Why big bellies could mean big trouble for brain function in later life

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Previously on this site I have presented evidence that the traditional way of assessing body weight ” the body mass index (BMI) ” is pretty useless as a marker for health. Part of the explanation for this lies in the fact that the BMI tells us nothing about the composition of the body. Neither does it tell us where any surplus weight is distributed. This is important because there is now a mass of evidence which links fat distributed around the midriff (abdominal obesity) with an increased risk of conditions such as cardiovascular disease and diabetes.

This week saw the publication of a study which suggests that having excess weight around one’s middle may be a risk factor for dementia too.

The study in question was published on-line in the journal Neurology. In this study, a measurement known as the ‘sagittal abdominal diameter’ (‘SAD’) was used to assess the extent of abdominal obesity (SAD is basically the distance between the front of the belly through to the back). Specifically, more than 6500 individuals had their SADs measured over a 9-year period (1964-1973). The presence of dementia was assessed an average of 36 years later. The relationship between individuals’ SAD measurements and risk of dementia was assessed, after accounting for so-called confounding factors such as age, sex and the presence of conditions such as high blood pressure, diabetes and cardiovascular disease.

The results of this study showed that:

Individuals with the highest SAD measurements were at a 272 per cent (almost 3-fold) increased risk of dementia compared to those with the lowest SADs.

In further analysis, the researchers took into account individuals’ BMI readings as well. Individuals with a high SAD will tend to have relatively high BMIs too. Taking BMIs into consideration helps to determine more accurately whether the link (if any) between SAD and dementia risk.

Having accounted for BMI measurement, the researchers found that:

Individuals with the highest SAD scores were at a 92 per cent increased risk of dementia.

In individuals with ‘normal’ weight (BMI of 18.5 ” 24.9), those with a high SAD measurement (> 25 cm) were not found to be at a statistically significant increased risk of dementia compared to those with a low SAD measurement (< 25 cm). Overall, what this study suggests is that abdominal obesity is linked with an increased risk of dementia. It's perhaps worth considering the mechanisms that might explain such an association. Some of the explanation may lie in the fact that increased abdominal size is often associated with metabolic syndrome. Previous research has linked metabolic syndrome with an increased risk of stroke " a condition which can lead directly to dementia (sometimes as a result of multiple strokes in which case the dementia is referred to as 'multi-infarct dementia'). Individuals with metabolic syndrome typically exhibit a degree of 'impaired glucose metabolism' (which essentially means they have difficulty keeping glucose levels controlled in the bloodstream). Impaired glucose tolerance (IGT) can ultimately lead to type 2 diabetes. Both IGT and type 2 diabetes have been linked with impaired brain function in later life [2]. A number of mechanisms have been postulated for how IGT and type 2 diabetes may increased dementia risk, including an increased tendency to inflammation as well as 'oxidative stress' (damage wreaked by substances known as 'free radicals') [3]. A key substance to the development of dementia may be the hormone insulin. In one study, individuals given insulin intravenously experienced a rise in the body levels of a protein known as 'amyloid beta' [4]. The significance of this? - Amyloid beta is the prime constituent of the 'plaques' found in the brains of individuals suffering from Alzheimer's disease. The long and short of all this research seems to be this: if you want to do what you can to help preserve your mental faculties as you age, it makes sense to take steps to keep blood sugar and insulin levels under control. For most of us, that will mean a diet relatively low in blood sugar destabilising carbs including many starchy staples bread, potato, rice, pasta and breakfast cereals.

1. Whitmer RA, et al. Central obesity and increased risk of dementia more than three decades later. Neurology 2008 [Epub 26 March 2008]

2. Fisman EZ, Tenenbaum A (eds). Impaired glucose metabolism and cerebrovascular diseases. In Cardiovascular Diabetology, Metabolic and Inflammatory Facets, Advances in Cardioology. Karger 2008;45:107-113

3. Whitmer RA. Type 2 diabetes and risk of cognitive impairment and dementia. Curr Neurol Neurosci Rep 2007;7(5):373-380

4. Kulstad JJ, et al. Differential modulation of plasma beta-amyloid by insulin in patients with Alzheimer disease.
Neurology. 2006;66(10):1506-10.

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