A new study shows that more than half of children and adolescents in the United States are not drinking enough water.

—Boys were 76% more likely to be dehydrated than girls, and non-Hispanic blacks were 34% more likely to be dehydrated than non-Hispanic whites.

“The good news is that this is a public health problem with a simple solution,” senior author Steven Gortmaker, professor of the practice of health sociology, said in a release. “If we can focus on helping children drink more water–a low-cost, no-calorie beverage–we can improve their hydration status, which may allow many children to feel better throughout the day and do better in school.”

The researchers looked at data from 2009-2012 from more than 4,000 children and adolescents ages 6 to 19, examining urine concentration to determine hydration. The data was from the National Health and Nutrition Examination Survey, an annual study conducted by the Centers for Disease Control and Prevention.

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Write to Tessa Berenson at tessa.berenson@time.com.