There are nutritional guidelines about what students must be served for school lunch, but what may be lacking are guidelines for how long a lunch period should last. Too often, kids aren't able to finish all the food they would have eaten if they had more time to eat.

Researchers from the Harvard T.H. Chan School of Public Health visited six elementary and middle schools with lunch periods that ranged from 20 to 30 minutes. They looked at the effect the length of a school's lunch period had on what the 1,001 students ate for lunch, analyzing their food selections and looking at how much was left on their plates when lunch period ended.

Children who had to wait in serving lines or those who arrived late to lunch were often left with as little as 10 minutes to eat their lunch.

When schools had lunch periods of 20 minutes or less, students ate 13 percent less of the main course, 12 percent less of the vegetables, and drank 10 percent less milk compared to students who had at least 25 minutes to eat. Those with less time to eat were also less likely to choose a fruit.

Children who had to wait in serving lines or those who arrived late to lunch were often left with as little as 10 minutes to eat their lunch. Naturally, food waste increased at schools with shorter lunch periods.

The team had expected that kids with less time for lunch would eat their entrée and drink their milk quickly, then throw away their fruits and vegetables. Instead, they found that they started eating everything, but just didn’t have enough time to finish eating.

Since many children rely on school meals for up to half their daily energy intake, it's important to give them enough time to eat their lunches, Juliana Cohen, lead author of the study, pointed out.

It may not be possible to increase the time for lunch in all schools, but the researchers believe strategies could be developed to get students through the lunch line more quickly. They suggest adding more serving lines or the using automated checkout systems.

The study is published in the Journal of the Academy of Nutrition and Dietetics.