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There has been an exponential increase in the number of people around the world who are obese and or have diabetes, both significant risk factors for dementia.

For many of us the battle of the bulge is seemingly the result of consuming too much fat, being too sedentary, or just simply eating too much.
As we get older our metabolic rate declines, so while we still experience hunger and enjoy our food, we need to reduce our total food intake to keep our weight in the normal range.

We know too that being overweight or obese puts us more at risk of other chronic illness and disease. We increase our risk of cardiovascular disease, stroke, diabetes and cancer.
But there is something else we need to be aware of as well.

Being overweight or obese doubles our risk of developing dementia or Alzheimer’s disease.
It is associated with having a smaller brain volume.

Having a smaller brain through obesity affects our brain function in specific areas:

In the hippocampus, the area specialised for learning and memory.
In the frontal lobes, our executive suite, concerned with planning, organisation and paying attention.
In the anterior cingulate gyrus, an area involved in decision-making, empathy and emotion.
In the thalamus, an area associated with coordinating other brain areas.

But it is the belly fat that puts us most at risk.
Having a big tummy with stick legs and arms actually puts you at greater risk than someone who is more generally rounded. The implication here, is that you don’t have to be particularly overweight to be at increased risk, if all the extra is sitting around your middle.

A study published last month showed this link. Over 700 subjects from the Framingham Heart Study were recuited. The mean age of the group was 60 years and 70% were women. Dr. S Seshadri and colleagues looked at the association of Body Mass Index (BMI), waist circumference, waist to hip ratio, CT measured abdominal fat and MRI brain scans looking at total brain volume and other variables, as well as the number of brain infarcts (mini areas of stroke) present. The results showed a strong correlation between central obesity (ie the fat in your abdomen) and the risk of dementia and Alzheimer’s disease.

Being obese in middle age, having an elevated systolic blood pressure (that’s the top reading of your blood pressure) and a high total cholesterol each are significant risk factors and individually double your relative risk of dementia. Put them together and you can end up with x6 the risk.

A meta-analysis of 10 international studies covering a 10 year period confirmed these findings, revealing that obesity increases the relative risk of dementia for men and women overall by an average of 42% compared to people of normal weight.

However after the age of 65years it appears that the trend is reversed. Annette Fitzpatrick from the University of Seattle reported the results of a study she undertook where she had around 300 people as subjects who did not have dementia. They had their weight at age 50 recorded and then repeated height and weight at age 65yrs to calculate the BMI’s at the different ages.

Over the 5-year follow up, those people classified as obese (BMI >30) were more likely to have developed dementia.
However the older subjects ie 65 years plus, who were underweight (BMI <20)


had an increased risk of dementia and the older subjects appeared to have gained a protective effect in later life.

So there appears to be an obesity paradox. There is a change in the predictive ability of BMI with dementia risk with age.

Why should this be?

The answer is yet to be revealed. There has been the suggestion that the onset of weight loss in older life may precede the onset of dementia. Studies have shown that women in particular will show weight loss about 10 years before the first signs of dementia become apparent.

However it is still the case that reducing obesity in midlife (especially central obesity) is crucial to reduce an individual’s overall risk of developing dementia or Alzheimer’s.

So meanwhile there is a big job ahead of us, to reduce obesity in those Western societies where being overweight has become the norm. The risk if we don’t, is that we will see increased mortality at an earlier age from cardiovascular disease and stroke and the added burden of the increased number of people developing dementia and Alzheimer’s disease.

Dr Jenny Brockis

Dr Jenny Brockis is a medical practitioner and internationally board-certified lifestyle medicine physician, workplace health and wellbeing consultant, podcaster, keynote speaker and best-selling author. Her new book 'Thriving Mind: How to Cultivate a Good Life' (Wiley) is available online and at all good bookstores.

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