Nearly one in three children are considered clinically overweight and obese. Compared with the year 1974, the proportion of children who were obese was five times higher in 2009. The dramatic rise is troubling in terms of health and general well-being — and now we can add academic performance as another reason to worry.

A study, published in the journal Child Development looked at more than 6,250 children from the Early Childhood Longitudinal Study-Kindergarten Cohort, a nationally representative sample that tracked children from kindergarten through fifth grade.

At five times during the study window, parents provided information about their families, teachers, children's social skills and emotional well-being. Children were also weighed and tested for academic performance.

According to the findings, when compared with children who were never obese, boys and girls who were obese from the start of kindergarten through fifth grade performed worse on the math tests through the test period. For boys whose obesity emerged later (in third or fifth grade), no such differences were found. For girls who became obese later, poorer math performance was temporary.

"The findings illustrate the complexity of relations among children's weight status, social and emotional well-being, academics and time," said Sara Gable, associate professor of nutrition and exercise physiology at the University of Missouri, Columbia, who led the study.

The study also reported that girls who were obese the whole time had fewer social skills, which explained part of their poorer math skills. Both boys and girls who were “persistently obese” were noted to experience more sadness, loneliness and anxiety — which also explained some of their math performance.

"Our study suggests that obesity in the early years of school, especially obesity that persists across the elementary grades, can harm children's social and emotional well-being and academic performance," Gable said.

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