Obesity and Dementia: Is There a Connection?

How’s your new year’s resolution going? If you’re one of the stereotypical Americans who made a resolution to get in shape and lose weight this year, then you might also be the stereotypical American who loses motivation for that goal a few weeks into the new year. A new study published in Neurology may help you to regain that motivation.

It’s not uncommon for us to hear in the news about the negative effects that obesity can have on the peripheral parts of our body—things like diabetes, heart disease, asthma, high blood pressure, and some cancers are all linked to being overweight or obese. But did you know that it can have effects on your mental health as well?

Hamer and Batty (2019), using 9,652 people aged 40-69, found a strong association between gray matter volume of the brain and both obesity measures of body mass index (BMI) and waist-hip-ratio (WHR). BMI gives a generally accepted measure of overall obesity level, while the WHR is an indication of “central obesity” (i.e. a big belly), the latter of which is generally correlated with more health problems than the former. The study grouped participants into 6 groups based on these measures: BMI less than 25 kg/m2 with and without central obesity, BMI 25-30 kg/m2 with and without central obesity, and BMI >30 kg/m2 with and without central obesity (visualized in figure below).

Body Mass Index and Waist-to-Hip in Relation to Gray Matter Brain Volume


Data are presented as means adjusted for age, gender, smoking habits, vigorous physical activity, alcohol consumption, education, major depression, heart disease and hypertension.

As shown above, the two obesity measures correlated differently with gray matter volume. While both higher BMI and WHR are both associated with lower gray matter volume, there is also an additive effect—while the “Overweight” and “Obese” BMI without central obesity groups had similar gray matter volumes, there was a significantly reduced gray matter volume in people with central obesity and “Obese” BMI compared to those with an “Overweight” BMI. It is also interesting to note that within both the “Normal” and “Overweight” BMI categories, there was no significant difference in gray matter volume between those with and without central obesity. The main take-away here being that a high BMI combined with a large belly is the worst position to be in if you’d like to optimize your brain health (or really any aspect of your health).

The authors also looked at the association between obesity measures and age, as shown below. As people aged, there was a significant decrease in gray matter volume regardless of BMI, but even within each age group, a higher BMI was associated with lower gray matter volume. Thus, when you have a higher BMI, you are essentially adding years onto the age of your brain.

Association Between Body Mass Index and Gray Matter Volume Relative to Age


Data are presented as means adjusted for age, gender, smoking habits, vigorous physical activity, alcohol consumption, education, major depression, heart disease and hypertension.

At this point, you may be asking, “why does it matter if you have less gray matter?” To answer that, we’ll take a look at a paper by Stout et al. (1996). Here, the authors looked at (among other things) gray matter volume in people with dementia and found that more severe cases of dementia were associated with lower gray matter volume. While the participants in the Hamer study did not have dementia, we can conclude that those with lower gray matter volume (i.e. more obese) were more likely to go on to develop it.

It is important to note that both of these studies are correlational studies, not causation. Thus, we cannot conclusively say that obesity causes you to lose gray matter volume. It could be the other way around. Or it could some other associated factor that is the culprit. It’s not really plausible to set up a causation study where you take normal weight people, randomly assign them to groups to either become obese or stay normal weight, and then see if gray matter volume changes as a result, so we’ll have to stick with the data that we have to make conclusions. Given that there are so many other health risks associated with being overweight or obese, the studies described here should just add some extra motivation to get you moving more and eating right if you fall into one of those categories.

Another limitation with the study is that it provides no clues as to if the loss of gray matter volume is reversible—assuming that obesity does cause your brain to shrink, does losing weight and getting healthy reverse the damage done to your brain? We don’t have an answer to that, but it would be an interesting study to do with your Brain Gauge! Maybe you need a little more motivation than just the number on your bathroom scale—you could use your Brain Gauge to track trends in your brain health as you shed pounds. As we’ve noted in other blogs, there is strong evidence that improving your diet and exercise habits do have positive impacts on your brain health, so it is a safe bet that you’ll see improvements in brain health as your body health improves. The correlations and associations between body fat, brain health, exercise, diet, and a number of other related factors may be complex, but the great thing is that understanding exactly what’s causing what doesn’t really matter when there is so much to gain and only fat to lose!


Hamer M, Batty GD. Association of body mass index, and waist to hip ratio with brain structure: UK Biobank study.

Stout JC, Jernigan TL, Archibald SL, Salmon DP. Association of dementia severity with cortical gray matter and abnormal white matter volumes in dementia of the Alzheimer type. Archives of neurology. 1996 Aug 1;53(8):742-9.

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