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Obesity is one of the most significant health burdens currently faced in the modern world, and its incidence is continually rising. It is linked to a growing number of other diseases such as type two diabetes, high blood pressure, coronary heart disease, asthma, cancers and sleep apnoea, to name a few. Before 2015, it was largely accepted that obesity could also be considered a risk factor for dementia. However, a 2015 study which used data from almost 2 million British people has joined the growing body of evidence challenging this notion, even going as far as providing evidence that obesity protects against dementia. So what can we learn from the current information, and what can we trust?
Defining The Disease
Before we go any further, it may be useful to define the diseases. First obesity, which can be defined as an excess of body fat, to the point where it causes adverse effects on health. Obesity is usually measured using the Body Mass Index (BMI) measurement scale, which takes the weight of a person (kg) and divides it by their height (m squared). With this scale, a value of 30 or over would be considered obese.
The incidence of obesity is growing substantially, with 500 million people considered to be obese across the globe in 2007. Worryingly, if we were to include people who are considered overweight (BMI of 25 of over), then this number triples to 1.48 billion people worldwide.
Dementia is a name given to a range of neurological disorders, characterised by memory loss, impaired reasoning ability and personality change. A few examples of dementia disorders include Alzheimer’s disease, the most common form of dementia, vascular dementia caused by damage from strokes, and frontotemporal dementia, where damage is limited to the front and side of the brain.
With an aging population and longer life expectancies, the incidence of dementia is growing, with an estimated 35.6 million people suffering with the disease in 2010. Along with increasing number of people becoming diagnosed, unsurprisingly the cost of treatment is also rising. The annual global healthcare costs associated with dementia was $604 billion in 2010.
A Link Between Obesity and Dementia?
With the large number of other disorders linked strongly to obesity, it would seem logical that dementia should also be linked to obesity. Numerous theories have been proposed, such as damage from oxidative stress or inflammation caused by obesity. There have also been numerous studies showing reduced cognition as a result of high fat diets.
In 2015, a review of the studies from the past decade showed nine investigations had looked into the risk of dementia associated with mid-life obesity. A further eight studies had investigated late-life obesity, two studies had investigated both, and five studies had investigated the effect of weight change on obesity. A comparison of the mid-life studies showed that obesity in mid-life (around the age of 40) increases dementia risk. Across the studies, there appears to be on average twice the risk of developing dementia following mid-life obesity compared to those who were not obese.
Some more significant results were deduced from measurements such as waist circumference. One study found that compared to people with a waist circumference of less than 83cm, there was twice the risk of vascular dementia in those with waist circumferences above 97cm.
Despite this evidence, the 2015 review also showed that from the ten studies investigating the effect of late-life obesity on dementia risk, there was a reduced risk of dementia in people considered overweight, and little to no extra risk of dementia in people considered obese. Further to this, the data from 5 “trajectory” studies (investigating weight loss and gain) showed that those who went on to develop dementia usually lost significant weight during this period. Conversely, those who lost weight more slowly appeared to have lower risks of dementia.
This important study, published in 2015 was conducted on the largest sample size by some margin (nearly 50 times larger that the largest sample size from previous studies). It appears to contradict the previous studies by suggesting that being obese during mid life could reduce dementia risk.
Following adjustment for a variety of factors (e.g. age, gender, smoking status) and assuming patients who died early had twice the chance of developing dementia, if the dementia rates are taken as a value of 1 in people of a healthy BMI, the rates would be 1.34 in people with a low BMI, 0.82 in people who were overweight, and less than 0.8 in people considered obese. This indicates that overweight or obese individuals had a lower rate of dementia than those of a lower BMI. Importantly, these results were statistically significant.
Despite the data appearing to be conflicting, it is worth bearing in mind that studies completed prior to 2005 had generally been conducted on small and varied sample sizes, varying from 108 patients to 12,076 patients. As a result, the confidence intervals (a measure that takes into account the variation in the data) were very large. Many of these studies also used significant selection pressures, often choosing people of specific genders or ethnicities. For example, one study only used male Asian-Americans. While this may have meant it was easier to recruit participants, it would also make it unrepresentative of the population at large.
Another key factor is that for all of these studies, BMI was the principal measure used to determine obesity due to its simplicity. However, it is not often considered an entirely reliable measure of obesity, as people who weigh more for other reasons (such as bodybuilders with large muscle mass) would weigh heavily, and thus produce a high BMI indicating they are obese, when they aren’t. It is also difficult to draw direct comparisons with data from some of the studies as they class obesity with different BMI ranges.
Several attempts have been made to make a more comprehensive measure of obesity. One attempt is the Visceral Adiposity Index, which uses the patient’s BMI, waist circumference measurement, their triglyceride (fat) and HDL cholesterol (often considered the “good” cholesterol) levels. The study that pioneered this measurement has confirmed with MRI imaging in some patients that it can be a reliable measure for determining obesity, but would be more complicated to investigate in patients. As yet, no studies using this measure to determine obesity have been performed relating to dementia risk.
The Take Away Points
The studies prior to 2015 had small sample sizes, or had limited samples to specific genders or ethnic groups, meaning they are likely to be less representative of the population at large.
Being underweight has consistently been shown to increase dementia risk.
People who lost a significant amount of weight in later life were also shown to have a significantly higher risk of dementia.
Both of these points suggests a poor diet, leading to weight loss could be to blame for increased dementia risk, and the protective effects of obesity are likely due to these people gaining adequate nutrient intakes from food.
However, this isn’t an excuse to not feel guilty about gaining weight. Obesity is still linked with a higher incidence of several other disorders. Some of these e.g. type-two diabetes have themselves been linked to a higher risk of dementia.
Consider checking to see if you are eating a balanced diet, and if you are particularly worried, it may be worth looking into supplementing vitamins to protect your precious grey matter.
Reaction Science article by Jennifer Hein, “Dietary Effects on Alzheimer's Disease Development”, available from http://reaction-science.com/articles/ed2-health-6.php
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