03/11/2014 12:08 EDT | Updated 05/11/2014 05:59 EDT

Obese girls tend to do worse in school, U.K. study says

Obese girls tend to have poorer grades in school compared to their healthier counterparts, but weight didn’t seem make a difference when it comes to boys, says a new study of British children.

The researchers say their study is the most comprehensive of its kind. They note that previous studies on obesity and academics were largely from the U.S. and also did not consider certain factors such as socio-economic status.

Scientists examined data from almost 6,000 participants from the Avon Longitudinal Study of Parents and Children, which began in the 1990s. This is an ongoing U.K. study of the influences on health and development of children.

Participants were classed according to their body mass index at ages 11, 13 and 16 and academic grades were analyzed from national tests at those ages.

The researchers, from Dundee, Bristol and Strathclyde universities in England, in collaboration with Georgia University in the U.S., also took into account factors such as mental health, IQ and the age at which the girls started menstruating.

In addition, the study considered socio-economic class and whether the girls' mothers smoked in the first three months of pregnancy.

Even with all the mitigating factors, researchers concluded thatobese girls tended to get grades in English, math and the sciences that were the equivalent of a D, compared to girls of healthy weight, who achieved grades that were equivalent to a C level.

The study, published in the International Journal of Obesity, says that “being obese at 11 predicted lower attainment by one third of a grade at age 16” for girls.

The authors say they could not find any academic differences among healthy weight boys versus obese boys.

Researchers admit there are other influences that they did not measure including the students’ self-esteem, how much school they missed and what kind of school environment they had.

Professor John Reilly of Strathclyde, the lead investigator, concludes: ''Further work is needed to understand why obesity is negatively related to academic attainment, but it is clear that teenagers, parents and policy-makers in education and public health should be aware of the lifelong educational and economic impact of obesity.'' 

In Canada, nearly 12 per cent of children aged 12 to 17 are obese, according to Statistics Canada.