A U.S. government study finds that immigrants are generally healthier than native-born Americans for the first few years after they arrive, but their health declines as they adapt to the American lifestyle.

The saying "When in Rome, do as the Romans do" speaks of assimilating into a new culture. But assimilating can mean adopting the bad along with the good. In just such an example, a new study from the U.S. Centers for Disease Control shows that immigrants from Asia, Africa, and Latin America lose the health advantages they come with about five years after migrating.

One of the study's authors is Achintya Dey, a native of India.

"In general, immigrant adults enjoy considerable advantages over their U.S.-born counterparts on many health measures, despite limited access to health care," she said. "However, the longer these immigrants live in the U.S., the more their health resembles their U.S.-born counterparts."

For example, the government report shows that obesity affected 22 percent of Hispanic immigrants living in the United States five years or longer compared to 16 percent residing in the country less than five years. The earlier immigrants were also likelier to have high blood pressure and heart disease than recent ones.

Black and Asian immigrant adults showed similar patterns of health decline with longer U.S. residence. Only white immigrants avoided this trend.

Achintya Dey says foreigners come to the United States healthier than the average American adult probably because people who leave their countries tend to be among the healthiest in their lands. But he points out that, after a few years, Americanization sets in.

"What happens over the years, they adopt the lifestyle, you know, diet change, they have less exercise or the tobacco use goes up," she said. "So their behavior and the lifestyle changes, and that could explain why there is a decline in their overall health."

The findings are based on U.S. national health surveys taken between 1998 and 2003.

"This study is a typical pattern that we're seeing now among immigrants," said Barbara Krimgold, of the Center for the Advancement of Public Health, a health policy research organization in Washington.

"Immigrants tend to be in neighborhoods with, say, lots more advertising of alcohol, lots more fast food, less access to fresh fruits and vegetables," she said.

The United States is not the only country where immigrant health deteriorates. A 2002 study found that the health of Asian immigrants in Canada also worsens. Its lead author, Mark Kaplan of Portland State University in Oregon, says immigrants' new, less healthy lifestyles present challenges to health policy makers with limited resources in ethnically diverse societies. He suggests directing health awareness programs at those who have lived in North America the longest, since the research shows that they are the unhealthiest.

"There is a lot of mobility around the world," Kaplan said. "So the question is, how do we design public health programs that effectively target those immigrant groups who are at greatest risk?"