“When school boards are going out and contracting with these vendors, what they're thinking is that they're going to improve the health of the students, that they'll get them to eat healthier. I don't think they're thinking of it as a tool to actually improve academic performance [but] we found that it is,” said Michael L. Anderson, an associate professor of economics at the University of California, Berkeley, and one of the study's co-authors. “Something that is basically cheap, that is going to improve student health, and that has test-score gains seems like it would be very attractive [to] policymakers.”

According to Anderson, who spoke as school meals received renewed attention due to President Trump’s proposed budget, this is the first large-scale study to examine how the overall nutritional quality of school meals affects student test-score achievement. In 2010, as part of a push to combat childhood obesity, the Healthy, Hunger-Free Kids Act was passed, resulting in more rigorous nutrition standards for school cafeterias. There is a body of recent literature that suggested a link between school meals and student test scores, but that research focused on improving access rather than the meals’ nutritional value.

To determine the link between food quality and student achievement, Anderson and his colleagues collected data from the California Department of Education on school districts’ meal vendors for the academic years from 2008-09 to 2012-13. Over that five-year period, 1,192 schools—about 12 percent of California public schools, including public charter schools—contracted for at least one school year with an outside company to provide lunch.

The team then hired the Nutrition Policy Institute, a research unit housed at the University of California, to score the nutritional content of vendors’ school lunches. Armed with sample school-lunch menus, NPI calculated the Healthy Eating Index (HEI), a U.S. Department of Agriculture measure of dietary quality for food items, for all of those companies’ meals. The average HEI score among all vendors with menu information was tabulated, and vendors with above-median scores were classified as healthy school-lunch providers. But there was still one crucial piece of information missing: how students at schools with healthy vendors stacked up against their peers at non-vendor schools on state tests.

In pursuit of that answer, the study’s authors compiled a database covering the same five-year timespan with school-by-grade-level test results on California’s Standardized Testing and Reporting exam, a statewide test given at the time to all public-school students in grades 2 through 11. Test score data from some 9,700 elementary, middle, and high schools found that contracting with a healthy meal vendor correlated with increased student performance by between .03 and .04 standard deviations—a statistically significant improvement for economically disadvantaged and non-disadvantaged students, Anderson said, adding that the size of the effect “is not huge … but it is notable.”