Lifestyle factors during childhood and adolescence have a decisive impact on the risk of contracting cancer later in life.
Scientists at the Karolinska Institutet in Sweden have compared the incidence of cancer in first- and second-generation immigrants in Sweden.
The findings are presented in three scientific articles in the coming issue of the International Journal of Cancer.
In two large-scale studies, the risk of developing cancer among first- and second-generation immigrants in Sweden has been analyzed to reflect the relative importance of where people grew up.
The material is based on information about everyone born in Sweden after 1931 and their parents, a total of 10.2 million individuals, who have been linked to the Swedish Cancer Register.
Of these, 613,000 people were first-generation immigrants who had come to Sweden at the age of 20-30 years. Among them, there were 32,000 individuals who later developed some form of cancer, the clear majority of them after at least ten years’ residence in Sweden.
The distribution across different types of cancer, which vary widely from one country to the next, is similar to that which is characteristic of their respective countries of origin.
The results show that the risk of contracting cancer is "determined" in most cases during childhood and adolescence, both via the environment patients grow up in and via the life-styles formed then.
The overall frequency of cancer among immigrants from the Nordic countries, the rest of Europe, and the United States was similar to that of Swedes, while immigrants from other parts of the world, above all Asia, exhibited a relatively lower number of cancer cases.
The study of second-generation immigrants comprises just under 600,000 individuals born in Sweden to immigrant parents.
The results of this analysis show that the differences that exist between first-generation immigrants, owing to differences in the childhood and adolescent environment and life-styles, largely disappear in the Sweden-born second generation.
In most cases, they have adopted the Swedish "cancer pattern." This further strengthens the indications that the crucial patterns in the risk of later developing cancer are established during childhood and adolescence.
The great challenge and opportunity presented by studies like these involving immigrants is that environmental factors and details in life patterns can be identified as significant in the development of different forms of cancer.
Analyses have also been performed on the entire material in the family cancer database, where some 90 percent of the individuals are Swedes. Of these, more than 600,000 parents have contracted cancer, as have 92,000 of their children.
By comparing the risk of cancer with close blood relations, common environments when growing up, and separate living environments as adults, researchers have been able to calculate the importance of nature and nurture in developing various forms of cancer.
Fifteen common forms of cancer have been studied. Of these, only thyroid cancer turned out to be clearly correlated with close blood relations.
Cancers of the endocrine glands, testicles, breast, cervix and intestines as well as melanoma exhibited some genetic influence, whereas stomach cancer and leukemia showed hardly any genetic background.
Having a common adult life environment without having had a common childhood and adolescence, which can be studied in spouses, often involves the same food and tobacco habits. Here the greatest correlation was shown for stomach and lung cancer, but also for cancer of the bladder and colon, fully analogous with the assumption that certain dietary habits influence the risk of cancer of the stomach and intestines, and that smoking increases the risk of lung and bladder cancer.
The genes identified in families with many cases of cancer of the breast, cervix, intestines and other organs can only be said to explain part of the familial connections.
In summary, this national study shows that lifestyle is absolutely the prime factor in the risk of getting cancer.
(Reference: Three articles in International Journal of Cancer, 99 (2002), appearing on May 10 but already available in a web version at this URL. Kari Hemminki, Xinjun Li, Kamila Czene, "Cancer risks in first generation immigrants to Sweden"(pp. 218-228); Kari Hemminki, Xinjun Li, "Cancer risks in second-generation immigrants to Sweden" (pp. 229-237); Kamile Czene, Paul Lichtenstein, Kari Hemminki, "Environmental and heritable causes of cancer among 9.6 million individuals in the Swedish family-cancer database"(pp. 260-266.))
[Contact: Professor Kari Hemminki]