A new study finds that the longer immigrants from Mexico, and their U.S.-born offspring, spend in the United States, the greater their odds of becoming obese.

Compared to similar individuals living in Mexico, researchers found the grandchildren of immigrants to the U.S. from Mexico were three times more likely to be obese adults.

"We just couldn't believe the fact that we found roughly a threefold increase from the one extreme... to the people on the other side," said the study's lead author Karen R. Florez, an associate social scientist at the non-profit research institute Rand Corporation, in Santa Monica, California.

Past research has found that immigrants to the U.S. are typically healthier in many ways than people in their ethnic groups who were born in the U.S.

In February, one study found that Hispanics born abroad had a much lower risk of stroke than their counterparts who were either born or spent most of their lives in the U.S.

Florez and her fellow researchers said it's also been established that U.S.-born Mexican Americans have greater odds of being obese than their family members who originally migrated from Mexico.

But the team wanted to extend that comparison to people who are still living in Mexico, in an attempt to tease apart and identify factors in the U.S. environment, or in the fact of being a migrant, that might influence obesity risk.

For the study, the researchers used one database from Mexico and another from the U.S. with information on 3,244 people's body mass index (BMI), a measurement of weight in relation to height.

In adults, a healthy BMI is between 18.5 and 25. Anything above a BMI of 30 is considered obese.

Each person in the study population was separated into groups depending on how long they had been "exposed" to the U.S., if at all.

For example, one group consisted of those who lived in Mexico and had no family in the U.S.; at the other extreme were U.S.-born Mexican Americans whose grandparents were the first to immigrate.

The researchers found that among those whose grandparents first immigrated to the U.S., about 32 percent of men and 36 percent of women were obese.

That's nearly double the proportion of men (17 percent) and women (14 percent) living in Mexico without any ties to the U.S. who were obese.

Overall, with a few exceptions, the results showed a trend of increasing obesity as "exposure" to the U.S. grew.

Kiarri Kershaw, an assistant professor of preventive medicine at the Northwestern University Feinberg School of Medicine in Chicago, points out, however, that while a larger percentage of the grandchildren were obese than their immigrant grandparents, the study did not say how long the grandparents had lived in the U.S.

Still, it is unique and valuable that the researchers included comparisons to people still living in Mexico, said Kershaw, who was not involved in the study, even though the findings are consistent with what is already known.

As for why a larger percentage of people may be obese the longer they or their families have been in the U.S., Florez and her colleagues write that one theory centers on the so-called food environment.

For example, the team notes in its brief report, published in the Archives of Internal Medicine, that a 12-ounce Coca-Cola in the U.S. has 240 calories with 65 grams of sugar, compared to the same drink in Mexico with 150 calories and 39 grams of sugar.

Kershaw said there could be other reasons too.

"I think it's complicated. It could be the food environment; it could be stress with acculturation or the loss of support networks when a person moves to a new country. I don't think that's clear," she said.