Cancer-Causing Factors

Introduction

Most of the advances in our knowledge of cancer-causing factors come from recent epidemiological studies. Epidemiology (a term coined by Hippocrates in 40 BC) is the branch of medical science studying the causes and distribution of a disease in a people or community. In all, about 40 different factors have been found to be contributory causes of the more common forms of cancer in man. Some of these, such as pollution or industrial chemicals and their effluents, are man-made and their toxic effects can be controlled by strict regulations. Others such as diet, alcohol consumption, smoking or sexual habits, are more personal and cannot be controlled in this way. Because their nature and exact identity is not yet generally understood, they can cause a great deal of unnecessary anxiety among the less well-informed.

People are not only over-anxious about cancer, they are also extremely gullible. Some start suspecting the presence of cancer-causing compounds even in their surroundings: in the air, water or their food. Many people are influenced by newspaper and television reports and fear a number of unfounded trivial risks, while ignoring less sensational but nevertheless real, scientifically established causes. The identification of such high-risk causes is the end result of careful observation which has pinpointed, for instance, an unusually high death rate from either a specific cause or among a particular group of individuals. These carcinogenic factors can be divided into two groups: the external or environmental ones and the intrinsic or genetic ones.

Environmental Factors

What recent epidemiological findings have clearly shown is that when we look at the distribution of cancer mortality world-wide, we can arrive at a number of very useful practical conclusions and some of them are quite surprising. There are, for instance, high and low-cancer risk populations in the world and having insufficient food actually reduces the incidence of certain forms of cancer. Of even more practical interest, and very reassuring in view of the misleading, almost hysterical publicity on the subject, is the realisation that at present, food additives, insecticides, herbicides and pesticides cannot play anything but a very minor role in the development of cancer. Similarly, chemicals and other contaminants seem of minor impact in comparison with the importance of what we actually eat. What can be concluded from all this is that since our environment is under human control we should be able to manipulate it in a way which will lead to a reduction of cancer.

Epidemiologists have been able to identify people in different parts of the world who are more likely to contract cancer. Stomach cancer was found to be highest in Japan, liver cancer in East Africa, cervical cancer in Colombia and Puerto Rico, and breast and bowel cancer in all affluent Western countries. Identification of these clues allows recommendations which, if followed, should lower these high rates of specific forms of cancer. These clues pointed to numerous factors, including the effects of hormones, of chronic irritation or inflammation, of ageing, and of the impact of stress or one’s emotional and psychological state. However, over half of the more common forms of cancer are related to diet, which makes the combined effect of food and drink a more important determinant of the risk of contracting cancer than even smoking.

The Food We Eat

Each item of food we allow to pass our lips throughout the day should therefore have been chosen with serious deliberation. This means that we must try to be consistently firm about the food we accept and the food we feel obliged to refuse.

By being careful about what and how much we eat we can reduce the probability of contracting one or more of the five killer diseases (heart disease, high blood pressure, strokes, diabetes and cancer), resulting from our greed and affluence, and from which less privileged nations do not generally suffer. In this context it surely is ironic that many Third World communities, so far not affected by the degenerative illnesses of the sophisticated West, should strive so desperately for greater affluence, which will bring in its wake the same toll of Western ill health!

There is a strikingly direct relationship between fat consumption and both breast and bowel cancer, which is reflected in the wide variation of these cancers between affluent and under-privileged nations. This variation tells us that with a suitable change in lifestyle, these common cancers should be avoidable in 80 per cent of cases. It is our incredibly short-sighted reluctance to break the habits of a lifetime which make such a life-saving improvement unlikely for many years to come.

Chemicals

Among the external cancer-causing factors there are also many chemical agents responsible for certain forms of cancer in the industrialised West.

Tobacco, particularly cigarette smoking, provides a sadly topical example of the role of such external factors. Lung cancer was half as common 40 years ago, when we smoked less, so we do know what to do to reduce it. Yet in a vast controlled study into the cause of lung cancer, Western societies are using themselves as experimental animals and sacrificing several million people in the process, instead of simply giving up smoking and reducing the possibility of this disease.

