This is one of a series - and one of the best - studies on aspirin and colorectal cancer. I used to think the effect was unlikely, but now I am satisfied that aspirin does reduce the risk of colorectal cancer." So said Sir Richard Doll, the doyenne of epidemiologists and the man who first proved the link between smoking and lung cancer.
He was speaking after the 1995 publication of the United States Nurses Study. It followed nearly 90,000 nurses from 1976; by 1995, 501 had developed bowel cancer. But the nurses who used aspirin four to six times per week were less likely to develop cancer of the colon. And the preventative effect was greater the longer the nurses had been taking regular aspirin.
There are a number of theories behind the action of aspirin on the bowel. Some cases of colon cancer are preceded by the development of multiple polyps -benign growths. Aspirin appears to be able to stop them from becoming malignant.
Bowel cancer is also linked with high levels of a chemical called prostaglandin in the bowel wall. We know that aspirin and other anti-inflammatory drugs inhibit an enzyme called cylyoxygenase (Cox) which is needed to produce prostaglandin. As a proven Cox inhibitor, aspirin helps to block the inflammatory chemicals which are associated with colon cancer.
Several other studies have consistently shown a dose-related reduction of both colon and rectal cancer in regular users of non steroidal anti-inflammatory drugs - NSAIDs - and aspirin. Regular NSAID usage has been shown to reduce risk of colon cancer by 35 per cent. Research involving over 660,000 people who took aspirin 16 times a month demonstrated a 42 per cent reduction in fatal colon cancer and reduction of a third in rectal tumours.
The longer you take anti-inflammatory drugs, the greater the reduction in risk. However, the jury is still out on the exact dose of aspirin which will bring about an optimal reduction in the incidence of bowel cancer.
Aspirin and NSAIDs are not without side effects. The principal problem is the increased risk of developing an inflamed stomach lining, which can lead to bleeding. Again, however, research suggests that the mortality risk from a gastrointestinal bleed caused by aspirin is outweighed by a reduction in mortality from colon cancer. This finding has not stopped some experts from recommending that long-term preventative drug use should be confined to people with a known elevated risk of developing bowel tumours.
A Western style lifestyle and diet have long been accepted as contributing to a high incidence of colon cancer in certain parts of the world. However, a Harvard Medical School study cast doubts on this theory when it tracked the fibre intake of 39,000 women and found no reduction in colon cancer among those who had a high fibre diet. A paper in the New England Journal of Medicine, which looked at the effects of diet on a group of patients with history of bowel polyps, found that adopting a diet that is low in fat and high in fibre did not alter the risk of recurrence of the pre-cancerous growths.
There are many other studies which have shown a definite link between a high fibre diet and a low incidence of bowel cancer. But researchers are now beginning to differentiate between the types of fibre we eat. The relative merits of insoluble fibres, such as whole-wheat products, and the soluble fibre found in fruit and vegetables, are being studied in an effort to resolve the conflicting evidence which has emerged.
Physical activity levels are another factor in the bowel cancer prevention equation. The nurses' health study showed that physical activity was associated with a reduced risk of bowel polyps. Much of the benefit came from activities of moderate intensity, such as brisk walking.
So, what are we to make of the current research on how best to prevent bowel cancer? The following is a personal recipe on how you might reduce your risk of colorectal cancer, based on my interpretation of the scientific evidence.
Increase your intake of fruit and vegetables - try to eat five servings each day.
Increase physical activity levels - brisk walking is ideal.
Reduce your intake of animal fats - eat more fish and poultry.
Take an aspirin every second day, having discussed with your GP whether it is safe for you to do so.
Contact Dr Houston at mhouston@irish-times.ie or leave messages on tel 01-6707711, ext 8511.