The road to a healthy diet is not a straight one. At every turn new doubts are raised, new solutions offered. Should we worry about fat or cholesterol? Butter or margarine? More carbohydrate or less? On one topic, however, there has never been any doubt. Fruit and vegetables are good for us and we should eat more of them - at least five servings a day. That is the best way to stave off heart disease, and, possibly, many other diseases associated with modern lifestyles.
The road to a healthy diet is not a straight one. At every turn new doubts are raised, new solutions offered. Should we worry about fat or cholesterol? Butter or margarine? More carbohydrate or less? On one topic, however, there has never been any doubt. Fruit and vegetables are good for us and we should eat more of them - at least five servings a day. That is the best way to stave off heart disease, and, possibly, many other diseases associated with modern lifestyles.
Now, even this shibboleth of healthy living is being subjected to scientific scrutiny - and found wanting. No one is yet saying fruit and vegetables are not good for us, and the best advice remains to eat more of them, but it may turn out that some are better than others.
Scientists believe that diet accounts for three-quarters of all cancers but the precise constituents that cause or prevent the disease remain a mystery. If they could be isolated and identified, dietary advice could be targeted more accurately and deficiencies remedied with specific foods or nutritional supplements.
The latest dietary constituents to come under the scientific microscope are lectins, proteins found in all plants and legumes, which are attracting increasing interest from researchers seeking the causes - and cure - for malignant disease. Some experts believe that they may provide the key that explains why a diet rich in fruit and vegetables protects against cancer.
There are hundreds of thousands of lectins in nature and they include some of the most potent toxins known to man. Their use as poisons has caused many scientists to ignore their potential health-giving role. In 1978, the Bulgarian dissident, Georgi Markov, was assassinated on Waterloo bridge by a man wielding a poisoned umbrella. The poison on the tip of the umbrella was risin - a lectin made from the castor bean.
Last year, lectins again attracted unwelcome publicity in the row over genetically modified food. Arpad Pushtai, the scientist at the Rowett Research Institute in Aberdeen who triggered the row, claimed genetically modified potatoes fed to rats produced abnormalities. The modification he made involved adding a gene for the snowdrop lectin.
But lectins may be about to emerge as the new hope in cancer prevention. Research to be published later this year will claim that lectins have a major role to play in stimulating, or dampening down, the uncontrolled growth of cells that is the marker of malignant disease.
Lectins are the latest in a range of constituents of the diet being examined by scientists for their cancer-preventing properties, which include anti-oxidants (vitamins A, C and E and selenium), minerals such as calcium, lignans in cereals, and isoflavones, hormone-like signalling agents. Only a tiny number of lectins have so far been studied. Research published last year in the journal Gut showed that lectins in broad beans and mushrooms helped halt the cancer process and slowed the growth of colon cancer cells.
Professor John Calam of the department of gastroenterology at Hammersmith Hospital, London, who led the research, said: "The broad bean lectin seems to calm things down. When we put it on the colon cancer cell line, proliferation decreased and differentiation increased, so we got less cancer.
"We know that diets high in fruit and vegetables protect against many types of cancer including bowel cancer. The lectin effect may be one of the reasons why."
In addition to broad beans and mushrooms, tomatoes and raspberries are also thought to contain "good" lectins which have a protective effect. Peanuts, however, contain a lectin that increases rates of cell-division which might be harmful and raw red kidney beans contain a poisonoust.
There is no evidence that an occasional packet of peanuts, eaten as part of a balanced diet, will do you any harm but slow cooking of kidney beans on a low heat - a favoured way of preparing chilli con carne - should be avoided as the temperature will not rise high enough to destroy the poisonous lectin.
Lectins could explain anomalous findings such as that from a study in the New England Journal of Medicine, which suggested that dietary fibre had no preventive effect against bowel cancer. The huge study, which followed the diets of 90,000 nurses over 16 years, went against the accepted wisdom that fibre protects against bowel cancer. Professor Rhodes said: "It is beginning to look as if it is some specific effect of individual foods that is protective rather than the mechanical effect of the fibre. There may also be some components of fibre that block the effects of lectin."
Cereals and wholemeal bread have been touted as the best source of fibre for people eating a Western diet, although the evidence that it is protective is "weak," according to Professor Rhodes. Vegetable fibre may turn out to be more protective, but the picture is complicated by other factors such as the consumption of red meat, which is thought to be a cause of bowel cancer.
Research carried out at Liverpool University has demonstrated the protective effect of mushroom lectins and the university has patented them for use in patients following eye surgery. One of the complications of treatment for glaucoma and detached retina is that the cells in the region of the operation can start growing rapidly and negate the effects of the surgery. The lectin blocks this proliferation of the cells.
Professor Rhodes has recently completed a study of four food lectins which has been submitted to a leading journal. He added: "We need to be looking at specific food components that are either protective or harmful. It is unlikely that the bottom line is going to be to eat masses of vegetables. It may matter which they are. In the end much more precise advice about diet will be given but until then the best advice is to eat more greens."
However, Dr Michael Hill, chairman of the European Cancer Prevention Organisation, said there were dangers in trying to target specific cancer preventing elements in the diet. "The worry is that we will put all our eggs in one basket. We have good evidence that fruit and vegetables are protective. There is tremendous interest in the isoflavones, anti-oxidants and now lectins. The truth is that they are probably all going to be important."
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