Obesity is a growing problem in developed countries. In the UK, a recent WHO studied indicated that one in four adults is obese (WHO obesity in the UK) and up to one in five children (WHO childhood obesity in the UK). Health minister Jeremy Hunt referred to the increasing number of children with severe weight problem as a national emergency (Interview with Jeremy Hunt on childhood obesity). Worryingly, obesity in children may not only be associated with increased risk for cardiovascular conditions, but may also hinder children’s academic progress. For instance, a cohort study of 5,000 children in Australia found that obesity was related to lower school performance for boys. That relationship persisted even when the researchers took other factors like family wealth into account (Black, Johnston, & Peeters, 2015, Taras & Potts Datema, 2005). Similarly, a study of 600 high-school students in the UK found that higher body-mass index (BMI), that is body weight relative to height, was negatively associated with school performance (Arora et al., 2013).
Obesity in childhood and adolescence is associated with lower cognitive performance
Lower school performance may be caused by differences in cognitive skills. Several studies investigated differences in cognitive performance in children and adolescents with obesity. Most of these studies focussed on executive functions. EF is used as an umbrella term for a set of inter-related abilities, including goal planning, attention, working memory, inhibition, and cognitive flexibility (Anderson, 2002; Diamond, 2013). Wirt and colleagues found a negative association between body weight, inhibitory control, and cognitive flexibility in a community sample of nearly 500 children (Cserjési, Molnár, Luminet, & Lénárd, 2007; Wirt, Schreiber, Kesztyüs, & Steinacker, 2015), even when controlling for family and lifestyle factors. Higher BMI was also associated with worse performance on executive function assessments in a sample of children with attention deficit hyperactivity disorder (ADHD) (Graziano et al., 2012). Adolescents show a similar association between obesity and cognitive performance. A study by Lokken and colleagues found impairments in attention and executive function in adolescents with obesity (Lokken, Boeka, Austin, Gunstad, & Harmon, 2009).
Together, these studies suggest that obesity in children and adolescents is associated with poorer performance on executive function tests (Liang, Matheson, Kaye, & Boutelle, 2014). These findings indicate that children and adolescents with higher body weight may find it more difficult to control their behaviour.
Obesity in childhood and adolescence is associated with structural and functional brain differences
Cognitive differences associated with childhood and adolescent obesity are also linked to structural and functional differences in the brain. Yau and colleagues compared 30 adolescents with obesity to a control group of 30 adolescents with normal weight matched for age, gender, and socio-economic status. The study found lower academic achievement, lower working memory, attention, and mental flexibility in the obese group that was associated with reduced cortical thickness in the orbitofrontal and anterior cingulate cortex (Bauer et al., 2015; Ou, Andres, Pivik, Cleves, & Badger, 2015; Yau, Kang, Javier, & Convit, 2014). These brain areas are generally associated with behavioural control. Further, the authors reported reductions in the microstructural integrity of several major white matter tracts, which may indicate that obesity is associated with differences in the efficiency of communication between brain areas (Stanek et al., 2011; Yau et al., 2014). Schwartz and colleagues took a closer look the relationship between body fat and white matter composition in a sample of 970 adolescents (Schwartz et al., 2014). The findings of the study suggested that white matter differences associated with obesity are linked to differences in the fatty acid composition of brain white matter. Further research is needed to interpret these results. But it could indicate that differences in diet associated with obesity could impact on cognitive performance by influencing the insulation of the brain’s wiring.
Brain function may also be affected by obesity. A series of studies by Kamijo and colleagues investigated functional differences in children with obesity using event-related potentials (ERP) (Kamijo, Khan, et al., 2012a; Kamijo, Pontifex, et al., 2012b; Kamijo et al., 2014). To obtain ERPs, the electro-encephalogram (EEG) is recorded while participants perform a cognitive task. The signal is then averaged to derive the electrophysiological response that is directly linked to a particular cognitive event. Children with obesity were found to perform worse on tasks that required inhibition of prepotent responses. The lower performance was associated with lower amplitude of an ERP response related to error monitoring and a less frontal distribution of an attention-related ERP. The ERP results may indicate a less efficient conflict monitoring system and differences in the neural organisation of the attention system in children with obesity.
Limitations: the chicken, the egg, and the confounding effect of the rooster
Like in many other areas of human cognitive neuroscience, studies of children and adolescents with obesity are based on correlations. This leads to some limitations of the conclusions that can be drawn from such work. For one, it is not possible to draw any firm conclusion about the causal relationship between the variables. In other words, it is not clear if obesity leads to differences in cognition or if cognitive differences predispose individuals to become obese. Secondly, the relationship between two variables may be influences by a third variable that has not been assessed. Some of these variables include other environmental influences that may both be associated with obesity as well as cognitive differences. These are likely include measures like family wealth and education (O’Dea & Wilson, 2006) among other influences that have not yet been investigated. Other physiological factors that are associated with obesity may also confound the relationship between obesity and cognition. For instance, differences in cardiovascular health or insulin metabolism in children with obesity may influence brain function and cognitive performance. However, some of the studies took these factors into account by matching control groups on cardiovascular health (Kamijo et al., 2014; Kamijo, Pontifex, et al., 2012b) or studying obese groups with a typical insulin response (Stanek et al., 2011). These studies found that differences in cognitive performance in the obese group were still observed when controlling for the influence of these factors.
Another factor that is rarely assessed it sleep apnea. A study by Tau found that differences in school achievement in children and adolescents with obesity were associated with obstructive sleep apnea. While this result does not invalidate other findings about cognitive performance deficits in children and adolescents with obesity, it highlights a potential mechanism by which obesity may impact on the cognitive performance.
The current literature suggests that obesity in childhood and adolescence is associated with cognitive differences in executive function and differences in the organisation of brain system related to executive function and cognitive control. Future research will be needed to identify the mechanism by which body fat content, brain physiology, and cognitive performance may be linked to address the unprecedented scale of weight problems in children and adolescents and their consequences.
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Picture credit: Charlie and the Chocolate Factory, Warner Bros Pictures, 2005