Foods from plant sources
Grains, or cereals, are the seeds and energy stores of cultivated grasses and the main types are wheat, rice, maize (corn), millet, sorghum, barley, oats and rye.
Wholegrains are grains and grain products made from the entire grain seed, which consists of the bran, germ and endosperm. They contain starch and protein as well as variable amounts of fibre, B vitamins and other micronutrients that are most concentrated in the germ and outer layers of the grain. The refining of wholegrains usually removes the germ and outer layers of the grain, thereby reducing the presence of fibre and micronutrients. Consumption of grains in refined forms, such as white rice, bread or pasta, is generally more common than consumption in wholegrain form.
Pulses (legumes) such as beans, lentils, peas and peanuts (groundnuts) as well as minimally processed grains are particularly concentrated sources of dietary fibre. However, vegetables, fruit, nuts and seeds also contain significant amounts of dietary fibre.
Vegetables can be separated into groups according to their individual starch content.
Starchy vegetables such as potatoes, sweet potatoes (yams), cassava (manioc), sago yams and taro contain higher levels of carbohydrate than non-starchy vegetables. Levels of other nutrients also vary between the two groups.
Examples of non-starchy vegetables include:
- carrots, beets, parsnips, turnips and swedes as well as green, leafy vegetables (such as spinach and lettuce)
- cruciferous vegetables (the cabbage family, for example, bok choy [pak choy], broccoli, cabbage and watercress)
- and allium vegetables (such as onions, garlic and leeks).
Grains and pulses (legumes) may be contaminated with mycotoxins such as aflatoxins, which are produced by certain moulds growing on agricultural crops.
People can be exposed to aflatoxins by eating contaminated foods. Although moulds that contaminate foods are usually destroyed by cooking, any toxins they produce may remain.
All naturally-occurring aflatoxins are classified as human carcinogens by the International Agency for Research on Cancer (IARC).
Aflatoxins are most problematic in countries with hot, damp climates and poor storage facilities; levels of aflatoxin contamination tend to be highest in sub-Saharan Africa and South-East Asia, as well as China, and rates of liver cancer are high in these countries.
Aflatoxin-contaminated foods are generally consumed in the countries where they are produced, but they may also be exported to neighbouring countries and intercontinentally.
OUR MAJOR FINDINGS ON THIS EXPOSURE
There is strong evidence that:
- wholegrains DECREASE the risk of colorectal cancer
- foods containing dietary fibre DECREASE the risk of colorectal cancer
- beta-carotene in foods or supplements is unlikely to have a substantial effect on the risk of prostate cancer
- foods contaminated by aflatoxins INCREASE the risk of liver cancer
- foods preserved by salting (including preserved non-starchy vegetables) INCREASE the risk of stomach cancer
While the evidence for links between individual cancers and non-starchy vegetables or fruit is limited, the pattern of association is consistent and in the same direction and overall, greater consumption of non-starchy vegetables or fruit protects against a number of aerodigestive cancers.
For wholegrains and foods containing dietary fibre the evidence shows that, in general, the more people consume, the lower the risk of some cancers.
– This is the opinion of the Expert Panel and forms the basis of our Recommendation on wholegrains, vegetables, fruit and beans
The Panel used the strong evidence on wholegrains and foods containing dietary fibre and the evidence on non-starchy vegetables and fruit when making Recommendations designed to reduce the risk of developing cancer.
Cancer Prevention Recommendation
Although contamination of foods with aflatoxins is a public health issue, individuals cannot necessarily influence whether foods are contaminated before being sold. It is therefore inappropriate to make a global recommendation on the consumption of foods contaminated by aflatoxins. Nevertheless, the Panel advises people against eating mouldy grains or mouldy pulses (legumes) and advises governments to ensure facilities for the safe storage of foods are made available in areas at risk of aflatoxin contamination.
A global recommendation about consumption of foods preserved by salting (including preserved non-starchy vegetables) has not been made, as these types of food are mostly consumed only in Asia. Nevertheless, the Panel advises that foods be preserved without using salt.
Wholegrains and colorectal cancer
Wholegrains are a rich source of various bioactive nutrients and non-nutrient compounds including vitamin E, selenium, copper, zinc, lignans, phytoestrogens and phenolic compounds and dietary fibre. Many of these compounds, which are largely found in the bran and germ of the grain, have plausible anti-carcinogenic properties. For instance, several phenolic acids have been shown in experimental studies to stimulate anti-oxidative activity. Alkylresorcinols, which are biomarkers of wholegrain wheat and rye intake, were shown to be inversely related to colorectal cancer risk in the European Prospective Investigation into Cancer and Nutrition (EPIC). Wholegrains may also protect against colorectal cancer by binding carcinogens and regulating glycaemic response.
Foods containing dietary fibre and colorectal cancer
In humans, different types of fibre can, to varying degrees, be fermented or metabolised by the colonic microflora, and this can influence the types and patterns of bacterial populations found in the colon. Microbial fermentation within the large bowel forms short-chain fatty acids, such as butyrate, that have been shown in experimental studies to have anti-proliferative effects for colon cancer cells. Other mechanisms by which greater dietary fibre intake may lower colorectal cancer risk include the reduction of intestinal transit time and increased faecal bulk, which would lessen the potential for faecal mutagens to interact with the colon mucosa, and a reduction of secondary bile acid production. High-fibre diets may also reduce insulin resistance, which is a risk factor for colorectal cancer. Overall there is moderate mechanistic evidence linking dietary fibre intake with a reduced risk of colorectal cancer.
Aflatoxins and liver cancer
Aflatoxin, and specifically aflatoxin B1, is a mycotoxin produced by moulds of the Aspergillus species, which contaminates many food crops stored in warm and moist conditions, a problem most evident in areas of Africa and Asia. Aflatoxin B1 is metabolised in the liver by members of the cytochrome P450 family, specifically CYP3A4 and CYP3A5, to its reactive intermediate, 8,9-exo-epoxide, which can form aflatoxin-N7-guanine adducts. The products of aflatoxin biotransformation in the liver are known to be highly genotoxic to the organ, and hepatocellular carcinomas from regions with high exposure to aflatoxin tend to bear a high mutation load in TP53 characteristic of aflatoxin adduct formation.
Greater intake of non-starchy vegetables and fruit
Fruit and non-starchy vegetables contain a large number of potential anti-tumorigenic agents, such as dietary fibre, carotenoids, vitamins C and E, selenium, dithiolthiones, glucosinolates and indoles, isothiocyanates, flavonoids, phenols, protease inhibitors, plant sterols, allium compounds and limonene. It is likely that a combination of these nutrients is responsible for the lower risk of certain cancers. Plants also provide a source of fibre in the diet, which may affect the colonic microbiota and host metabolism to alter cancer risk.
Foods preserved by salting and stomach cancer
Animal models have shown that high salt levels alter the viscosity of the mucous protecting the stomach and enhance the formation of N-nitroso compounds. In addition, high salt intake may stimulate the colonization of H. pylori, the strongest known risk factor for stomach cancer. Finally, in animal models, high salt levels have been shown to be responsible for the primary cellular damage that results in the promotion of stomach cancer development.
Download this chapter of the Third Expert Report for further details on the mechanisms associated with wholegrains, vegetables and fruit and certain cancers.