Obesity is a global health burden, a serious risk factor for development of metabolic disorders, cardiovascular diseases and many other conditions. But some researchers believe that in addition to affecting physical health, obesity can damage the brain and compromise intelligence.

Brain imaging studies have documented multiple structural and functional abnormalities in the brains of obese individuals, which are already evident in adolescence.  Moreover, research findings indicate that even obesity in childhood is associated with lower intelligence scores. But this is not all. According to some investigations, there is causality in the opposite direction, meaning that lower IQ at childhood results in increased prevalence of obesity in adulthood.

Scientific studies have investigated the association of IQ and obesity in large cohorts. For instance, a group of researchers analyzed data in a prospective, longitudinal study and investigated whether becoming obese is associated with a decline in intelligence from childhood to later life. More than one thousand children were included and tracked until their fourth decade of life. Anthropometric measurements (i.e., body weight and height) were carried out at birth and at 12 occasions later in life, at the ages of 3, 5, 7, 9, 11, 13, 15, 18, 21, 26, 32, and 38. The intelligence quotient (IQ) scores were assessed at the ages of 7, 9, 11, and 38. As the results demonstrated, the participants who became obese had lower IQ scores at adulthood in comparison with the participants whose body mass index (BMI) remained within the normal range. However, the obese participants did not experience a severe decline in their IQ over lifetime, meaning that they had lower IQ scores even in childhood, in comparison with normal weight controls.

Another population-based study followed babies born in the same week of 1950 in the United Kingdom for more than half a century. More than 17 thousand babies were included and their intelligence was assessed at the ages of 7, 11 and 16, while the obesity level and BMI were evaluated at 51. The results indicated negative effects of childhood intelligence on adult BMI and obesity level. In addition, it turned out that more intelligent children had healthier dietary habits and were exercising more frequently as adults.

Considering the negative association between childhood obesity and intelligence, one review study questioned the direction of this causality. After careful examination of longitudinal population based studies, this review study suggested that the direction of causality goes from having low intelligence that results in weight gain and obesity. It also claimed that excess weight gain did not cause a decline in IQ. The study found no strong evidence that obesity impairs cognitive functions or leads to cognitive decline, while it established proof that poor intelligence in childhood leads to weight gain in adulthood.

Still, not all scientists agree with these conclusions. For instance, a group of researchers investigated the impact of obesity on cognitive functions in children with sleep-disordered breathing. They included three groups of children in the study: children with obstructive sleep apnea, children with obstructive sleep apnea and obesity, and children without any of these conditions (normal control). The aim was to assess the total, verbal, and performance IQ scores in these children. The total and performance IQ scores turned out to be significantly lower in the children with obstructive sleep apnea and obesity, in comparison with the other two groups. In addition, BMI negatively influenced the total IQ score in obese children (with obstructive sleep apnea). This study clearly demonstrated that obesity can lead to higher cognitive impairments.

Since childhood IQ and obesity are linked, others investigated whether maternal pre-pregnancy obesity can impact the child’s neurological development. More than 30 thousand women were included; their pre-pregnancy BMI was calculated and the children’s IQ scores were assessed at 7 years of age. The results indicated that women with a BMI of around 20 kg/m² had children with the highest IQ scores. In contrast, maternal obesity (BMI 30 kg/m²) was associated with lower total and verbal IQ scores. More importantly, excessive weight gain during pregnancy accelerated this association.

All of these findings confirm that there is a link between childhood intelligence and body weight later in life. But what is the mechanism underling this phenomenon?

According to some studies, higher intelligence (IQ) in childhood predicts a better socio-economic status later in life (a higher educational level with a better income). In addition, higher educational attainment seems to reduce the risk of obesity, probably based on better dietary habits (more healthy food choices). This might partly explain how a lower IQ in childhood can lead to weight gain and obesity later in life. When it comes to the impact that excess weight gain has on intelligence, it seems that more research is needed to confirm this association and elucidate the underlying mechanisms. One of the possible explanations for this association is that hormones produced by fat cells may damage brain cells. Another possibility is that excess body weight may jeopardize cerebral blood vessels and, thus, impair brain functions.

Although the cause of obesity-lowered intelligence scores is not entirely clear, it is evident that the link exists. Since obesity is a rising global health concern, its negative effects should also be investigated in terms of its impact on cognitive functions and intelligence. This is especially important when we consider that even pre-pregnancy obesity leads to lower IQ in children.

References

Belsky, D.W., Caspi, A., Goldman-Mellor, S., Meier, M.H., Ramrakha, S., Poulton, R., Moffitt, T.E. (2013). Is obesity associated with a decline in intelligence quotient during the first half of the life course? American Journal of Epidemiology. 178(9): 1461-1468. doi: 10.1093/aje/kwt135

Kanazawa, S. (2013). Childhood intelligence and adult obesity. Obesity (Silver Spring). 21(3): 434-440. doi: 10.1002/oby.20018

Kanazawa, S. (2014) Intelligence and obesity: which way does the causal direction go? Current Opinion in Endocrinology, Diabetes and  Obesity. 21(5): 339-344. doi: 10.1097/MED.0000000000000091

Vitelli, O., Tabarrini, A., Miano, S., Rabasco, J., Pietropaoli , N., Forlani, M., Parisi, P., Villa, M.P. (2015). Impact of obesity on cognitive outcome in children with sleep-disordered breathing. Sleep Med. 2015;16(5): 625-630. doi: 10.1016/j.sleep.2014.12.015

Huang, L., Yu, X., Keim, S., Li, L., Zhang, L., Zhang, J. (2014). Maternal prepregnancy obesity and child neurodevelopment in the Collaborative Perinatal Project. International Journal of Epidemiology. 2014;43(3): 7837-92. doi: 10.1093/ije/dyu030

Chandola, T., Deary, I.J., Blane, D., Batty, G.D. (2006). Childhood IQ in relation to obesity and weight gain in adult life: the National Child Development (1958) Study. International  Journal of Obesity. 30(9): 1422-1432. DOI: 10.1038/sj.ijo.0803279

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