Our bodies are made up of billions of cells organised into groups with specialised functions. e.g. heart, liver, lung, brain etc. All of our cells are born, reproduce (with some exceptions) and die. The lifespan of cells is sometimes days, other cells may live for months or even longer. The life cycle of cells is not left to chance and the birth, reproduction and death of cells is rigorously and meticulously controlled. Each time a cell reproduces, there is a very small chance that a genetic error will occur. Many of these errors are detected and reversed but some or not. The more times a cell reproduces, the more chance there is for errors to occur.
It is probably an accumulation of such errors, which is at least in part responsible for aging and the increased risk of developing cancer as we get older. Cancer is an abnormal proliferation of cells where either too many cells are being born, or they fail to die. Often, both of these factors seem to be involved. Because the cells are growing too fast or are failing to die, uncontrolled tumours (cancer) develops and these cells often spread to other sites, where they continue their abnormal growth and interfere with the function of normal tissues and organs. Any factors which increase cell growth and reproduction are therefore usually associated with an increased risk of genetic errors and cancer formation.
Within the last ten years or so, it has become apparent that obesity (too much fatty tissue) increases the risk of cancer and taking regular exercise, reduces the risk of cancer. It is now thought that obesity is second only to cigarette smoking as a preventable cause of cancer. Indeed, approximately one in five of all cancer is directly attributable to obesity and this figure rises to around one in two cancers in postmenopausal women. The types of cancer linked to obesity include colon, breast, prostate, kidney, oesophageal, endometrial, melanoma, thyroid cancer, leukaemia, certain forms of lymphoma and multiple myeloma.
Not only does obesity increase the incidence of cancer but the outcome of cancer appears also to be worse in obese individuals. It is also likely that there is a dose effect, in that the more obese you are the higher the risk and the poorer the outcome. Furthermore, if you are of normal weight and then become obese in later life, you similarly increase your risk of cancer, implying a direct cause and effect between obesity and cancer. For reasons that are not fully understood, excess fatty tissue in the abdominal area, seems to be particularly bad in terms of your risk of cancer.
How does obesity cause cancer?
Needless to say we don't yet have all the answers to this but there are a number of mechanisms that seem to be important. We know that obesity also causes type 11 diabetes, which is an inability to use glucose properly as a fuel, and this in turn increase the levels of certain hormones and growth factors, such as insulin. It is suggested that these factors drive abnormal cell growth and enhance cell survival. Secondly, obesity is associated with alterations in other hormone levels (particularly sex hormones such as oestrogen, progesterone and testosterone)) and these altered hormone levels also seem to drive cell growth and survival. Thirdly, the immune system does not like excess fatty tissue accumulation and cells of the immune system invade this fatty tissue and in the process produce a large variety of products which drive cell growth and retard cell death. It is also thought that one of the normal functions of the immune system is to seek out and destroy cancerous cells and in obese people this function of the immune system may also be impaired.
All of this may seem like bad news if you are overweight or obese. However, all is far from lost and some very achievable remedies are available. While we know that obesity causes cancer, we also know that moderate levels of exercise directly prevents cancer and furthermore, as regular exercise reduces obesity this also indirectly lowers cancer risk. The direct effects of exercise on reducing cancer risk appears to be particularly associated with breast, colon, endometrial, lung and prostate cancer, but probably has a direct effect on the risk of other cancers as well. If you add in the effects of exercise in reducing obesity, then regular exercise can have a large positive impact in reducing cancer risk.
Aside from its effect in reducing obesity, how does exercise directly affect cancer risk?
Again, the answer to this is not fully understood but mechanisms such as reduction in cell growth factors with exercise, improved hormone balance, improved immune function, improved cardiovascular function, reduced inflammation and a better capacity to deal with physical and psychological stress are all probably important.
This is all good news if you want to reduce your risks of developing cancer but what if I already have cancer?
Well there is more good news regarding the effects of exercise. There is now a widespread consensus that moderate amounts of physical activity improves quality of life following cancer diagnosis. Regular exercise is particularly good for dealing with cancer related fatigue and psychological well-being, improving cardiovascular function, and reducing the risk of additional complications such as type two diabetes and atherosclerosis. There is even some preliminary information emerging that regular moderate levels of exercise may improve survival in some cancers such as breast and colorectal cancer. If you have cancer, exercise will certainly help but talk to your doctor first and make sure you don’t have particular medical reasons for avoiding certain types of exercise.
Although cancer is in many ways a feared disease the outlook for many types of cancer has improved dramatically and cure is now possible in many cases. In particular, there are now some straightforward steps we can take to significantly reduce our cancer risk and help our quality of life post cancer diagnosis. Stop smoking, eat appropriately, reduce alcohol consumption, exercise well (30 minutes per day) and take steps to incorporate some form of daily relaxation and down time into your life. Although not all cancer risk is under our direct we can all take steps to significantly reduce our chances of developing cancer.