Kids in middle schools eat more fruits and vegetables when the salad bar is in the lunch line than when it's outside the line, according to a new study.

"We were surprised, not that there was a difference in amounts of fruits and vegetables that students took and consumed, but by the size of the difference that placement had on students' choices," said lead author Marc A. Adams of Arizona State University.

"Our study measured actual weights of food items and was not affected by students' perceptions or memories," he added.

The researchers compared the amount of fresh fruit and vegetables taken, consumed and wasted by 533 Phoenix area middle-school students. Half of the students went to schools with salad bars in the serving line before the point of purchase and half went to schools where the salad bar was elsewhere in the cafeteria, after the point of purchase.

The students went through the lunch line and selected their items as usual, and when they were done getting food the researchers weighed the fruit or vegetable items on their trays.

After lunch, the research staff collected student trays to measure fruit and vegetable waste.

More than 98 percent of students at schools with salad bars in the lunch line self-served some fruit or vegetables, compared to 23 percent of kids in other schools.

Those with salad bars in the line also consumed more than four times more fruit and vegetables than other students, and threw more fruit and vegetable items away, the research team reported in the Journal of the Academy of Nutrition and Dietetics.

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"A single cross-sectional study cannot definitively quantify the difference salad bar position may make," said Yvonne Terry-McElrath of the University of Michigan.

"Clearly, both serving and consuming of (fruits and vegetables) increased dramatically based on salad bar position in the Adams et al. study, which is encouraging," Terry-McElrath told Reuters Health by email.

"Based on our results, if schools have the space available, we recommend that schools place salad bars inside of the lunch line in the path of students before they pay," Adams told Reuters Health by email. "Once students exit the serving line, most will not seek out additional opportunities to take fruits and vegetables because it might mean breaking away from friends or navigating busy cafeterias with short lunch periods; only highly motivated students will seek out salad bars."

Salad bar placement usually depends on the amount of space available in the cafeteria, he said.

"There doesn't seem to be a downside to making changes like this, but some school facilities will have a harder time than others," said Wendi Gosliner of the University of California, Agriculture and Natural Resources Nutrition Policy Institute, who was not part of the new study.

"If more evidence were to suggest that placement had this big of an effect on students' consumption, that could warrant making even difficult changes to move the salad bars," Gosliner told Reuters Health by email.

"The National School Lunch Program serves about 30 million students daily, and if most schools adopted a default placement of salad bars inside of the service line, millions of students could increase their fruit and vegetable consumption each day," Adams said. "Placement is one of those simple modifications that can help nudge students to make the best choices to improve their fruit and vegetable intake without asking or telling them to do so."