The International Agency for Research on Cancer estimates that in 2020, about 19.3 million new cancer cases were diagnosed globally and this was accompanied by 10.0 million deaths. Cancer incidence and mortality rates varies my geographic region with Australia, New Zealand and Ireland ranked as the top countries with highest cancer incidence and mortality rates. In the USA alone, it is estimated that 1 in 2 women and 1 in 3 men will develop cancer in their lifetime.
Cancer is a disease of more than 100 hundred different types and most common types of cancers in the western world are lung, breast (women), prostate (men) and colon & rectal and skin cancers. Each cancer has parallel yet distinct origin with a lethal tendency to escape from the original site, enter the bloodstream, and spread to other sites in the body through a process known as metastasis. Although one hundred years of intense biomedical research has yielded tremendous breakthroughs including the discovery of radiotherapy and chemopreventative drugs that can be used in treatment for cancer patients, cancer as a disease remains largely incurable.
This is because cancer is a complex and a formidable disease. Cancer occurs when the process of normal cell division goes awry, leading to uncontrolled and unchecked cellular growth. We now know that cancer at the cellular level is a genetic disease caused by DNA damages in the cell. DNA is the genetic material for all known living organisms with the exception of some viruses which use ribonucleic acid (RNA) as their genetic material. All the information needed for normal cell processes, such as growth and division, are contained in these DNA sequences, on genes. Human beings have about 25,000 genes encoded in their DNA. Thus, any damage to DNA that affects the expression of these genes can lead to cancer. Many external as well as internal factors that cause DNA damage can result in cancer and the accumulation of these DNA damages over several years that leads to cancer. Consequently, one of the major risk factors associated with most types of cancer is age.
Despite these broad generalizations of risk factors associated with cancer, there are individuals who are more or less susceptible to cancer. For instance, some individuals can smoke their entire life and yet do not develop lung cancer. On the other hand, some nonsmokers do develop lung cancer on exposure to “second hand” tobacco smoke. This is due to small mutations that occurs in high frequency at different population or racial/ethnic group levels known as single nucleotide polymorphisms (SNPs). These small differences in the DNA can affect an individual’s metabolism positively, such as enhancing removal of toxins in the body that can damage DNA, or negatively, exacerbating the individuals’ inability to accomplish such a task. These genetic variants can also influence an individual’s response to medical intervention. Hereditary genetic predisposition has been associated with high risk of cancer in several individuals in a particular family. For instance, 1 in 40 Ashkenazi Jewish women with breast cancer has mutation in a gene called BRCA1. Thankfully, genetic predisposition to cancer accounts for less than 10% of all causes of cancer. Overall, high cancer incidence is the result of gene-environmental factors.
Cancer etiology is due to several factors both inside the body and in the environment. Exposure to a wide variety of natural and man-made substances in the environment accounts for at least two-thirds of all the cancer cases. Environmental factors include lifestyle choices such as tobacco smoke, excessive alcohol consumption, poor diet, lack of exercise, excessive sunlight exposure, and sexual behavior, as well as increased exposure to certain viruses. Other factors include exposure to certain drugs, hormones, radiation, viruses, bacteria, and environmental chemicals that may be present in the air, water, on workplace. Most chemicals are not carcinogenic, but a wide variety of chemicals and chemical classes can cause cancer in animals and humans. As a result, cancer is a complex genetic disease that invariably requires multiple genetic lesions to breach these safeguards. Even in individuals with inherited mutations in critical protective genes such as tumor suppressors, cancer usually does not develop for many years or decades. Yet, the inexorable accumulation of mutations during a lifetime ensures that some form of malignant disease will eventually develop in many if not most humans.
Epidemiology, which is the study of disease in the population, has been instrumental in identifying several risk factors for different cancers. These studies either compare the risk of certain cancers in people with and without certain characteristics (cohort studies) or compare the histories of people with and without cancer (case control studies) to provide information on a wide range of possible risk factors. In addition, laboratory experiments using animal and human cells in culture or in animal models, helped confirm some of these risk factors. The environment, reproductive life, diet, and smoke are four important risk factors associated with cancer. Exposure to carcinogens, hormonal modifications influenced by childbirth and birth control, and exposure to viruses underlie these lifestyle factors. These factors can exert pressure on different groups of cells that can subvert normal regulatory processes. These lifestyle factors in principle can be altered to prevent many cancers.
The first recognized and recorded instance of an environmental cancer due to environmental influence was reported in 1775 by an English physician who concluded that the high incidence of nasal and scrotum cancer in chimney sweeps was due to chronic exposure to soot. Not only where a person works but also where a person relaxes can contribute to the risk of developing cancer. For instance, unprotected exposure to the sun exposes the skin to UV radiation which can directly alter DNA to cause mutations. Exposure to asbestos is linked primarily to lung cancer, whereas exposure to benzidine, a chemical found in certain dyes, is associated with bladder cancer, and smoking is linked to cancers of the lung, bladder, mouth, colon, kidney, throat, esophagus, lip, stomach, cervix, liver, and pancreas.
