(CNN)Changes to school nutrition standards that pushed more fruits, vegetables, whole grains and low-fat dairy products significantly decreased kids' and teens' body mass index after the standards were implemented in 2010, a new study finds.
The new study comes as the United States again considers updates that would put more limits on added sugars and sodium in school meals.
The study, published in JAMA Pediatrics, followed 14,121 US youths ages 5 to 18 from January 2005 to March 2020. The study didn't include data following widespread school shutdowns due to the Covid-19 pandemic.
The researchers found an overall decrease in annual body mass index, or BMI, in the period following implementation of the Healthy, Hunger-Free Kids Act. BMI is a measure of body fat based on height and weight.
Previous research had shown that school-provided meals were linked with childhood obesity in the years before stricter nutritional guidelines.
"This [study] is providing the evidence base to continue to have strong school nutrition standards for kids. We still have an overweight and obesity problem in the US and policies that strengthen the nutrition standards in school meals are needed to help improve the health of our children," said Dr. Lauren Au, an assistant professor at UC Davis' Department of Nutrition who studies the effectiveness of school nutrition programs and was not involved with the new study.
The overall BMI decrease was seen across ages and income levels, which researchers say is significant. Children from higher income families -- who may be able to purchase foods outside of what is provided to them through the free or reduced-price meal program -- experienced equal benefits from the nutritional qualities in schools.
Additionally, according to the study, prior to the implementation of the Healthy, Hunger-Free Kids Act, BMI increased in teenage years. However, that trend reversed after the nutritional changes were made.
"The Healthy Hunger Free Kids Act does still have impact in adolescents when they have such differences in dietary preferences, and the ability to purchase their own foods compared to younger children," Au said.
According to the US Centers for Disease Control and Prevention, about 20% of children and adolescents ages 2 to 19 are obese, which can lead to lifelong health complications including high blood pressure, type 2 diabetes and breathing problems such as asthma and sleep apnea.
The National School Breakfast and School Lunch Program, both of which were reformed by the Healthy, Hunger-Free Kids Act of 2010, provide meals at low or no cost to more than 30 million children. These meals make up an estimated 50% of the calories in a child's day.
"I think when you're looking at these population level, large scale evaluations, what might look like a small effect in any one child at any one moment, actually means a lot at the broader level over time," said Dr. Aruna Chandran, an author of the study and epidemiologist at Johns Hopkins Bloomberg School of Public Health.
More school nutrition updates expected
Efforts to improve school nutrition standards are ongoing. The US Department of Agriculture recently proposed new school food guidelines, which included reduction of added sugars and sodiu