Some of these chemical agents actually interact with others to produce much greater effects. For example, British lung cancer incidence in non-smoking town dwellers and non-smoking country dwellers was found to be similar. But among smokers, lung cancer mortality in large towns was twice that in rural areas, presumably as a result of interaction between smoking and air pollutants. Similarly, such raised lung cancer incidence also occurs in asbestos workers who smoke, while asbestos workers who do not smoke tend to get asbestosis, a chronic lung disease rather than lung cancer.

In smokers who drink alcohol, the interaction between smoking and alcohol leads to a higher incidence of cancer of upper digestive and respiratory passages, probably the result of damage to the cell-lining covering these passages.

In Egypt, the combination of schistosomiasis (a parasitic bladder infection) and smoking is likely to cause bladder cancer.

Genetic Factors

Compared with the very important role of these external (environmental) agents, the influence of intrinsic (genetic) factors in carcinogenesis pales into insignificance. Heredity (familial predisposition) plays only a minor role in cancer of the stomach, intestine, lung and breast. This will be dealt with in the appropriate section later, but let us now make some general observations on heredity.

When we are dealing with family susceptibility the important question to ask is whether the disease is actually due to some altered genetic pattern, or whether it is not much more likely that it is caused by environmental factors such as life-style or eating habits which affect all members of the family. Equally important is the question: which factor is more significant, the environmental or the genetic one? The answer, from the point of view of prevention, is that the environmental factors must be more important, because they are under our control, whereas the genetic factors are not. To give a practical example: fair-skinned, blue-eyed people are genetically predisposed to skin cancer on excessive exposure to ultraviolet sunlight. Although we can’t change our skin, we can control our amount of exposure to sunlight!

It stands to reason that each one of us should be on the lookout for factors within our own control, which might promote such risks and try to avoid them. For instance, even light smokers, whose parents have lung cancer, run a fifteen-fold increase in risk of developing a lung tumour.

Other Cancer-Causing Factors

Apart from this type of inherited proneness to cancer (which, as stated, plays only an insignificant role in the general incidence of cancer), there is another kind which is started by a pre-existing illness. Examples include ulcerative colitis, which may lead to bowel cancer, pernicious anaemia which may lead to stomach cancer, and a special type of anaemia (called the Plummer Vinson syndrome) which is caused in thin frail, elderly -  women by a diet poor in iron and which usually culminates in throat cancer.

It is worth noting that in these cases cancer can mostly be prevented by dealing with the original disease. This has been dramatically demonstrated quite recently in tropical liver cancer. A fungus called Aspergillus flavus produces, under humid tropical conditions, a cancer-causing mould called aflatoxin which can contaminate groundnuts and grain if stored in unsatisfactory conditions. When these contaminated peanuts are eaten by people who have been previously infected by the hepatitis B virus, they will develop liver cancer in large numbers. Such outbreaks occur mainly in the under-privileged nations of East Africa, where storage facilities still remain very primitive.

Infection with the hepatitis B virus is probably the commonest cause of liver disease world-wide. There are over 200 million people with this infection, of whom many will die from liver damage, including liver cancer.

Preventing the virus-induced hepatitis B by means of vaccination has only just become possible and it protects the liver against cancer. There is now real hope that this infection and the disease it causes will eventually be wiped out for good.

Most of the identified industrial cancer-causing substances have operated over at least ten years before being discovered. During this long period the interaction between two mutagens, like smoking and asbestos, occurs. Smoking is the initiator that induces the early stages while asbestos is the promoter which acts on the later steps.

Similarly, smoking will promote upper respiratory and digestive tract cancer in habitual alcohol consumers; or cervical cancer in women with previous herpes genitalis and/or human papilloma virus infections; or bladder cancer in Egyptians with a parasitic bladder infection.

This two-stage progression in the development of cancer offers a great opportunity for effective cancer prevention. By avoiding the known promoter, its triggering action on the actual malignant process can be blocked, thereby preventing the disease altogether.