A new study on students' eating habits before and after USDA updated school meal standards finds that kids are selecting healthier foods with fewer calories per gram for lunch.

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The study, funded in part by the Robert Wood Johnson Foundation, assessed the nutritional and caloric content of more than 1.7 million school meals selected by students more than a year before and after the updated standards went into effect.

Study authors attribute students' healthier selections to the increased availability and variety of fruits and vegetables in school meals.

Healthy choices of fresh fruit, salads and vegetables at Washington-Lee High School in Arlington, Virginia for lunch service on Wednesday, October 19, 2011. USDA Photo by Bob Nichols.

"This study is the latest in a long list of evidence which shows that stronger school meal standards are leading to healthier habits in schools," USDA Secretary Tom Vilsack said Monday. "Children are eating more fruits and vegetables and consuming more nutrients, making them better prepared to learn and succeed in the classroom."

The researchers analyzed foods selected by 7,200 students in three middle and three high schools located in Washington state. Fifty-four percent of the district's students qualify for free or reduced-price meals.

The study was conducted between January 2011 and January 2014 during the months that schools were in session, RWJF said, 16 months before and 15 months after implementation of the updated standards.

This study is different from previous studies in that is includes high school students and encompasses a larger number of students over a longer period of time.

The research found that the overall nutritional quality of the foods chosen by students increased by 29% and the calorie content per gram decreased by 13%.

To determine the nutritional quality of the foods students chose, researchers assessed the amount of calcium, vitamin C, vitamin A, iron, fiber and protein in each item. They determined the mean of percent daily value provided in all the foods selected each day, averaged per month for six nutrients per 1,000 calories. They assessed the calorie content of the foods served by calculating the energy density (amount of calories per gram) of each food item.

Related: Senate Ag Committee Explores School Lunch Implementation

"This study shows that the updated school meal standards are effective. Students chose more nutritious meals that had fewer calories per gram after the standards were implemented," said Donna B. Johnson, Ph.D, the study's lead author and associate director of the Center for Public Health Nutrition at the University of Washington School of Public Health. "Schools are offering more fruits and vegetables and a greater variety of fruits and vegetables—we found that to be the primary reason students were selecting healthier lunches."

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The study found no statistically significant difference in school lunch participation: 47% of students participated in the school lunch program before the healthier standards were in place and 46% of students participated after they were implemented.

The researchers measured monthly school meal participation as the average number of daily meals served in all of the six secondary schools in the district divided by overall student enrollment in those schools.

"The updated standards and healthier offerings did not affect participation in the meals program," Johnson said.

The updated nutrition standards – enacted by Congress in 2010 under the Healthy, Hunger-Free Kids Act and implemented by the USDA in fall 2012 – are based on recommendations from an Institute of Medicine panel of experts.

The HHFKA was up for reauthorization in 2015, with a Sept. 30 deadline. Though Congress did not act, funding continues.

"After decades of a growing obesity epidemic that harmed the health and future of our children and cost our country billions, we are starting to see progress in preventing this disease," Vilsack said. "Now is not the time to take as step backwards in our efforts to do what is right for our children's health.

"I urge Congress to reauthorize the child nutrition programs as soon as possible and to maintain the high standards set by the Healthy, Hunger-Free Kids Act," he said.