It’s Not You, It’s Your Brain: How Body Mass Index and Body Fat Percent correlate with Brain Structure.

Obesity has become a worldwide epidemic. In Canada alone, the prevalence of obesity had doubled between 1981 and 2007/09 with over one in four Canadian adults are obese (Public Health Agency of Canada and Canadian Institute for Health Information, 2011). Obesity has been associated with changes in brain structure, especially neural networks involved in cognition, motivation, impulse control, and salience processing (Raji et al., 2010; Taki et al., 2008). Understanding the association between obesity and specific brain structure will shed some light on obesity-related cognitive and behavioural differences and, consequently, pave the way to treatment and prevention.
The National Post recently reported the findings of a study that found overweight and obese people having significantly less grey and white matter in key areas of the brain, offering a biologically plausible explanation for why overweight people tend to have reduced cognitive functioning, greater impulsivity, and altered reward processing (Kirkey, 2016). Reporting on changes across the whole brain, people with a higher Body Mass Index (BMI) or Body Fat Percent (BFP) had slightly more grey matter, while no significant difference in white matter was found between lean and overweight people. Specific brain networks, however, showed heavier people having less white matter in the salience network and dorsal striatum, brain regions involved in reward processing and habitual behaviour, respectively. The news article quoted the lead researcher’s comment that, “it stands to reason that these changes could further affect the ability of overweight individuals to exert self-control and maintain healthy lifestyle choices.” He further commented that the study could not answer whether differences in brain structure make certain individuals prone to becoming obese or poor diet, lifestyle, and/or body composition cause these differences. More and larger studies were warranted (Kirkey, 2016).
The news article published by the National Post corresponded well to the study published in Frontiers in Neuroscience (Figley, Asem, Levenbaum, & Courtney, 2016). The study demonstrated that body composition (i.e. BMI and BFP) was positively correlated with global gray matter volume, while no correlation with global white matter volume was found, after corrected for age, gender, and total intracranial volume. The structural imaging data indicated that individuals with higher BMI and/or BFP exhibited reductions in white matter volume and microstructure surrounding the dorsal striatum and regions of the default mode network, executive control network, and salience network (Figley et al., 2016). It is important to note that the study did not directly answer whether the observed structural and functional brain differences were likely to cause decreases in cognitive performance and self-control in overweight and obese individuals. Interpreting the study findings in the context of previous research might, however, provide some insight (Figley et al., 2016). Given the role of the mesolimbic pathways of the dopaminergic system in reward processing, motivation, and willpower, the reductions of gray and white matter throughout these pathways induce lower than normal reward responses and consequently, maintain addictive behaviours, including overeating or eating high-fat, high-sugar foods, to elicit similar pleasure responses (Blum, Thanos, & Gold, 2014; Kolb, Whishaw, & Teskey, 2016).
Although the recent study failed to answer whether differences in brain structure made certain individuals prone to becoming obese or vice versa, it contributed to the understanding of obesity as a complex disease. Recognizing obesity as a chronic disease will help reduce the stigma often associated with the condition and stop regarding obesity as a lifestyle/behaviour problem, but a medical condition that requires effective prevention and lifelong treatment strategies (Rich, 2015). 

Oleh: Monica Purnama,MSc., Calgary-Alberta, Canada


References
Blum, K., Thanos, P. K., & Gold, M. S. (2014). Dopamine and glucose, obesity, and reward deficiency syndrome. Frontiers in Psychology , 5 (919). Retrieved November 15, 2016, from http://journal.frontiersin.org/article/10.3389/fpsyg.2014.00919/full
Figley, C. R., Asem, J. S., Levenbaum, E. L., & Courtney, S. M. (2016). Effects of body mass index and body fat percent on default mode, executive control, and salience network structure and function. Frontiers in Neuroscience , 10 (234). Retrieved November 14, 2016, from http://journal.frontiersin.org/article/10.3389/fnins.2016.00234/full
Kirkey, S. (2016, July 6). Does fat affect your brain? Study finds obese have less grey and white matter in key areas. Retrieved November 15, 2016, from National Post: http://news.nationalpost.com/health/does-fat-affect-your-brain-study-finds-obese-have-less-grey-and-white-matter-in-key-areas?__lsa=ea52-9391
Kolb, B., Whishaw, I. Q., & Teskey, G. C. (2016). An introduction to brain and behavior (5th ed.). New York, NY: Worth Publishers.
Public Health Agency of Canada and Canadian Institute for Health Information. (2011). Obesity in Canada: A joint report from the Public Health Agency of Canada and the Canadian Institute for Health Information. Retrieved November 14, 2016, from Public Health Agency of Canada: http://www.phac-aspc.gc.ca/hp-ps/hl-mvs/oic-oac/assets/pdf/oic-oac-eng.pdf
Raji, C. A., Ho, A. J., Parikshak, N. N., Becker, J. T., Lopez, O. L., Kuller, L. H., et al. (2010). Brain structure and obesity. Human Brain Mapping , 31, 353-364.
Rich, P. (2015, October 9). CMA recognizes obesity as a disease. Retrieved November 15, 2016, from Canadian Medical Association: https://www.cma.ca/En/Pages/cma-recognizes-obesity-as-a-disease.aspx
Taki, Y., Kinomura, S., Sato, K., Inoue, K., Goto, R., Okada, K., et al. (2008). Relationship between body mass index and gray matter volume in 1,428 healthy individuals. Obesity , 16, 119-124.