This year, about 25,000 people in the United Kingdom will find out that they have colorectal cancer, i.e., cancer that occurs in the bowel (the bowel is often referred to as the large intestine and includes the colon and a terminal six-inch segment called the rectum). Although people tend to think that colorectal cancer is a 'men's disease', the truth is that it strikes women just slightly more often than men. The average man or woman in the UK has about a 1 in 20 chance of developing colorectal cancer during his/her lifetime, with those who have a personal or family history of colorectal cancer, polyps, or inflammatory bowel disease at an even higher risk. If one of your parents (or a brother or sister) has had colorectal cancer, your chances of developing the disease roughly double (one in 10 such people will come down with colorectal carcinoma).

If you have experienced rectal bleeding, blood in your stools, abdominal pain, or a change in bowel habits, you should notify your doctor immediately; all are potential symptoms of colorectal cancer. However, some individuals with colorectal cancer notice only a general fatigue, often due to iron-deficiency anaemia associated with bleeding within the bowel. Unfortunately, the disease can also be present without symptoms, which means that a regular colon-cancer screening programme is imperative.

Cancer researchers believe that colorectal cancer is the second most preventable cancer, after lung cancer. Usually, colorectal cancer begins as a polyp or collection of polyps, which are small, benign clusters of cells on the inside lining of the colon. Over a period of several years, some of these polyps may grow larger and become cancerous. That's why doctors often advise their patients to participate in a regular polyp screening programme - which includes the removal of those polyps which do crop up. Such programmes are lifesavers, because they often detect cancers when they are still quite small and manageable; researchers estimate that as many as 90 per cent of all colorectal cancers could be prevented if the entire population participated in a regular polyp-removal programme. Sigmoidoscopes and endoscopes are utilized to detect polyps visually within the colon; a good screening programme should also include regular stool blood tests and digital rectal examinations.

What increases your risk?
Research has shown that high-fat and/or low-fibre diets seem to increase the risk of colorectal cancer. Other factors such as obesity, as well as alcohol and tobacco use, enhance the likelihood of colorectal cancer. That's why doctors and colorectal-cancer researchers often recommend a diet which is low in fat and rich in fruits, vegetables, and fibre. Experts contend that up to 40 per cent of all colorectal cancer cases could be prevented by following a good diet, avoiding obesity, shunning tobacco, and consuming alcohol only in moderation.

The good news for athletes is that regular exercise has also been linked with a decreased risk of colorectal cancer. In a recent study carried out at Chang Gung University in Taiwan, men who exercised regularly had an 83-per cent lower risk of colon cancer, compared with men who were sedentary (International Journal of Cancer, vol. 82, pp. 484-489, 1999). This research supports several previous investigations which also linked strenuous exercise with a reduced chance of developing colorectal cancer - regardless of gender. No one knows exactly how much exercise is needed to lower the risk, or even whether moderate training (about 30 miles per week for runners, for example) is better than low-level training (10 to 15 miles per week), but it's reasonable to think that every endurance athlete who trains fairly regularly is gaining at least some protection.

The mechanism underlying the exercise and colon-cancer connection is unknown, although researchers speculate that athletes tend to defecate more often than sedentary individuals (exercise tends to enhance intestinal motility). This more-frequent-defecation pattern decreases the period of time that potential carcinogens within the colon are actually in contact with the lining of the large intestine.

H2O flushes cancer
Interestingly enough, the same Taiwanese study determined that drinking rather copious quantities of water also decreased the risk of colorectal cancer. In the research, men who consumed the most water had a 92-per cent lower risk of developing colorectal cancer, compared to men who drank the least amount of water. The mechanism for this may be similar to the one associated with exercise: High water intakes tend to soften up gut contents, decreasing the transit time of food through the intestines and also diluting the concentrations of potential carcinogens. Incidentally, drinking large amounts of water has also been correlated - not surprisingly - with a lower risk of bladder cancer.

Another new study has connected a third factor - fish consumption - with improved chances of avoiding colorectal cancer. In this research, a total of almost 8,000 individuals (3220 males and 4770 females) were monitored for disease over a 13-year period (from 1983 through 1996). Frequency of fish consumption for these persons was categorized into three levels: no or occasional consumption of fish (less than one serving per week), one serving per week, and two or more servings per week. Odds ratios were calculated over the 13-year period to estimate the risk of various types of cancer for members of the three groups, and the researchers controlled for education, body mass index, area of residence, alcohol consumption, and tobacco smoking, as well as fruit and vegetable intakes ('Fish Consumption and Cancer Risk,' American Journal of Clinical Nutrition, vol. 70, pp. 85-90, 1999).

As it turned out, even moderate consumption of fish (one serving per week) conferred protection against many forms of cancer, and increased fish intakes (two or more weekly servings) generally added to the protective effect. For example, high fish consumption (two or more servings per week) reduced the risk of colon cancer by 40 per cent and the chance of rectal cancer by 50 per cent, compared to eating less than one serving per week. High fish consumption also reduced the chances of oral-cavity or pharyngeal cancers by 50 per cent, the risk of oesophageal cancer by 40 per cent, the chance of stomach cancer by 30 per cent, and the risk of pancreatic cancer by 30 per cent.

It must be the fatty acids
Large fish intakes also lowered the chances of laryngeal cancer by 30 percent, endometrial cancer by 20 per cent, ovarian cancer by 30 per cent, and multiple myeloma (cancer of the bone marrow) by 50 per cent. The risks of developing cancers of the liver, gallbladder, breast, bladder, kidneys, or thyroid, or for developing lymphomas, were not influenced by fish eating.

Why might fish consumption protect against colorectal - and many other forms of - cancer? The exact mechanism is not known, but the fat from fish contains rich lodes of the so-called n-3 fatty acids, which are important components of cell membranes, appear to have anti-inflammatory effects, and inhibit the growth of many different types of tumour cells. One of the best things you can do to protect yourself against cancer is to increase your intake of n-3 fatty acids, the ones found in fish, and decrease your ingestion of other forms of fat ('Missing Anti-Proliferative Effect of Fish Oil on Rectal Epithelium in Healthy Volunteers Consuming a High-Fat Diet: Potential Role of n3:n6 Fatty Acid Ratio,' European Journal of Cancer Prevention, vol. 4, pp. 231-237, 1995). Of course, it's also possible that individuals who eat fish tend to follow a healthier lifestyle overall, compared to red-meat eaters, and that the healthy lifestyle - not the fish - underlies the cancer protection.

Unfortunately, there is little evidence to support the idea that increased fish consumption can reduce the risk of breast cancer (although running and other forms of regular exercise have been linked with a lower breast-cancer incidence). There is also little correlation between fish intake and prostate-cancer risk. However, increased fish consumption has also been linked with a reduced chance of cardiovascular problems and mortality ('Fish Consumption and the 30-Year Risk of Fatal Myocardial Infarction,' New England Journal of Medicine, vol. 336, pp. 1046-1053, 1997).

Jim Bledsoe