By Julie Steenhuysen

CHICAGO (Reuters) - Tough choices tempt kids at every turn -- whether it is soda in school, junk food ads on TV or the fast-food chain around the corner -- and school policies limiting physical activity only make matters worse, U.S. researchers said on Tuesday.

This throng of temptations may explain why childhood obesity has reached epidemic proportions, they said.

The collection of studies, published in the American Journal of Preventive Medicine, together suggest environmental factors and policies conspire to challenge the health of children in America.

"We have in our schools and communities a perfect storm that will continue to feed the childhood obesity epidemic until we adopt policies that improve the health of our communities and our kids," Frank Chaloupka, an economics professor the University of Illinois at Chicago, said in a statement.

While too many calories and too little exercise explain how children become obese, the research looks at environmental factors that contribute to these behaviors, and suggests policy changes that could make healthy choices easier.

"The environment that they live in matters," said Lisa Powell of the University of Illinois at Chicago, who studied restaurant and food store options in the neighborhoods and food-related television advertising aimed at teens.

She said when people cannot get to supermarkets but instead must rely on the convenience stores that proliferate in many poor neighborhoods, families end up eating less healthy food.

Lower-income neighborhoods also tend to have a higher proportion of fast-food restaurants, and black urban neighborhoods have the highest percentage of fast-food restaurants.

"The general environment around them is not really conducive to a healthy lifestyle," she said. "It is not surprising that we would in turn see an increased likelihood of overweight."

NO SAFE HAVEN

When teens are at home, they see a barrage of advertisements for fast food and sweets, Powell said.

She and colleagues studied more than 200,000 ads on top-rated shows viewed by teens aged 12 to 17 in 2003 and 2004. Powell found more than a quarter of the ads were for fast food, sweets and beverages -- items well within a teen budget.

Overall, fast-food advertising comprised 23 percent of all food-related ads seen by teens.

At school, teens have ready access to high-fat, sugary foods and drinks, according to a study by Lloyd Johnston and colleagues at the University of Michigan.

Johnston found the majority of middle schools (67 percent) and high schools (83 percent) had contracts with a soft-drink company.

While high schools are more likely to offer soft drinks, they are less likely to require physical education, Johnston found in a separate study. While 87 percent of 13- to 14-year- old students surveyed attend schools that require physical education, only 20 percent of 17- to 18-year-olds face physical education requirements.

"Historically, people have thought of obesity in terms of individual willpower, but there is a great abundance of environmental influence that contributes as well," he said in a telephone interview.

"Communities and schools need to be looking at what they are doing and trying to improve it," he said.