The study observed 6,000 students in the U.K. between the ages of 11 and 16. Of these, 71 percent were considered obese, 13 percent overweight, and 15 percent were considered to be a healthy weight.
They found that for girls who were obese at age 11, the difference in academic achievement was on average one-third of a grade, enough to drop them to a D compared with an average attainment of a C grade.
Researchers accounted for factors including socioeconomic status, mental health, IQ, and age of menarche for the study, and found they did not change the relationship between obesity and poor grades.
Being obese was not the only factor. The age of obesity onset mattered as well.
"Being overweight or obese at age 16 was not as detrimental for attainment if participants had been a healthy weight at age 11, suggesting that potentially, it is long term overweight/obesity that is the most problematic." said the study.
The relationship between obesity and poor academic performance was less clear in boys.