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School-age kids' body mass index (BMI) z-scores fell following the implementation of the Healthy, Hunger-Free Kids Act of 2010 (HHFKA), researchers reported.

Compared with the decade prior to the HHFKA being put into place, there was a significant decrease in annual BMI z-scores among U.S. youth from the ages of 5 to 18 (-0.041, 95% CI -0.066 to -0.016) in the 3.5 years after the law passed, according to Aruna Chandran, MD, MPH, of Johns Hopkins Bloomberg School of Public Health in Baltimore, and colleagues.

A significant decline in BMI was apparent for both younger children ages 5 to 11 (-0.034, 95% CI -0.059 to -0.009) as well as teens ages 12 to 18 (-0.045, 95% CI -0.071 to -0.018), the researchers wrote in .

The shift also held for both lower and higher income households, those with a median annual income below $50,000 (-0.038, 95% CI -0.063 to -0.013) and at or above that threshold (-0.041, 95% CI -0.066 to -0.016). "This income cutoff roughly aligns with the income eligibility for free and reduced-price meals in the 2016 to 2017 school year," the researchers pointed out.

Both males (-0.037, 95% CI -0.062 to -0.012) and females (-0.046, 95% CI -0.070 to -0.020) also saw significant BMI drops between the pre- and postintervention periods.

"School meals and snacks represent a key opportunity for interventions to combat the childhood obesity epidemic given the high rates of participation in school meal programs and the significant proportion of caloric intake that youths receive at school," wrote Chandran's group, noting that more than 20% of U.S. youth have obesity.

The HHFKA -- "the first national legislation to improve the dietary quality of school meals and snacks in over 2 decades," according to Chandran's group -- applied to food sold on site at school, from breakfasts and lunches to snacks in vending machines. It required food for sale at schools to fall in line with 2010 U.S. Department of Agriculture (USDA) guidelines, which recommended food include more whole grains, fruits, vegetables, and low-fat dairy products with less sodium and fat. It also established age-specific recommendations for school meal serving sizes.

This study really "underscores the important impact of the [National School Lunch Program and School Breakfast] on reducing excess weight gain among lower-income children, who are unquestionably among the most vulnerable for food insecurity, obesity, and obesity-related chronic diseases," added an by Lauren Fiechtner, MD, MPH, of Mass General Hospital for Children in Boston, and colleagues.

They added that this particular study's real-world evidence on an intervention to improve weight outcomes advances the field, as "this natural experiment accounts for the complex social and political conditions that the program operates in -- an obvious advantage over clinical trials."

Fiechtner's group also suggested the nutritional guidelines for school-sold food should now be updated to reflect the most recent , with particular attention paid towards swapping out fruit juices for whole fruits and bumping up fiber intake.

Implemented in phases, the HHFKA first started to roll out into schools during the 2012 to 2013 school year after Congress passed it in 2010. By the 2015 to 2016 school year, 93% of school districts were in compliance, according to a . The time periods compared in the study were January 2005 to August 2016, before implementation, and afterward from September 2016 to March 2020.

Chandran's group pulled data from the nationwide Environmental Influences on Child Health Outcomes program. Data from 14,121 youth from 50 cohorts from the ages of 5 to 18 were ultimately included. The average first BMI measurement was at age 8.8 years.

Among the cohort, 51% were male, 62% were white, and most resided in the West (37%) or Northeast (28%).

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