Feb 13 (Reuters) - People in the United States who live at higher altitudes where the air is thinnest are less likely to be obese than those in low-lying areas, according to a U.S. study.
Using data for more than 400,000 people, researchers who published their results in the International Journal of Obesity found that people who lived closest to sea level were four to five times more likely to be obese, compared to people who live well above sea level in Colorado.
"I was surprised by the magnitude of the effect," said lead author Jameson Voss, from Uniformed Services University of the Health Sciences in Bethesda, Maryland. "I wasn't expecting such a consistent pattern as what was emerging."
About 36 percent of U.S. adults are obese, according to the U.S. Centers for Disease Control and Prevention (CDC). Rates vary across the nation, however, with a higher percentage of obese adults in southern states. Western states, such as Nevada and Colorado, report the fewest obese adults.
The reasons behind the difference are unclear, said Voss and his colleagues, but one possible explanation is differences in elevation - which can affect appetite hormones, growth and how many calories the body burns.
For the study, the researchers combined information from several databases, including a telephone health survey of 422,603 U.S. residents from 2011.
They had information on 236 people who lived at the highest altitude of at least 3,000 meters (about 9,800 feet) above sea level. They all lived in Colorado and tended to smoke less, eat healthier and exercise more.
The researchers also had information on 322,681 people who lived in the lowest altitude range, or less than 500 m (about 1,600 feet) above sea level.
After taking into account other factors that could influence the results, the researchers found that adults living in the lowest altitude range had a Body Mass Index (BMI) - a measure of weight against height - of 26.6. That compared to a BMI of 24.2 for people who lived in the highest altitude range.
A healthy BMI falls between 18.5 and 24.9, and obesity is a score of 30 or more, according to the CDC.
Voss and his colleagues also found that a drop in the risk of a person being obese was tied to every 200 meter increase in elevation.
"It provides some evidence that these associations persist over the long term," he said.
Cynthia Beall, a professor of anthropology at Case Western Reserve University, said it's common for travelers to high elevations to burn more calories in their first few weeks and that it would be interesting to see whether that would sustain.
But she cautioned that the new findings don't prove the higher altitude prevented people from being obese, noting that other researchers have found that people living in Colorado's higher altitudes more to lower altitudes when they become sick with chronic illnesses.
Beall said it would be interesting to take a closer look at the people living in Colorado and see whether their obesity prevalence would change if they moved to a lower altitude.
"I know from my own self that when I come back to my own activities and diet, I come back to my own weight too." SOURCE: http://bit.ly/WEWACs (Reporting from New York by Andrew Seaman at Reuters Health; editing by Elaine Lies)