reduced risk of colon cancer with increased consumption of dietary fibre. However, the results from cohort studies have been inconsistent. A large review of the relationship between diet and cancer concluded that the inconsistency reflects the heterogeneous nature of dietary fibre and the inadequate measuring techniques, rather than that there is no protective effect2.

Others argue that the study of nutrients or individual food constituents may mask the total health effect of wholegrains. The influence on wholegrain consumption may depend on the presence or absence of many constituents and their interactions. Few studies have assessed the effect of grain consumption and risk of cancer.

Wholegrain foods and cancer - the evidence

Epidemiological studies
Case control studies which provided information relating to wholegrain consumption were reviewed to determine whether consumption of wholegrain foods (such as wholegrain breads and pasta, wholegrains and high fibre-cereals) reduces the risk of cancer3.

• Overall, the risk of cancer in those with a high intake of wholegrain foods was 34% lower than those with a low intake of wholegrain foods.
• Nine out of ten studies showed that high intakes of wholegrain foods reduced the risk of colorectal cancer by 21% compared to those with low intakes of wholegrain foods.
• Seven studies showed that high intakes of wholegrain foods significantly reduced the risk of gastric cancer by 43% compared to low intakes of wholegrain foods.
• The risk of seven hormone-related cancers (breast, prostate, ovarian and endometrial) was reduced by 14%, 10%, 37%, 45% (the last two were statistically significant) in those with high intakes compared to those with low intakes of whole grain foods.

A review of 39 studies providing information on intake of wholegrain foods and colorectal cancer found 26 studies showed a protective effect, five showed no effect and four showed a positive effect (of which two were in developing countries) 4. When the same data was reviewed in terms of the cereal fibre intake (only 19 studies provided data on cereal fibre intake) 16 showed a protective effect, three showed no effect and none showed that cereal fibre increased the risk of colorectal cancer.

These studies suggest that the more wholegrain foods are consumed the greater the reduction in cancer risk.

The impact of refined grains on cancer risk is inconsistent. Some case-control studies have reported a positive association between cancer and intake of refined cereals. This may be as a result of diets high in refined carbohydrate being deficient in protective dietary constituents. However, a cohort study in Japan, where rice and wheat are usually consumed as refined grains, did not demonstrate increased risk of colorectal cancer.

Randomised controlled trials
The Polyp Prevention study conducted in Australia found that adding wheat fibre to the diet prevented the growth of large adenomas and hence possibly the development of colon cancer5.

Well designed cohort studies and randomised controlled trials are required to confirm the results of experimental and animal studies which suggest that grains reduce the risk of cancer. However, the evidence to date suggests that it may be worthwhile encouraging consumption of whole grain foods.

Pulses and cancer

There are few studies investigating the link between consumption of pulses and cancer risk. The "World Cancer Research Fund" examined the evidence from 58 epidemiological studies on the topic2. Half reported a decreased cancer risk with a higher intake of pulses and half reported an increased risk. It is therefore difficult to determine the role of pulse consumption in the prevention of cancer based on this data. Populations with high consumption of pulses are generally in poorly developed countries and increased cancer risk may be attributed to other dietary or environmental factors. Given the known nutritional attributes of pulses, it is worthwhile encouraging increased consumption especially in countries where consumption is poor.

References
1. McIntosh G. Cereal foods, fibres and the prevention of cancers. Aust. J. Nutr. Diet. (2001) 58 Suppl.2.
2. World Cancer Research Fund/American Institute for Cancer Research. Food, Nutrition and the Prevention of Cancer: a Global Perspective. Washington DC: American Institute for Cancer Research, 1997
3. Jacobs DR, Marquart L, Slavin J et al. Whole-grain intake and cancer: an expanded review and meta-analysis. Nutrition and Cancer 1998;30(2):85-96
4. Hill MJ. Cereals, dietary fibre and cancer. Nutrition Research 1998;18(4):653-659
5. MacLennan R, Macrae F, Bain C, et al. Randomized trial of intake of fat, fiber, and (-carotene to prevent colorectal cancer. J Natl Cancer Inst 1995;87:1760-6

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