About the research
In the west we have long been obsessed with weight and its effect on our health. The focus, however, is usually on the unhealthy effects of weight extremes (very high or very low), but research on how our weight affects our mental health has been limited. Some researchers from University College London and University of Southampton have been using survey data on adult psychiatric health to examine the link between body mass index (BMI) and the probability of experiencing a common mental health disorder.
The aim of this research was to describe the relationship between BMI and common mental disorders across the full range of BMI values and not just categories such as ‘overweight’ or ‘obese’. The researchers were also interested in any differences across genders and age groups.
The gender and age differences were dramatic. For young men the results were U-shaped: those who are underweight or obese are more likely to suffer from mental disorders. For women, a higher BMI tended to increase the possibility of a common mental disorder. For older people the link between BMI and the probability of having mental disorders is weaker. The researchers hope these results will inform clinicians of the different patterns of psychological difficulties over- or underweight men and women may have.
The researchers extracted the following variables from the data: body mass index (BMI), the presence of any common mental disorder, age, gender, and highest educational qualification. The relationship between BMI and mental disorders was investigated using a logistic regression analysis and cubic splines were incorporated to allow the relationships to be non-linear. Cubic splines are a very flexible way to include continuous variables as predictors in regression. They are much more flexible than adding quadratic terms such as BMI squared, and avoid the loss of information that comes from chopping a continuous variable up into categories.
McCrea, R.L., Berger, Y.G. and King, M.B. (2012) ‘Body mass index and common mental disorders: Exploring the shape of the association and its moderation by age, gender and education’, International Journal of Obesity, 36(3), pp. 414-421. doi: 10.1038/ijo.2011.65