Above average increases in body fat from 9 to 15years of age had a negative impact on academic performance, independent of physical activity.

Aim: The associations between body fat levels and physical activity with academic performance are inconclusive and were explored using longitudinal data. Methods: We enrolled 134/242 adolescents aged 15, who were studied at the age of nine and agreed to be followed up from April to May 2015 for the Health behaviours of Icelandic youth study. Accelerometers measured physical activity, body mass indexes (BMI) were calculated and dual-energy X-ray absorptiometry scans assessed the participants’ body composition at nine and 15. Their language and maths skills were compared to a growth model that estimated the academic performances of children born in 1999. Results: Higher than normal body fat levels between the ages of nine and 15 were negatively associated with maths performance, but the same association was not found for Icelandic language studies. These were Pearson's r = −0.24 (p = 0.01) for BMI and Pearson's r = −0.34 (p = 0.01) for the percentage of body fat. No associations were found with changes in physical activity. Conclusion: Children who put on more body fat than normal between the ages of nine and 15 had an increased risk of adverse academic performance that was independent of changes in physical activity.