Information sheet: Foods from plant sources and cancer

Key messages and recommendations

• There is strong evidence that foods containing dietary fibre (wholegrain cereals, vegetables, fruit, legumes/beans, nuts and seeds) protect against bowel cancer.
• There is strong evidence that wholegrains protect against bowel cancer.
• There is strong evidence that non-starchy vegetables and fruit protect against aerodigestive cancers (head, neck and oesophageal).
• There is strong evidence that foods preserved by salting (including preserved non-starchy vegetables) increase the risk of stomach cancer
• Australians are not consuming adequate amounts of fruit, vegetables, wholegrains or dietary fibre as recommended by the Australian Dietary Guidelines.

Background

Research shows that most diets that are protective against cancer are rich in foods of plant origin. This includes foods such as wholegrains, fruit, vegetables (including legumes), seeds and nuts. Unprocessed foods of plant origin contain high amounts of nutrients, some of which are associated with reducing cancer risk, as well as dietary fibre.^[1]

Wholegrains are grains and grain products made from the entire grain; the bran, germ and endosperm. Wholegrains contain fibre, B vitamins and other micronutrients that are most concentrated in the germ and outer layers of the grain. Non-starchy vegetables include carrots, beetroot and turnips; green, leafy vegetables (spinach and lettuce); cruciferous vegetables (bok choy, broccoli and cabbage) and allium vegetables (onions, garlic and leeks). Legumes include peas, beans and lentils.

Dietary fibre is defined as the fraction of edible parts of plants that is resistant to digestion and absorption in the small intestine and is usually partially or completely fermented in the large intestine.^[2] Adequate dietary fibre is essential for proper functioning of the gut and has also been associated with risk reduction for a number of chronic diseases, including some cancers.^[3]

Increased consumption of plant foods may also have a secondary effect on decreasing cancer risk by helping to avoid excess weight gain and decreasing consumption of red and processed meat.

Biological mechanism

The following mechanisms have been proposed to explain the association between plant foods and decreased cancer risk.^[1]

Foods containing dietary fibre

• Fermentation of fibre in the bowel produces short-chain fatty acids including butyrate that has been shown to inhibit bowel cancer cell growth.
• Increased faecal bulk resulting in decreased intestinal transit time and therefore reducing exposure of the bowel lining to potential carcinogens.
• Lowering insulin levels, thereby preventing insulin resistance which is a risk factor for bowel cancer.

Wholegrains

• Bioactive nutrients and compounds including vitamin E, selenium, copper, zinc, lignans, phytoestrogens and dietary fibre mostly found in the bran and germ of the grain have likely anti-carcinogenic properties.
• Also see proposed mechanisms for foods containing dietary fibre as outlined above.

Non-starchy vegetables and fruit

• Many components found in fruit and vegetables including carotenoids, vitamins A, C and E, selenium, phenolic acids, flavonoids and glucosinolates, dithioltheones, indoles, isothiocyanates, protease inhibitors, plant sterols, allium compounds and limonene have anti-tumourigenic effects, meaning that they inhibit or prevent tumour growth. It is likely that a combination of these nutrients is responsible for the lower risk of some cancers. ^[1]
• Also see proposed mechanisms for foods containing dietary fibre as outlined above.

Epidemiological evidence

The World Cancer Research Fund reports there is strong evidence to suggest that consumption of wholegrains and foods containing dietary fibre protects against bowel cancer.^[1] Data on cancer risk and dietary fibre intake come predominantly from studies on consumption of fibre from foods (either foods that naturally contain fibre or that have fibre added), as opposed to supplements, so no effect can be confidently attributed to fibre that is not from food (either foods that naturally contain fibre or that have fibre added).^[1] While evidence for links between individual cancers and non-starchy vegetables and fruit was limited, it was consistent and in the same direction leading to a conclusion that greater consumption of non-starchy vegetables and fruit probably protects against a number of aerodigestive cancers. Aerodigestive cancers are those of the mouth, larynx, nasal cavity, salivary glands, larynx and oesophagus.^[1]

There is strong evidence of a link between foods preserved by salting and an increased risk of stomach cancer but not a separate conclusion for preserved non-starchy vegetables. Foods preserved by salting refers mainly to high-salt and salt-preserved foods, including pickled vegetables and salted or dried fish, as traditionally prepared in East Asia.^[1]

Grains and pulses (legumes) may be contaminated with mycotoxins such as aflatoxins, which are produced by certain moulds growing on agricultural crops. 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. Consuming foods contaminated by aflatoxins increases the risk of liver cancer.^[1]

Current consumption levels

Dietary survey data from 2011-12 shows that Australians are consuming less than the current recommended level of dietary fibre with men eating 24.8 grams of fibre per day and women about 21.1 grams per day.^[4]

Just under 30% of dietary fibre intake comes from cereals and cereal products, about 14% comes from fruit and about 22% comes from vegetables (including legumes).^[4]

The National Health Survey 2017-2018 found that just over half (51.3%) of Australian adults met the guidelines for the recommended daily serves of fruit (2 or more serves), while one in thirteen (7.5%) met the guidelines for serves of vegetables (5-6 or more serves for men depending on age, and 5 or more for women). Only one in twenty (5.4%) adults met both guidelines.^[5]

Dietary recommendations

Cancer Council supports the Australian Dietary Guidelines which recommend a diet rich in plant-based foods, including:

Eating at least two serves of fruit and five serves of vegetables, including legumes, daily.

• Eating at least four serves of wholegrain or wholemeal foods every day.
• Eating two to three serves of nuts or seeds (no added salt) every day.

More specifically, according to Australian recommendations on dietary fibre, women should aim to consume 25 grams of dietary fibre per day, men should aim for 30 grams.^[3]

Links

Australian Dietary Guidelines

Nutrient Reference Values - Dietary Fibre

World Cancer Research Fund – Wholegrains, vegetables and fruit and the risk of cancer.