MONDAY, July 7 (HealthDay News) -- The kinds of foods that students can purchase at their public middle schools or high schools are far less healthy than the food available to children attending elementary schools, a new study suggests.

The researchers based their conclusion on a tally of the number of vending machines installed at 395 schools spread across 129 school districts in 38 states, as well as on a nutritional analysis of the kinds of foods stocked in the machines or offered up a la carte in school cafeterias and snack bars.

"The food environment changes as you move from elementary schools to high schools," said study author Daniel M. Finkelstein, a researcher with Mathematica Policy Research, in Cambridge, Mass. "And the main difference between the lower and higher grades was the greater availability of unhealthful foods and beverages for older students."

The findings are reported in the July issue of Pediatrics.

Finkelstein said the purpose of the study was to take a nutritional snapshot of current food offerings in public schools -- not to gauge exactly what students were purchasing or consuming. The research also didn't try to explain what is driving the nutritional shift between the elementary school and the high school level.

The researchers analyzed questionnaires and food checklists completed in 2005 by school principals and food managers as part of the third School Nutrition and Dietary Assessment study. Random on-site food inspections were also done in some schools.

The conclusion: The school "food environment" is significantly healthier in lower grades.

While vending machines were found in just 17 percent of elementary schools, they were found in 82 percent and 97 percent of middle schools and high schools, respectively.

Finkelstein noted that, while vending machines aren't automatic sources of unhealthful foods and drinks, they are -- along with a la carte cafeteria options -- known to be sources of low-nutrient foods. In other words -- junk food.

In fact, the study found that 91 percent of high school vending machines sell at least some unhealthy foods.

The study also found that a la carte food and beverage options were served in 71 percent, 92 percent, and 93 percent of elementary, middle and high schools, respectively. Almost 80 percent of these options were determined to be unhealthful.

In 75 percent of high schools, students weren't allowed to leave campus during lunch. Yet less than half of all the elementary, middle, or high schools surveyed had a "wellness policy" in place, or took advantage of a government-sponsored fruit and vegetable program. And less than 20 percent of schools offered children the option to buy a low-fat lunch, the study found.

But, Alice H. Lichtenstein, director of the Cardiovascular Nutrition Lab at Tufts University's USDA Human Nutrition Research Center in Boston, said the findings need to be put into perspective.

"I think we put an overemphasis on the food students can access in school, because the majority of a teen's waking hours is not spent in school," she said. "But in any case, comparing elementary school children and high school children is comparing apples and oranges. They're two very different groups of students.

"High school students spend a lot more time in school," Lichtenstein added. "And they have more disposable income. So it's not surprising that they're going to be offered more vending machines. So, yes, it would be nice if in all public schools everything in them would be healthy choices. But I'm not sure that this comparison gets at particularly good information."

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