STOCKHOLM (Reuters) - Lifestyle during the first 20 years of life is a more important factor than genetics when defining the risk of cancer, two Swedish studies on first- and second-generation immigrants show.

"Birth in Sweden sets the Swedish pattern for cancer development irrespective of the nationality of descent," said the abstract of one of the studies due to be published on May 10 in the International Journal of Cancer.

The survey of some 600,000 immigrants, who came to Sweden in their 20s and became parents in this country, showed that their cancer risks did not differ significantly from the cancer risks of the populations in their native countries.

But second-generation immigrants born in Sweden had a different risk profile than their parents and had a similar cancer incidence as native Swedes, another study of some 600,000 immigrants, mainly from Europe and North America, showed.

"Internationally, there are clear differences between cancer risks. But these differences disappear within one generation, so environment must clearly be a significant cancer factor," Professor Kari Hemminki of the Karolinska Institute university hospital told Reuters Monday.

As the first 20 years of a person's life were so important in defining the risk of incurring cancer, possible preventive means should be aimed during that period, he said.

For some types of cancer it was clear why the risks were different between the generation who spent their first 20 years outside Sweden and their children born in Sweden.

For example, the risk of lung cancer decreased among second-generation immigrants because Swedes tended to smoke less than people in many other countries.

The higher risk of stomach cancer among first-generation immigrants compared with their children and native Swedes could be linked to eating habits, vitamin deficiencies and use of salt--all factors linked to this type of cancer, Hemminki said.

Darker skinned second-generation immigrants were as prone to contract skin cancer as blond Swedes, and much more so than their own parents, due to a similar sun bathing style among youths regardless of origin.

The studies were made using the Swedish Family Cancer Database, which contains information on all people born in Sweden after 1931 and their parents.

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