Wholegrains, Vegetables & Fruit and Cancer Risk

Following on from the previous blog, which looked at how meat, fish and dairy products are related to cancer risk1 , we’ll end this series by looking at the cancer risks involved in eating wholegrains, vegetables and fruit.

The Continuous Update Project (CUP), which analyses research on what causes cancer and how it can be prevented, operates under the auspices of the WCRF/AICR (World Cancer Research Fund/American Institute for Cancer Research). The following information is taken from their Third Expert Report 2 released in 2018.

The major findings of this Report are:

There’s 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 3 INCREASE the risk of liver cancer
  • foods preserved by salting (including preserved non-starchy vegetables 4 ) INCREASE the risk of stomach cancer
  • non-starchy vegetables and fruits DECREASE the risk of aerodigestive 5 cancers and other cancers

There’s limited evidence that:

  • low intake of non-starchy vegetables INCREASES the risk of colorectal cancer
  • preserved non-starchy vegetables INCREASE the risk of nasopharyngeal cancer
  • low intake of fruit INCREASES the risk of stomach cancer and colorectal cancer
  • non-starchy vegetables DECREASE the risk of mouth cancer, pharyngeal cancer, laryngeal cancer, nasopharyngeal cancer, oesophageal cancer, lung cancer (in people who smoke or used to smoke), breast cancer
  • fruit DECREASES the risk of oesophageal cancer and lung cancer (in people who smoke or used to smoke)
  • citrus fruit DECREASES the risk of stomach (cardia 6 ) cancer
  • non-starchy vegetables and fruit DECREASE the risk of bladder cancer
  • food containing carotenoids 7 DECREASES the risk of lung cancer and breast cancer
  • foods containing beta-carotene 8 DECREASES the risk of lung cancer
  • foods containing vitamin C 9 DECREASE the risk of lung cancer (in people who have smoked tobacco) and colorectum cancer
  • foods containing isoflavones 10 DECREASE the risk of lung cancer (in people who have never smoked tobacco)

The following is a chart which shows the above information in a more graphic format.

Full details are available at as a downloadable PDF file 11 , including references indicated in superscript throughout the above chart.

Foods from plant sources

Wholegrains

Wholegrains are grains and grain products made from the entire grain seed, which consists of the bran, germ and endosperm. Wholegrains 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.

When wholegrains are refined, the germ and outer layers of the grain are usually removed, thereby reducing the presence of fibre and micronutrients.

Eating refined grains , such as white rice, bread or pasta, is generally more common than consumption in wholegrain form, and there are negative health implications 12 13 seemingly not covered in this Report, for some reason.

Pulses

Pulses (legumes) such as beans, lentils, peas and peanuts (groundnuts) as well as minimally processed grains are particularly concentrated sources of dietary fibre.

Of course, vegetables, fruit, nuts and seeds contain significant amounts of dietary fibre too.

Vegetables

Vegetables can be separated into groups according to their individual starch content:

  • starchy vegetables
    • potatoes, sweet potatoes (yams), cassava (manioc), sago yams and taro etc
      • these contain higher levels of carbohydrate than non-starchy vegetables
  • non-starchy vegetables
    • root veg – carrots, beets, parsnips, turnips and swedes etc
    • green, leafy veg – spinach and lettuce etc
    • cruciferous (the cabbage family) veg – bok choy, broccoli, cabbage and watercress etc
    • allium veg – onions, garlic and leeks etc

Levels of other nutrients  vary between the above two groups.

Aflatoxins

Grains and pulses (legumes) may be contaminated with mycotoxins 14 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) 15 .

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.

Salty veg and fruit

The Panel didn’t bother giving any definite recommendations regarding foods preserved by salting (including preserved non-starchy vegetables), since these types of food are mostly consumed only in Asia. However, they do state that foods should be preserved without using salt.

Mechanisms involved in cancer risk avoidance and cancer

Wholegrains and colorectal cancer

Wholegrains are a rich source of various bioactive nutrients and non-nutrient compounds including:

  • vitamin E 16
  • selenium 17
  • copper 18
  • zinc 19
  • lignans 20
  • phytoestrogens 21
  • phenolic compounds 22 , and
  • dietary fibre 23

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 24 , 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.

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 23
  • carotenoids 25
  • vitamin C 26
  • vitamin E 16
  • selenium 17
  • dithiolthiones 27)
  • glucosinolates 28
  • isothiocyanates 29
  • flavonoids 30
  • phenols 31
  • protease inhibitors 32
  • plant sterols33
  • allium compounds 34 , and
  • limonene 35

It’s 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 36  and host metabolism to alter cancer risk.

Foods preserved by salting and stomach cancer

There is good evidence that salt preservation can cause stomach cancer 37 . The following are suggested mechanisms:

  • high salt levels can alter the viscosity of the mucous protecting the stomach and, hence, enhance the formation of N-nitroso compounds 38 39
  • high salt intake may stimulate the colonisation of Helicobacter pylori (also known as H pylori, previously known as Campylobacter pylori, which is a gram-negative, microaerophilic bacterium usually found in the stomach) which is the strongest known risk factor for stomach cancer 40 41
  • high salt levels have been shown to be responsible for the primary cellular damage which results in the promotion of stomach cancer development 42 43

There’s more information available 44 on the process by which cancer develops. You may also be interested in a blog I wrote about the fascinating subject, intimately related to cancer development, of angiogenesis and the work of Dr William Li 45 .

Joe’s Comments

It’s no surprise that there are loads of benefits in relation to cancer risk when consuming wholegrains, fruit and veg. For me, it’s a simple matter – avoid the risks and stick to the plants. Any argument against this struggles to find a justification for including meat, fish and dairy into the diet, as can be seen by the wealth of evidence in the last two blogs that the latter are, to say the least, a mixed blessing, if blessing they be! This isn’t even just a matter of avoiding cancer risks, eating wholefood plants is a proactive way of ensuring you get optimal health benefits from whatever you put in your mouth.

The Panels’ recommendations about avoiding salt as a preservative make a lot of sense to me. I would go further and suggest that you cut out added salt altogether, as supported by Dr Michael Greger 46 . For those who want to kick the salt habit, I wrote a blog 47 which contains lots of healthy alternatives that don’t cause any health problems – indeed, quite the reverse.

A final memory from my childhood. A darling old chap called Fred used to sing this song to me when I was about 5 or 6 years old after he carried me on the back of his Honda 90 to stay with him at industrial Haverton Hill in Middlesbrough. For some reason this song always made me cry as a young child. Why am I’m sharing this with you? I don’t know – maybe it’s just that I recognise that vegetables have been important to me all my life in one way or another…

 


References

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