A study published within The Lancet Diabetes and Endocrinology journal, which links being underweight in middle and old age with dementia, should be interpreted with caution by both patients and healthcare professionals.
The study is based on data taken from nearly 2 million patients between 1992 and 2007. Participants were 40 years old or older in 1992 and the median age was 55. The number of people that developed dementia over the 15 year study period was 45,507 people which represent 0.24% of all the patients reviewed.
The results showed that the most at risk were people that were underweight (note: BMI under 20 kg/m2), with these people having a 34% higher risk of developing dementia than those that were of a healthy weight at the start of the study. In addition to this, the study showed that people that were very obese (BMI over 40 kg/m2) had a 29% lower risk than people of a healthy weight.
Dementia takes decades to develop
The research appears to show that being overweight or obese has a protective effect against dementia, however, care is needed in how the results are interpreted. Dementia is a condition that takes many decades to develop and the 15 years study period is simply not long enough to be able to state that being overweight has a protective effect.
It is noteworthy that whilst type 2 diabetes is relatively common in people from 45 years of age, dementia only starts to become more commonplace in people over 65 years of age. This therefore suggests a lag between the conditions of around 20 years, longer than the study period. Whilst this note makes the assumption that the dementia and type 2 diabetes are strongly linked, research does indeed suggest there to be a strong link between the conditions.
Dementia and loss of insulin production in type 2 diabetes
The current understanding of dementia is that the condition develops following the build up of beta-amyloid plaques within the brain. A note that has particular relevance to type 2 diabetes is the fact that loss of insulin producing capabilities within people with a relatively long duration of type 2 diabetes has also been linked with the development of amyloid plaques within the pancreas.
Another point that requires careful consideration in interpreting the study is to do with how type 2 diabetes was treated during the 1990s and prior to this decade. During this time, type 2 diabetes was rarely treated with insulin injections as it is now. As a result, those who had had type 2 diabetes for a number of years would start to lose the ability to produce their own insulin. As insulin production decreases over time, and without insulin injections being taken to compensate, the body loses weight. These days, people with type 2 diabetes are much more likely to be given insulin than the generation before and therefore the steady loss of weight in the decades following type 2 diabetes diagnosis does not tend to happen with as much regularity as it used to.
The study did not detail whether those patients that were underweight at the start of the study and went on to develop dementia had been overweight, obese or otherwise developed type 2 diabetes prior to the start of the study. Because of this, it is difficult to draw firm conclusions from the study.
Is being overweight really a protective factor?
The key question that needs to be asked is: “What preceded the low body weight in those that developed dementia during the study period?” The other question that arises from the study is what the results would be if the study period were to be extended by up to another 20 years? Would those that were obese within the 1990s maintain a low risk of dementia or would they be shown to have a much higher risk 20 years further down the line?
Until these questions are answered, they key message for health remains that maintaining a healthy weight and following an active lifestyle is the best option for reducing your risk of dementia in older age.

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