MONDAY, Oct. 6 (HealthDay News) -- Three-quarters of middle schools have vending machines where snacks and sugared drinks are sold, a new study finds.

The research demonstrates that there are vending machines in most middle schools, and "that those vending machines don't always have the healthiest choices," said study author Amy Virus, a registered dietitian with the Center for Obesity Research and Education at Temple University in Philadelphia.

The results come from a nationwide sample of 42 middle schools, and researchers discovered that most of these vending machines offer food and beverage choices that contain as much as 320 calories an item, Virus said.

The findings were expected to be presented Monday at the Obesity Society's annual meeting, in Phoenix.

In a nation where one out of three children is considered overweight, these added calories put children at risk for obesity and type 2 diabetes, Virus noted. Middle school students are at particular risk, because they are going through puberty, their physical activity and dietary habits are fluctuating, and they are increasingly responsible for any behavior changes, Virus added.

At the schools surveyed, the majority of vending machines contained sugar-added drinks such as soda, Gatorade and iced tea, Virus explained. The drinks contained anywhere from 60 calories to 320 calories. Fruit juices also found in the machines averaged 140 calories to 320 calories. It is a misconception that 100 percent fruit juice is a healthy snack choice, she said. "Although juice can fit into a meal, drinking excess juice outside a meal gives you the extra calories," she explained.

These findings are part of a larger study aimed at reducing obesity in middle school students. The next step will be to eliminate 100 percent fruit juice from these schools' vending machines, change snack and desert foods to 200 calories or less, and change chips to reduced fat or baked snacks. More water will be placed in the machines if recommendations are followed.

But these steps may not be enough, according to Roberta H. Anding, a clinical dietitian at Texas Children's Hospital. The 300-calorie-a-day increase in children's diets since the 1970s comes from more refined carbohydrates and fats, she said. Schools need to make the right choices to implement these changes.

"If the focus is just on calories, it might give a stamp of approval to a reduced-calorie snack that is trans fat- and white flour-rich", she said. "Research suggests that those who eat whole grains instead of highly processed food decrease the risk of diabetes. These whole grains are rich in magnesium, which also is protective against diabetes."