Carcinogens and Cancer page 2
Free radicals
Free radicals (or oxidants) are highly reactive forms of oxygen or nitrogen that are produced by the body's metabolism, environmental factors, inflammation and sometimes by infection. Molecules are made up of atoms held together by chemical bonds. Each chemical bond consists of a pair of electrons. When a bond is broken, two molecular fragments (or atoms) are left over, each of which contains one unpaired electron. These atoms are highly charged and highly unstable because of the unpaired electron. These charged, highly unstable, and very reactive particles are known as free radicals.
Free radicals usually exist only for a small fraction of a second and spend their short lives racing around looking for molecules to combine or react with. These reactions are called oxidation, a process that is very similar to the formation of rust on metal. When a free radical combines with a molecule, it can damage the molecule and consequently the cell it belongs to. The damage can occur to cell membranes, proteins, lipids (fats), or to the cell's DNA. It is this damage to the cell's DNA that eventually leads to cancer. It is also believed that free radicals cause damage to cells that results in what we know as aging.
The body has about 1 trillion cells, and each cell is estimated to receive around 10,000 free radical hits each day. Part of the reason there are so many oxidative reactions is that each reaction causes a chain reaction. Thus, one free radical potentially leads to the creation of hundreds of thousands of others. The good news is that we have an antioxidant defense system that protects us from this barrage of cellular attacks. Antioxidants react with free radicals and end the chain reactions that could lead to cellular damage. If we did not have a good defense mechanism, our body would break down very quickly.
It is important to note that most of the mutagens mentioned in this section, such as ionizing radiation, heavy metals, cigarette smoke, alcohol, fat (see below for alcohol and fat), iron and other chemicals, generate free radicals. Free radicals are also produced as a result of exercise. However, regular exercise enhances the antioxidant defense system and therefore protects the body against damage that would otherwise be caused by these free radicals.
Other risk factors – Lifestyle
Although carcinogens must cause a genetic mutation to a cell in order to kick-start the development of cancer, there are several other factors that can lead to the disease. Some of these factors are not carcinogens by definition, but they definitely do their part in carcinogenesis (the development of cancer).
Smoking is the leading cause of cancer. The American Cancer Society estimates that over 180,000 people will die from tobacco-related cancer in 2003. This is around one third of all cancer deaths. Besides generating certain cancer-causing chemicals in the smoke you inhale, it also causes different kinds of cellular oxidation including damage to cells' DNA. Smoking also reduces plasma levels of folic acid, which can debilitate the immune system and lead to cancer. Any way you look at it, the statistics speak for themselves – if you smoke, you have a much higher risk than if you do not. Although most people don't know it, smoking increases the risks of other cancers besides that of the lung, such as pancreatic, cervical, stomach and kidney cancers as well as myeloid leukaemia. Any amount of smoking increases your cancer risk dramatically. Significant second-hand smoke exposure can double the lung cancer risk for a non-smoker.
Diet is probably the most important risk factor. Diets high in fat are associated with increased cancer risk, especially of the prostate, colon, rectum and breast. In women, a high fat diet raises estrogen levels in the body, and high estrogen has been found to increase breast cancer risk. Diets low in fiber are associated with higher colon cancer risk (as well as other cancers). Diets high in sugar and simple carbohydrates are conducive to cancer. Diets high in antioxidants are associated with significantly reduced levels of cancer in general. There are many other dietary factors that influence your odds of developing cancer. We don't go into detail here because diet, and its relation to cancer risk, is what this book is mostly about and we talk about it extensively further on.
Excessive alcohol consumption has also been linked to increased cancer risk, especially of the liver, mouth, esophagus, stomach, colon, rectum, lung and breast. Those who drink and smoke have an even higher risk than the sum of the risks of either just smoking or just drinking excessively. Anything more than an average of two drinks per day is considered excessive.