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In addition to the well-known impact on risk for disorders such as diabetes and reduced life-expectancy, the effects of obesity may extend to psychological function. The so-called obesity epidemic may be causing decline in cognitive function through direct and indirect impacts on brain functioning. An expanding waistline thus appears to link to decreasing ability to learn and remember.
Rates of obesity continue to rise. Obesity is generally defined as a Body Mass Index (BMI) equal to or greater than 30. In the U.S. in 2010, all states had an obesity prevalence of at least 1 in 5 adults — 36 states had a prevalence of at least 1 in 4 adults . In the U.K., the Department of Health finds 1 in 4 adults are obese with rates to increase to around 1 in 2 by 2050 if no action is taken .
Whilst some countries have high rates of malnutrition, data from the World Health Organisation highlights that this is not just a “Western” epidemic. For example, a number of South Pacific countries far exceed the 33.9% of adults who are obese in the United States: Nauru, an island country in Micronesia in the South Pacific, tops the bill with 78.5% of adults who are obese; American Samoa (an unincorporated territory of the United States located in the South Pacific Ocean, southeast of the Independent State of Samoa) has an adult obesity prevalence of 74.6%; Tokelau, a territory of New Zealand, has 63.4%; Tonga, a sovereign state and an archipelago in the South Pacific Ocean, has 56%; Kiribati, an island nation located in the central tropical Pacific Ocean, has 50.6%. Other non-Western countries also have a high prevalence of adult obesity. For example, Saudi Arabia has an adult obesity prevalence of 35.6%; Egypt 30.3%; Chile 21.9%; and South Africa 21.6%. The obesity-epidemic is truly global in scale.
Many of the risks of obesity are well known — cardiovascular diseases, diabetes, musculoskeletal disorders and some cancers to name but a few. Perhaps less well known is the ongoing research into the impact to our brains, and therefore our psychology and cognitive function. Research showing links between diet and dementia have been widely publicised. For example, the BBC reported early results showing that a larger meal size increases risks of dementia. Whilst these early results are potentially interesting, it is important to note that BMI may link to other types of cognitive decline and in both younger and older adults.
In a recent review, researchers from the Department of Psychology at the University of Alabama explored research showing both an impact on cognitive function and potential causal mechanisms . The review is different from some we have previously reported — this is a narrative review, meaning that there is no published set of criteria explaining how articles were selected for inclusion and no attempt to systematically report all relevant studies. Rather, in this case, experts in the field have selected, summarised and synthesised studies they consider to be of interest. Whilst this type of narrative review is less scientifically rigorous and findings remain tentative, it nonetheless functions to provide an overall picture of the state of research linking obesity and cognitive decline.
The authors summarised studies finding a link between obesity and “frontal-subcortical” cognitive abilities. In plain English this covers goal-directed behaviours. In more detail, these are complex cognitive processes that seem to control other more basic processes. These controlling processes are known as “executive functions”, for example the ability to control attention, abilities to plan behaviors including movement when in a new situation and regulate impulses to ensure behaviour is socially acceptable. These complex processes require involvement from many brain areas, suggesting that the physical impact of obesity on the brain is wide-ranging.
A great deal of this research has concerned itself with dementia and of peer-reviewed studies (the study reported by the BBC above is yet to be peer-reviewed) such as that by Whitmer et al’s 2008  show that the link between obesity and dementia depends on age. Specifically, obesity in midlife predicts later risk of dementia, whereas obese older adults are not more likely to have dementia. It appears that obesity in midlife may cause damage that builds up to increased risk of dementia.
So far, so good — but what are the mechanisms by which obesity could influence the brain and cognitive function? One suggestion is “global atrophy” — generalised damage to the brain caused by obesity. This may be caused indirectly — so obesity causes diabetes or hypertension and it is these conditions that go on to decrease cognitive function. This pathway remains unclear. A more direct link between obesity and cognitive function is via “adipokines”. Adipose tissue is another term of body fat. These tissues contain protein molecules called “adipokines” (also referred to as “adipocytokines”) that allow communication between cells and circulate around the body. Adipokines may enter the brain and cause structural abnormalities, such as increased amounts of white matter, which are linked with lower cognitive function.
We are all well aware of the dangers of modern convenience lifestyles; that smoking, drinking and overeating are bad for us and that exercise and maintaining a healthy weight are good for us. It appears that these good behaviours are necessary not only to keep ourselves fit and alive, but also to keep our brains fit and functioning well. And the good news in all this — research suggests that when obese or overweight people lose weight, this can improve cognitive abilities, including working memory . So think of both your body and your mind when you are planning your next meal.
- Obesity and Overweight for Professionals: Data and Statisitics: U.S. Obesity Trends. Centers for Disease Control and Prevention (CDC). Accessed 2012 Mar 11.
- Obesity. Department of Health. Accessed 2012 Mar 11.
- Sellbom and Gunstad. Cognitive Function and Decline in Obesity. J Alzheimers Dis. 2012 Jan 18. [Epub ahead of print]
- Whitmer et al. Central obesity and increased risk of dementia more than three decades later. Neurology. 2008 Sep 30;71(14):1057-64. Epub 2008 Mar 26.
- Gunstad et al. Improved memory function 12 weeks after bariatric surgery. Surg Obes Relat Dis. 2011 Jul-Aug;7(4):465-72. Epub 2010 Oct 30.