Immigrant children of all backgrounds get even less vigorous exercise than their U.S.-born counterparts, the largest study of its kind suggests.

Plenty of earlier evidence shows that U.S. children are pretty inactive. The new study of nearly 70,000 children simply found even lower levels of activity among immigrants.

Almost 18 percent of foreign-born children with immigrant parents got no vigorous exercise on any days of the week, and 56 percent didn't participate in organized sports.

By contrast, 11 percent of U.S.-born children with American parents got no vigorous exercise, and 41 percent didn't participate in sports.

Given the obesity epidemic and immigrants accounting for about 13 percent of the U.S. population, the authors said it is important to know whether there are ethnic differences in physical activity and sedentary behaviors. They were led by Dr. Gopal Singh, a researcher at the U.S. Department of Health and Human Services's Maternal and Child Health Bureau.

Here's how the researchers explain their results: Immigrant families surveyed were on the whole poorer than nonimmigrants and lived in less safe neighborhoods. That means they likely had less time for exercise and sports, and worse access to places to engage in those activities.

But also, many immigrant parents place a high emphasis on reading, language lessons, studying and other inactive pursuits.

Interestingly, earlier research found that immigrants tend to be less overweight and obese than people born and raised in the United States. That difference tends to wear off with longer exposure to U.S. culture including junk food and television.

The new study also found that immigrant children generally watched less TV than American-born kids, although it did not look at obesity levels.

"Many of our American norms are not healthy," said Dr. Sarah Armstrong, a Duke University childhood obesity expert. "Could we just teach them our good habits, and not our bad?"

Armstrong said it was the largest study by far to look at the topic.

The study appears in Monday's Archives of Pediatrics and Adolescent Medicine. It is based on 2003-04 telephone interviews with parents of children aged 6 to 17, including white, black, Hispanic and Asian immigrants.

Singh said the results among Hispanics were particularly striking: nearly 23 percent of children in families where both parents were born in Spanish-speaking countries got no vigorous physical activity. Also, two-thirds of them didn't participate in organized sports.

Dr. Mita Sanghavi Goel of Northwestern University said those results are troubling because of high rates of obesity and diabetes — both related to inactivity — among Hispanics, the nation's largest immigrant group.

"That just highlights how important it is to intervene early and set healthy lifestyle patterns early on," Goel said.

Rates for other immigrants who got no vigorous activity were 13 percent for blacks, almost 10 percent for whites and 7 percent for Asians. For no participation in sports, the rates were 49 percent for blacks, 38 percent for Asians and 32 percent for whites.

The authors said more research is needed to verify results in Asians because relatively few Asians were studied.

Among all immigrant groups combined, 65 percent got regular physical activity, versus 75 percent of U.S.-born children whose parents were both born in America.

Regular physical activity was defined as getting at least 20 minutes of vigorous exercise such as running, swimming and basketball at least three days weekly. That was the minimum amount the government recommended when families were surveyed.

Newer government advice recommends an hour of moderate-to-vigorous exercise most days. Just last month, a study found that fewer than a third of U.S. 15-year-olds got even that minimum amount.

The authors acknowledged that parents may not always know exactly how much activity their children get, a potential limitation of the new study.