Wednesday March 12 2014
Putting on a few pounds does not make you less intelligent
"Study links obesity in teenage girls with lower academic results," BBC News reports. Previous studies have reported that child and adolescent obesity has a wide variety of adverse consequences in both the short and long term.
A large study of UK secondary school pupils has now looked at whether being overweight or obese at the age of 11 influences educational attainment on SAT tests at 11 and 13 years of age and GCSE grades achieved at 16 years of age.
The researchers found an association between obesity at 11 years of age and poorer academic achievement in GCSE exams five years later in girls, even after adjustment for a wide range of factors that could have influenced the results (confounders).
They say that the difference in academic achievement was equivalent to one-third of a grade at age 16, which would be sufficient to lower average attainment to a grade D instead of a grade C. However, the association between obesity and academic attainment was less clear in boys.
The reasons for an association between obesity and academic achievement in girls are unknown, but causes may include the health effects of obesity that could lead to girls missing school.
Where did the story come from?
The study was carried out by researchers from the universities of Dundee, Strathclyde, Bristol and Georgia. It was funded by the UK Medical Research Council, the Wellcome Trust, the University of Bristol and the BUPA Foundation.
The study was published in the peer-reviewed International Journal of Obesity. The article is open access, meaning it can be accessed for free from the journal's website.
The results of this study were well reported in the media.
What kind of research was this?
This was a cohort study that aimed to determine whether being obese at 11 years old was associated with poorer academic attainment at 11, 13 and 16 years old. It also aimed to look for factors that might explain the relationship seen.
Cohort studies are the ideal study design to address this sort of question, although they cannot show a cause and effect relationship as there could be other factors (confounders) that are responsible for any association seen.
What did the research involve?
The researchers looked at the association between weight status at 11 years old and academic attainment assessed by national tests at 11, 13 and 16 years of age in 5,966 children who were part of the Avon Longitudinal Study of Parents and Children.
Children were only included in the study if they did not have a psychiatric diagnosis or special educational needs.
The researchers measured the weight and height of children when they were 11 and 16 years old, and their body mass index (BMI) was also calculated.
Healthy weight was defined by BMI "z-scores". Z-scores show how different an individual's BMI is from the average in the UK population (more precisely, it shows the number of standard deviations different it is).
There are no agreed cut-offs in the UK, but the World Health Organization (WHO) defines being overweight as more than one standard deviation higher than average and obesity as more than two standard deviations. This study has used a lower level to define obesity. In this study:
- normal weight was defined as a BMI z-score more than 1.04
- being overweight was defined as a BMI z-score of 1.04 to 1.63
- being obese was defined as a BMI z-score of 1.64 or more
Academic attainment was assessed from performance in English, Maths and Science on Key Stage 2 tests at age 10/11, Key Stage 3 tests at 13/14 and GCSEs at age 15/16. The researchers concentrated their analyses on English attainment.
The researchers looked at the association between weight status and academic attainment. Boys and girls were analysed separately, as differences have been seen in academic attainment between the sexes.
The researchers also looked at whether the association seen between weight status and academic attainment could be explained by:
- depressive symptoms at age 11
- IQ at age 8
- age of girls when their periods started
The researchers adjusted their analyses for a large number of potential confounders.
What were the basic results?
At 11 years of age, 71.4% of children were a healthy weight, 13.3% were overweight and 15.3% were obese.
After adjusting for all potential confounders, girls who were obese at the age of 11 had lower English marks at 13 and 16 years of age compared with girls who were a healthy weight. There was no significant difference in marks for overweight girls compared with healthy weight girls.
The association between obesity and English marks were less clear in boys, with a significant association only being seen for obesity at the age of 11 and lower English marks at 11.
Neither depressive symptoms nor IQ explained the relationship between weight status at 11 and academic attainment at 16 years old for boys or girls. The age at which girls had their first period also did not explain the relationship between weight status at 11 and academic attainment at 16 years old for girls.
The researchers also looked at changes in weight status. Changes in weight status had no effect on English attainment at GCSE in boys.
However, girls who were overweight or obese at age 11 and 16, or who changed from being overweight at 11 to obese at 16 had poorer English attainment at GCSE than girls who remained a healthy weight.
Girls who were a healthy weight who became overweight and girls who became a healthy weight from being overweight had no difference in English attainment at GCSE than girls who remained a healthy weight.
How did the researchers interpret the results?
The researchers conclude that, "For girls, obesity in adolescence has a detrimental impact on academic attainment five years later". They say that, "Being obese at 11 predicted lower attainment by one-third of a grade at age 16. In the present sample, this would be sufficient to lower average attainment to a grade D instead of a grade C."
The researchers go on to say: "Mental health, IQ and age of menarche [period] did not mediate this relationship, suggesting that further work is required to understand the underlying mechanisms. Parents, education and public health policy makers should consider the wide-reaching detrimental impact of obesity on educational outcomes in this age group."
This large UK-based cohort study has looked at whether being overweight or obese at age 11 influences educational attainment on SAT tests at 11 and 13 years of age and GCSE grades achieved at 16 years of age.
It found an association between obesity at 11 years of age and poorer academic achievement in GCSE exams five years later in girls, even after adjustment for a wide range of confounders.
The researchers found that depression, IQ and the age girls started menstruating could not explain the association. The association between obesity and academic attainment was less clear in boys.
The reasons for an association between obesity and academic achievement in girls are unknown. The researchers suggest that obese girls may miss school because obesity affects physical and mental health. They also suggest that obese children's academic attainment may suffer because they tend to be stigmatised by other children or teachers, or because excess fat might affect brain function.
Strengths of this study include the large sample size, cohort design, adjustment of analyses for a wide range of confounders, and the fact that BMI and educational attainment were objectively assessed. However, it should be noted that the researchers defined childhood obesity at a lower BMI level than that used by the World Health Organization.
However, as the researchers noted, despite the adjustment for a range of confounders, cohort studies cannot prove a cause-and-effect relationship, as there might be other confounders that are responsible for the association seen. They also point out that many of the confounders were only measured at one point and they were unable to adjust for change, such as changes in depressive symptoms.
The researchers suggest that future studies should explore the influence of other potential factors that could explain the association seen, including self-esteem, absenteeism, school environment and the role of the teacher.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.