Before the U.S. Department of Agriculture set strict standards for nutrition for federally reimbursable lunch programs, less than two percent of middle or high schools would have measured up.

The absence of certain standards was associated with youth obesity, according to a new study. Full implementation of the program, which should be happening now, may have a notable impact on adolescent health, though this study did not address implementation of the program, the authors write.

“By the time USDA standards get fully implemented, it will be really dramatic, there will be a very, very different nutritional environment in these schools,” said lead author Yvonne M. Terry-McElrath, a research associate at the Institute for Social Research at the University of Michigan in Ann Arbor.

Beginning in the fall of 2012, the USDA required that schools participating in the federally reimbursable National School Lunch Program adhere to certain nutritional standards designed to limit levels of fat, sodium, sugar, and calories.

Starting this year, the same standards also apply to any other food source on campus, like snack bars and vending machines.

For the new study, researchers analyzed data from three nationally representative surveys between 2007 and 2012 including more than 50,000 public school students in the 8th, 10th and 12th grades.

The vast majority of the schools do participate in the National School Lunch Program, Terry-McElrath said.

Students reported their height and weight and school administrators reported whether students had access to certain foods and beverages at school lunches or from nearby food sources.

Researchers chose four easy-to-observe USDA standards from the new requirements: no sugar-sweetened beverages, no whole or two percent milk, no candy or full-fat snacks, and no french fries. The USDA also suggests, but does not require, that fruits or vegetables be available wherever food is sold.

“The standards that we’re looking at come nowhere near the full USDA requirements,” Terry-McElrath told Reuters Health by phone.

Almost a third of high schools had none of the USDA components. Fewer than two percent of middle schoolers and one percent of high schoolers attended schools with all five components.

“It is not an easy thing to make such a large change in the nation’s school nutrition environment,” Terry-McElrath said. “Our research shows that this will be a very significant change” requiring a long transition period, she added.

“There will be significant challenges in many schools to adhere to these standards,” said Leslie A. Lytle of the Gillings School of Global Public Health at the University of North Carolina at Chapel Hill.

Lytle wrote an editorial on the new results.

“Over the past decades, our schools have moved toward a model where individual schools don't have cafeteria workers who know how to prepare food, and where there are not kitchen facilities for preparing food,” she said. “Without onsite kitchens and trained staff, schools will be challenged to find healthy, quick foods to offer in their competitive food venues.”

Overall, the USDA components were not associated with students reporting themselves overweight or obese.

However, high school students were around ten percent less likely to report being overweight or obese if their schools had fruits and vegetables available wherever foods were sold, if higher fat milk was not available, and if three or more USDA components were available at their school.

Nonwhite and Hispanic students were 20 to 35 percent less likely to report being overweight or obese if sugar-sweetened beverages were not available at school, according to results in JAMA Pediatrics.

Until now, there has been no financial incentive for schools to make changes to the foods they offered, and there may have been financial reasons to offer less healthy foods, Lytle said.

“Now that federal reimbursement is attached not only to what is served on the 'regular' lunch line as well as what is offered in a la carte, snack lines, vending and school stores, it is much more likely that schools will pay attention to food offered throughout the larger school environment,” Lytle told Reuters Health by email.

But school changes alone may not make a big difference for kids’ health, she said.

“Schools are just one place where children make food choices, eat and learn about what, how and why to eat,” Lytle said. “It will take change at many levels before we see an impact on kids' health.”