Here is an essay on ‘Lifestyle and Cancer’ which states – How environmental and lifestyle factors play a prominent role in triggering the development of cancer.
One of the more striking findings to emerge from epidemiological studies of cancer incidence is the extent to which particular kinds of cancer arise with differing frequencies in different parts of the world (Figure 5). For example, liver cancer is especially prominent in Africa and Southeast Asia, stomach cancer is frequent in Japan, and prostate, colon, breast, and lung cancers are common in the United States.
These differences are based on age- adjusted rates, which mean that the likelihood of developing a particular kind of cancer for a person of a given age, say 50 years old is significantly affected by the country in which that person lives.
If a 50-year-old person’s risk of developing various types of cancer depends on where he or she lives, it suggests that the causes of cancer are to be found in factors that differ between countries rather than in the process of aging per se. So we must look to variables that differ between geographic locations, and not simply aging, to find the causes of cancer.
In theory, these determining variables might involve either environmental factors that vary between countries, differences in the genetic makeup of people living in those countries, or some combination of the two.
To investigate the relative roles played by environment and heredity, scientists have studied cancer rates in people who move from one country to another. In such cases, a person’s heredity stays the same but his or her environment changes, so the relative contributions of the two factors can be evaluated.
Some of the most striking data involve the experience of Japanese immigrants to the United States. In Japan, the incidence of stomach cancer is greater and the incidence of colon cancer is lower than in the United States. When Japanese families move to the United States, these differences begin to diminish and their children acquire cancer risks that are more typical of those observed in their new location (Figure 6).
Hence the risk of developing these cancers must be determined largely by environmental factors rather than by inherited susceptibility. This conclusion does not mean that heredity plays no role in determining cancer risk, but rather that the role played by heredity is simply smaller than that of the environment.
Calculations based on these and other types of epidemiological data suggest that environmental factors account for about 80% to 90% of our cancer risk, with the remaining 10% to 20% coming from hereditary factors. (The term environmental is being used here in its broadest sense and encompasses not just the physical environment, but also the social and cultural environment and its associated effect on lifestyle choices.)
If the risk of developing cancer is determined to a large extent by environmental factors, how do we explain the dramatic increase in cancer rates seen in older people?
The answer to this question involves at least two components. First, the longer a person lives, the greater the accumulated lifetime exposure to those environmental agents that cause cancer and thus the greater the chance that cancer will eventually arise. Second, the development of cancer is usually a prolonged, multistep process that evolves over a period of several decades, making the disease less likely to appear in younger people.