Eat a diet rich in wholegrains, vegetables, fruit and beans

Overview of evidence

One of our Cancer Prevention Recommendations is to make wholegrains, vegetables, fruit, and legumes and pulses (such as beans and lentils), a major part of your usual daily diet.

There is evidence that eating wholegrains, fibre, vegetables, and fruit can help protect against certain cancers, weight gain, and overweight and obesity.

Goal

• Consume a diet that provides at least 30g per day of fibre from food sources.
• Include foods containing wholegrains, non-starchy vegetables, fruit, and legumes and pulses (such as beans and lentils) in most meals.
• Eat a diet high in all types of plant foods, including at least five portions or servings (at least 400g or 15oz in total) of a variety of non-starchy vegetables and fruit every day.
• If you eat starchy roots and tubers as staple foods, try to eat non-starchy vegetables, fruit, and legumes and pulses (such as beans and lentils) regularly too.

Evidence

There is strong evidence that:

• Wholegrains DECREASE the risk of colorectal cancer
• Foods containing dietary fibre DECREASE the risk of colorectal cancer and protect against weight gain and overweight and obesity

The evidence for links between other individual cancers and consumption of non-starchy vegetables or fruit is limited, but in this instance the Panel looked at the body of evidence overall to draw their conclusions. They agreed that the pattern of association and the direction of effect were both consistent enough to make a recommendation.

This was that, overall, a diet that includes greater consumption of non-starchy vegetables or fruit probably protects against several aerodigestive cancers (such as mouth, pharynx, and larynx, and oesophageal) and some other cancers. In particular, people should avoid a diet with very low intake of these foods.

Relatively unprocessed foods of plant origin are rich in nutrients and dietary fibre. Higher consumption of these foods – instead of processed foods high in fat, refined starches and sugars (for example, white bread and pasta, biscuits, cakes, and pastries) – results in a diet higher in essential nutrients and more effective for regulating energy intake relative to energy expenditure. This protects against weight gain, overweight and obesity, and, therefore, obesity-related cancers.

• Grains, or cereals, are the seeds and energy stores of cultivated grasses, such as 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.
• Legumes and pulses (such as beans, lentils, peas, and peanuts (groundnuts)), as well as minimally processed grains, are concentrated sources of dietary fibre. Vegetables, fruit, nuts, and seeds also contain significant amounts of fibre.
• Vegetables can be separated into groups according to their individual starch content.
  • Starchy vegetables include potatoes, sweet potatoes (yams), cassava (manioc), sago yams, and taro contain higher levels of carbohydrate than non-starchy vegetables.
  • Non-starchy vegetables include carrots, beetroot (beets), parsnips, turnips, and swedes. Green, leafy vegetables (such as spinach and lettuce), cruciferous vegetables (such as broccoli, cabbage, and watercress) and allium vegetables (such as onions, garlic, and leeks) also belong to this group.

Additional information

Grains, as well as legumes and pulses, may be contaminated with mycotoxins such as aflatoxins, which are produced by certain moulds growing on agricultural crops.

Aflatoxins, which are classified as human carcinogens by the International Agency for Research on Cancer (IARC), INCREASE the risk of liver cancer.

Aflatoxins are most problematic in regions with hot, damp climates and poor food storage facilities. Levels of aflatoxin contamination are highest in sub-Saharan Africa, South-East Asia, and China. Rates of liver cancer are high in these countries. Aflatoxin-contaminated foods are most commonly consumed in the countries where they are produced, but they may also be exported to neighbouring countries and internationally.

Mechanisms

Wholegrains are a rich source of various bioactive nutrients and non-nutrient compounds. These  includevitamin E, selenium, copper, zinc, lignans, phytoestrogens and phenolic compounds, and dietary fibre. Many of these compounds have anti-carcinogenic properties.  Wholegrains may also protect against colorectal cancer by binding carcinogens and regulating glycaemic response.

Dietary fibre can be fermented or metabolised by the colonic microflora, forming short-chain fatty acids, such as butyrate, that have anti-proliferative effects for colon cancer cells. Dietary fibre intake also reduces intestinal transit time and increases faecal bulk, lessening the potential for faecal mutagens to interact with the colon mucosa. High fibre diets may also reduce insulin resistance, which is a risk factor for colorectal cancer.

Fruit and non-starchy vegetables contain many 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’s likely that a combination of these nutrients is responsible for the lower risk of certain cancers found among those who consume higher levels of fruit and non-starchy vegetables.

Implications for other diseases

The Goals and Recommendation on wholegrains, vegetables, fruit, and legumes and pulses (including beans, lentils, and peas) are based on evidence of cancer risk.

At the same time, they are supported by evidence on mortality (both cardiovascular and non-cardiovascular diseases and conditions) and risk of developing type 2 diabetes.

Many other, broadly similar recommendations have been issued by a range of authoritative international and national organisations.