A colleague recently said to me, "Cancer has become a desperate scourge. In my own locality I know one or two people with cancer on every road. And, no wonder. We don't know what we are eating nowadays. Our food is full of chemicals." These sentiments are common, but widely respected studies estimate that pollution overall accounts for only one per cent of human cancers.
Cancer is a major problem. One in three of us will contract cancer and one in five of us will die from cancer. It is mainly a disease of the aged. Elderly people are now more numerous in the population than earlier this century and, because of this fact alone, cancer is more common today. When discussing cancer trends, it is essential to speak in terms of age-adjusted rates. These rates, taking into account that absolute numbers of cancer deaths are increasing of other factors that increase longevity, tell us how well overall we are doing against cancer.
The feeling is widespread that cancer incidence and deaths from cancer are continually increasing, but this feeling is wrong. With the exception of respiratory cancer, largely caused by cigarettes, age-adjusted cancer incidence is generally not increasing and age-adjusted cancer death rates are slowly falling. The graph below illustrates this.
An agent that can cause cancer is called a carcinogen. A chemical is tested for carcinogenic potential by administering it to rats at near-toxic doses and following up to see if cancer develops. About half of all chemicals tested, whether natural or synthetic, are carcinogenic at these doses. There is good reason to suspect that many chemicals are carcinogenic only at these very high doses. However, this is not taken into account by the testers who take a positive result to unambiguously identify a carcinogen whose potential to cause cancer is assumed to be proportional to dose, decreasing to zero effect only at zero dose. If many chemicals are carcinogenic only at high doses, we are greatly exaggerating the risk at the very low doses to which the public is exposed.
Bruce Ames, the widely respected American biochemist, points out that the great bulk of the chemicals ingested by humans is natural, both by weight and number. The widespread suspicion of an epidemic of cancer caused by synthetic industrial chemicals is mistaken.
There is widespread concern about residual pesticides in food. According to Ames, 99.99 per cent of the pesticides in food are natural, present in plants to repel insects and other predators. We cannot eat food without ingesting these natural pesticides, half of which test positive as rodent carcinogens. Public and regulatory interest focuses on the 0.01 per cent that are synthetic pesticides. However, reducing our exposure to this tiny fraction, while ignoring the 99.99 per cent background levels, cannot be expected to reduce cancer rates.
If nobody smoked, drank alcohol or ingested a synthetic chemical, and if everybody ate a perfect diet and adopted a perfectly healthy lifestyle, would cancer disappear? Unfortunately not, but the incidence of cancer would go down by perhaps 80 per cent. Our cells are chemical machines. They work well but they are not perfect. Mistakes are made and damage is caused by unavoidable background agents such as natural radiation. The cell has mechanisms to repair damage and to neutralise ingested natural plant pesticides, but, over time, mistakes and damage build up, causing background cancers.
Diet accounts for about one third of cancer risks in the developed world, smoking accounts for another third and the remainder can be attributed to hormone imbalance, pollutants in air, infections, some high risk occupations, over-exposure to sun, hereditary factors, and background cancers.
Even a cursory glance at the causation factors shows that most cancer is preventable. As regards diet, the standard recommendation of eating, in moderation, a wide variety of foods distributed across the four food groups is very good advice. In particular one should eat plenty of fruit and vegetables. The quarter of the population with the lowest intake of fruit and vegetables has twice the cancer rate of the quarter with the highest intake. Vigorous physical exercise also reduces the risk of cancer. Excessive intake of alcohol is a risk factor for some cancers.
Smoking tobacco is responsible for up to one third of all cancer deaths. Smoking also causes about one quarter of all heart disease. Everybody knows that cigarettes are deadly. The number of people who continue to smoke, and, in particular, the numbers of young people who take up smoking is a major scandal. Synthetic air pollutants are much feared by the public, but the evidence is that they present only a small risk. Indoor air quality is of greatest concern as people spend 90 per cent of their time indoors. The most important carcinogen in air is probably the natural radioactive gas radon. It seeps into houses from the underlying soil and it may cause up to 10 per cent of Ireland's lung cancers, which total about 1,500 per year. The Radiological Protection Institute of Ireland (RPII) is surveying the country in order to identify areas with higher levels of indoor radon. Parts of Galway, Mayo, Clare, Cork, Wicklow and Louth have been identified as high radon areas, although individual high-radon houses can occur anywhere in the country.
The long expensive search for a cure for cancer has been largely fruitless. Cancer prevention works and it costs little or nothing. Eat sensibly with plenty of fruit and vegetables, don't smoke, don't drink to excess, treat the sun with caution, and take plenty of exercise. And finally, be happy - it reduces risk factors for all diseases.
William Reville is a senior lecturer in Biochemistry at UCC.