Elissa McIntosh, used with permission
Source: Elissa McIntosh, used with permission

This guest post was contributed by Elissa McIntosh, a doctoral student in Clinical Science within the University of Southern California's Psychology Department.

Losing weight perennially tops the list of New Year’s resolutions. We all have different reasons for this pledge. Maybe we want to fit into our skinny jeans or have more energy to play with our family. Have you ever heard someone say, “I want to lose weight to improve my memory?”

In the last few decades, rates of obesity and related conditions such as type 2 diabetes have skyrocketed in the United States and other developed countries. Childhood obesity rates have risen at a particularly alarming clip: Today, about one in five school-age children is obese, and 31% of children are overweight or obese. This is a problem, because obesity is linked with a host of negative health outcomes. It is well known that obesity and diabetes are related to vascular issues such as hypertension, heart disease, and stroke. However, it is less known that obesity is related to our thinking skills (cognition), and even future risk for dementia.  

John Hain, Creative Commons license
Source: John Hain, Creative Commons license

Research suggests that childhood obesity negatively affects cognition. A team of researchers at the University of California San Diego, led by June Liang, reviewed the research on this topic by examining 67 published studies. They found that obesity was related to worse performance on several thinking skills and behaviors, including inhibition (ability to regulate your emotions and behaviors), cognitive flexibility (ability to switch between different tasks), attention, and visuospatial skills. Similarly, childhood obesity appears to hurt standardized testing scores in math and reading. Of concern, children who are overweight are more likely to be overweight in adulthood, pointing to the need to try to combat obesity in children.

Obesity appears to compromise cognition in adults as well as children. In a study led by Séverine Sabia at University College London, researchers studied the relationship between body mass index (BMI) across the lifespan and cognitive function in late mid-life. To do this, the researchers measured BMI in early adulthood (25 years old), early middle life (mean age = 44), and late mid-life (mean age = 61). The study found that being obese at two or three of these timepoints was associated with worse performance on tests assessing memory and executive functioning.   Although “executive function” sounds like the job performance of a Wall Street CEO, it actually refers to a cluster of cognitive skills related to planning and controlling behavior, like organizing a project or prioritizing tasks. The researchers concluded that chronic obesity negatively impacts your thinking skills in midlife.   

Midlife obesity doesn’t just affect the brain in the moment; it also seems to increase the risk for dementia. One twin study led by Weili Xu at the Aging Research Center at Karolinska Institute in Sweden reported that twins who were overweight or obese at midlife were at greater risk for developing dementia, with obese people being at the greatest risk. When controlling for twin pairs in the study, the relationship between BMI and late-life dementia was weakened suggesting that genetic and shared early life environmental factors contribute to the relationship between obesity and dementia. Similarly, a Finnish study by Anna-Maija Tolppanen showed increased BMI in middle age was associated with increased dementia risk, independently of other risk factors such as hypertension and diabetes. These studies are consistent with a large and growing body of research demonstrating that midlife obesity negatively affects cognition and increases risk for dementia. Studies investigating relationships between obesity and cognition and dementia in older adults have reported similar findings.  

Since cognitive skills are really brain functions, we might expect there to be a relationship between thinking abilities and the structure of the brain – that is, its size, shape, and organization. That is exactly what many researchers have found. Neuroimaging studies have shown that obesity is related to smaller brain volumes, including in the areas of the brain related to thinking skills like memory. So, what do these imaging studies tell us? Well, considering the relationship between obesity and cognition, these imaging findings likely mean that obesity-related processes may be adversely affecting brain development or brain structure.  While the exact mechanisms for this relationship are unknown, obesity is associated with several factors that may negatively affect brain structure such as elevated cortisol, lack of exercise, inflammation, hypertension, and type 2 diabetes mellitus. Since these conditions often co-occur within an individual, it is difficult to point out exactly what is driving the relationship between obesity and poor cognitive skills. However, using statistical methods, studies have shown that obesity is still related to smaller brain volumes when accounting for these other factors that may impact brain volume. Taken together with the research looking at obesity and dementia, it is likely that obesity-related brain atrophy may increase risk for cognitive decline and dementia.

If you’ve read this far, you’ve heard about a bunch of scary research showing that obesity is related to poor thinking skills and smaller brain volumes. Yikes! But, there’s an upside! Lifestyle factors, such as exercise, can improve our cognitive functions. Research shows that increases in physical activity improve thinking skills. Why might this be? Exercise is thought to improve cognition through various mechanisms. First, aerobic exercise releases a chemical called brain-derived neurotropic factor (BDNF) that stimulates neurogenesis. In plain English, the brain releases a chemical that promotes production of new brain cells. Exercise also reduces insulin resistance and inflammation, which are known to negatively affect cognition and health more generally. Exercise also may improve cognition by indirectly affecting our mood, sleep, and stress levels. Poor sleep, depression, anxiety, and stress are all known to negatively affect brain function. Exercise has been repeatedly shown to improve mood states, stress, and sleep.

So, what’s the take home message here? Obesity is linked to poor cognition and smaller brain volumes. Also, obesity is associated with increased risk for dementia. Despite this bad news, there is hope. Exercise across the lifespan can ameliorate the negative effects of obesity on cognition. A group of researchers in Finland have demonstrated that leisure-time physical activity in midlife and after is associated with decreased risk for dementia and Alzheimer’s disease. Even among older adults with mild cognitive impairment, exercise has been shown to improve cognitive symptoms like memory issues. In conclusion, it’s never too late to start exercising and eating healthier. Your brain will thank you later!

References

Liang, J., Matheson, B., Kaye, W., & Boutelle, K. (2014). Neurocognitive correlates of obesity and obesity-related behaviors in children and adolescents. Int J Obes (Lond), 38(4), 494-506. doi:10.1038/ijo.2013.142

Raji, C. A., Ho, A. J., Parikshak, N. N., Becker, J. T., Lopez, O. L., Kuller, L. H., . . . Thompson, P. M. (2010). Brain structure and obesity. Hum Brain Mapp, 31(3), 353-364. doi:10.1002/hbm.20870

Sabia, S., Kivimaki, M., Shipley, M. J., Marmot, M. G., & Singh-Manoux, A. (2009). Body mass index over the adult life course and cognition in late midlife: the Whitehall II Cohort Study. Am J Clin Nutr, 89(2), 601-607. doi:10.3945/ajcn.2008.26482

Tolppanen, A. M., Ngandu, T., Kareholt, I., Laatikainen, T., Rusanen, M., Soininen, H., & Kivipelto, M. (2014). Midlife and late-life body mass index and late-life dementia: results from a prospective population-based cohort. J Alzheimers Dis, 38(1), 201-209. doi:10.3233/jad-130698

Tolppanen, A. M., Solomon, A., Kulmala, J., Kareholt, I., Ngandu, T., Rusanen, M., . . . Kivipelto, M. (2015). Leisure-time physical activity from mid- to late life, body mass index, and risk of dementia. Alzheimers Dement, 11(4), 434-443.e436. doi:10.1016/j.jalz.2014.01.008

Xu, W. L., Atti, A. R., Gatz, M., Pedersen, N. L., Johansson, B., & Fratiglioni, L. (2011). Midlife overweight and obesity increase late-life dementia risk: a population-based twin study. Neurology, 76(18), 1568-1574. doi:10.1212/WNL.0b013e3182190d09

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