About one in three children in the U.S. are now overweight, and since the 1980s the number of children who are obese has more than tripled. But a new study of 26.7 million young children from low-income families shows that in this group of kids, the tidal wave of obesity might finally be receding.

Being obese as a child not only increases the risk of early-life health problems, such as joint problems, pre-diabetes and social stigmatization, but it also dramatically increases the likelihood of being obese later in life, which can lead to chronic diseases, including cancer, type 2 diabetes and heart disease. Children as young as 2 years of age can be obese—and even extremely obese. Early childhood obesity rates, which bring higher health care costs throughout a kid's life, have been especially high among lower-income families.

"This is the first national study to show that the prevalence of obesity and extreme obesity among young U.S. children may have begun to decline," the researchers noted in a brief report published online December 25 in JAMA, The Journal of the American Medical Association. (Reports earlier this year suggested that childhood obesity rates were dropping in several U.S. cities.)

The study examined rates of obesity (body mass index calculated by age and gender to be in the 95th percentile or higher—for example, a BMI above 20 for a 2-year-old male—compared with reference growth charts) and extreme obesity (BMI of more than 120 percent above that of the 95th percentile of the reference populations) in children ages 2 to 4 in 30 states and the District of Columbia. The researchers, led by Liping Pan, of the Division of Nutrition, Physical Activity and Obesity at the U.S. Centers for Disease Control and Prevention, combed through 12 years of data (1998 to 2010) from the Pediatric Nutritional Surveillance System, which includes information on roughly half of all children on the U.S. who are eligible for federal health care and nutrition assistance.

A subtle but important shift in early childhood obesity rates in this low-income population seems to have begun in 2003. Obesity rates increased from 13.05 percent in 1998 to 15.21 percent in 2003. Soon, however, obesity rates began decreasing, reaching 14.94 percent by 2010. Extreme obesity followed a similar pattern, increasing from 1.75 percent to 2.22 percent from 1998 to 2003, but declining to 2.07 percent by 2010.

Although these changes might seem small, the number of children involved makes for huge health implications. For example, each drop of just one tenth of a percentage point represents some 26,700 children in the study population alone who are no longer obese or extremely obese. And if these trends are occurring in the rest of the population, the long-term health and cost implications are massive.

Public health agencies and the Obama Administration have made battling childhood obesity a priority, although these findings suggest that early childhood obesity rates, at least, were already beginning to decline nearly a decade ago. Some popular prevention strategies include encouraging healthier eating (by reducing intake of highly processed and high-sugar foods and increasing fruit and vegetable consumption) and increased physical activity (both at school and at home).

The newly revealed trends "indicate modest recent progress of obesity prevention among young children," the authors noted. "These finding may have important health implications because of the lifelong health risks of obesity and extreme obesity in early childhood."