In addition, the importance of the environment can be seen in the difference in cancer rates throughout the world and change in cancer rates when groups of people migrate from one country to another. For example, when Asians, who have low rates of prostate and breast cancer and high rates of stomach cancer in their native countries, immigrate to the US, their prostate and breast cancer rates rise over time until they are nearly equal to or even greater than the higher levels of these cancers in the US. Likewise, their rates of stomach cancer fall, becoming nearly equal to the lower US rates. Lifestyle factors such as diet, exercise and obesity are thought to play a major role in the trends for breast and prostate cancer, and infection with the bacterium Helicobacter pylori is an important risk factor for stomach cancer.
Another early observation was that nuns are more likely to develop breast cancer than other women. It is now known that having children reduces breast cancer risk for women. The age of a woman at the time of giving birth for the first time and the age of a woman at the initiation and termination of her menstrual cycles also influence certain cancer risks. Hormonal contraception and fertility treatments can also affect cancer risk because they alter a woman’s ovulation schedule. Sexual promiscuity can also contribute to increased risk of cancer- sexually transmitted human papillomavirueses can be found in all cervical cancers worldwide. It is not surprising therefore that nuns have a low incidence of cervical cancer whereas prostitutes have the highest incidence of cervical cancer.
The incidence of a specific cancer varies greatly between different populations in different geographical locations, with diet being one of the most influential factors. For instance, stomach cancer is the predominant cancer in the Chinese population and a minor cancer in the population of the USA. Interestingly, the risk of stomach cancer in Chinese people who have migrated to the USA decreases only if they adopt the American diet, but not if they retain an Eastern diet. Asian men have the lowest incidence of prostate cancer if they remain in Asia. However, the incidence of prostate cancer is significantly higher in 2nd and 3rd Asian migrants in the USA as a consequence of adopting the western diet. The Mediterranean diet, which is rich in fresh fruit, vegetables, and red wine has been promoted to be beneficial in reducing cancer risk. Given the mounting evidence linking fruit and vegetable consumption to reduction in cancer risk (as well as reduction in obesity and other health problems), every effort to inspire a high degree of awareness, such as designing memorable slogans such as the “5-A-Day” campaign, must be at the top of every nation’s health agenda.
The clearest example of lifestyle factors underlying a specific cancer is the discovery that smoking causes lung cancer (it is also implicated in pancreas, bladder, kidney, mouth, stomach, and liver cancer). Smoking accounts for 30 percent of all cancer deaths. At least 81 carcinogens have been identified in cigarette smoke. Smoking became particularly fashionable in Europe and the USA during the 1st and 2nd world wars and resulted in an epidemic of lung carcinoma. After vast public education campaigns and a subsequent reduction of smoking, lung cancer rates have fallen dramatically. This is a clear demonstration that some future cancer deaths can be avoided by changes in lifestyle factors.
One’s immune system and stress level as well as spiritual wellness can also affect a person’s susceptibility to cancer. The importance of the immune system for health is illustrated by the frequent observation that individuals with defective immune response to chronic infection are susceptible to serious, often life-threatening infections as evidenced by the emergence of the acquired immunodeficiency syndrome (AIDS) in the 1980s. Typically, the immune system recognizes these abnormal cells and kills them before they produce a tumor. Defects in the development and functions of the immune system results in increased susceptibility to infections, reactivation of latent infections (such as cytomegalovirus, Epstein-Barr virus, and tuberculosis) in which the normal immune response keeps the infection in check but does not eradicate it, and increased incidence of certain cancers. Dietary bioactive food components that interact with the immune response have considerable potential to reduce the risk of cancer.
In conclusion, the recent declines in cancer incidence and mortality rates are mainly due to improved preventive measures. The decline in stomach cancer is attributed to refrigeration, and a decline in lung cancer is attributed to decrease in smoking as the public becomes increasingly knowledgeable about the harmful effects of smoke. Furthermore, declines in colorectal, cervical, prostate, and breast cancer mortality reflect the effect of improved screening, leading to early detection and diagnosis. An estimated one third to one half of cancers are preventable. At least one-third of cases of cancer are caused by environmental factors. Many of these cancers are linked to lifestyle factors that can be modified such as cigarette smoke, excessive alcohol consumption, poor diet, low physical activity, and being overweight or obese.
A third of cancer deaths can be prevented by eliminating tobacco smoke. After tobacco, overweight or obesity appears to be the next most important preventable cause of cancer. In addition to lifestyle choices, precautions can be taken in the home and workplace to reduce exposure to other harmful exposures. In the past 25 years enormous progress has been made in defining the molecular events that take place as a normal cell becomes malignant and finding the critical genes thought to be involved in cancer. These scientific advances clearly indicate that what we put in our mouths and what we do every day impact our risk of cancer. Thus, knowledge about cancer prevention measures that address modifiable risk factors such as tobacco smoke, alcohol consumption, diet, occupational exposure, infection and physical activity can help ease cancer burden.
(Read more: Cancer Causes and Controversies- Understanding Risk Reduction and Prevention; Praeger, 2011).