Obesity and the Brain

Is there a link between obesity and cognitive functioning?

Posted Jun 24, 2013

Though obesity can lead to medical problems including heart disease, diabetes, sleep apnea and depression, can it affect cognitive performance as well?    According to a research study  recently published in Neuropsychology, that may actually be the case.    John Gunstad of Kent State University and Kelly M. Stanek of the University of Alabama led a team of researchers examining the interaction of Body Mass Index (BMI) and cognitive functioning across the human lifespan.   Their conclusions have unsettling implications for the current obesity pandemic as well as the disturbing rise in dementia cases among older adults.

Based on previous research, obesity has been linked to increased risk of Alzheimer’s disease and stroke.  Weight gain has also been linked to long-term decline in cognitive performance even when dementia is not part of the picture.    Neuroimaging studies of morbidly obese people (BMI greater than 40)  suggest that they are at greater risk for brain atrophy though these results are still controversial for younger age groups.   As people grow older however, they become more vulnerable to brain-related disorders which may make the effect of obesity even more important.

In their latest study,  Stanek, Gunstad, and their colleagues examined data collected from the Brain Resource International Database (BRID) as well as data taken from a national study of morbidly obese people receiving bariatric surgery to help weight loss.  In total, 732 people ranging in age from 18 to 88 and with BMI scores ranging from 19 to 75 were used in the study.    Along with medical history and demographic data,  the researchers also looked at how the people in the study responded on different neuropsychological tests of cognition, including memory, attention and executive functioning.

As the researchers predicted, age was inversely correlated with performance on most of the cognitive tests.    Obesity was also strongly associated with decreased attention, fine motor speed, and processing speed across the entire lifespan but the relationship between BMI and other cognitive abilities appeared to be more complicated.   While the overall correlation between BMI and executive functioning  was not significant,  it did become a factor when age was introduced into the picture.   According to the study results, executive functioning deficits were greater for older adults who were obese than for older adults with a lower BMI.    These results were still significant when other factors such as medical history were taken into account.

Overall, these findings suggest that obesity can increase the risk of different cognitive problems as people grow older and also supports previous research by Stanek and Gunstad.   The interaction between age and BMI in reduced executive functioning  seems especially important since both aging and obesity have separate effects on brain structure and cognition.    As people grow older, they become more vulnerable to different factors affecting brain functioning than they were when younger.  That can include things such as head injuries, medical conditions, and, apparently, obesity as well.

In exploring why obesity affects brain functioning, Stanek et al.  suggest that obesity can affect the brain’s white matter or the glial cells and myelinated axons that relay signals between neurons in the different parts of the brain.    Though brain imaging studies have not turned up strong evidence for obesity affecting white matter so far,  a 2011 study by Stanek and her colleagues found  a negative relationship between BMI and structural integrity in specific brain regions such as the corpus callosum and the fornix.   This relationship appears to be linked to age as well with older patients showing greater impairment linked to obesity.   

The relationship between cognitive impairment and obesity is not necessarily limited to older adults however, since research has found cognitive problems in severely obese adolescents as well.   Since there have been relatively few research studies looking at obesity and brain functioning so far, the actual  relationship between BMI and cognition is still unclear.

There is also the problem of measuring obesity and what constitutes a healthy weight in humans.  While BMI is still widely used, critics have rightly pointed out that it may not be the best way to measure body weight in normal individuals.   Not only does BMI often fail to show the difference between lean muscle mass and fat mass, but some medical conditions such as sarcopenia can distort body mass figures in older adults.     Measuring hip circumference and abdominal fat may be more useful in studying the link between obesity and cognitive problems.  

While the impact that obesity has on cognitive functioning appears relatively modest so far, at least for adolescents and younger adults, there are still important questions to be answered about obesity and the brain.   Can even modest cognitive problems linked to obesity help explain why overweight people have trouble losing weight?    At least one recent research study has already shown that executive functioning and memory deficits can identify those patients undergoing weight loss surgery who are unable to stick to weight loss programs afterward.

Better treatments for obesity are certainly needed since the  current obesity epidemic has already become a major social problem.   In the United States alone,  35.7% adults are obese while an additional 33.1% are overweight according to a 2010 survey with over 6% of Americans meeting the criteria for extreme obesity (BMI greater than 40 kg/m2).    Would understanding the link between obesity and brain functioning lead to better treatments for obesity as well as provide help for older adults at risk for dementia?     The answers provided by research in this area may well be more critical than you might think.