It is well known that obesity is associated with various health problems, such as an increased risk of diabetes, stroke, heart disease and certain types of cancer, but in recent years, scientists have started to also observe links between this disease and dementia. Although the relationship is still controversial, several studies have found that being obese during middle age increases the risk of dementia in later life, and one even found that being overweight can also increase the likelihood of developing this condition.

Now, in a real turn-up for the books, a new study has found that obese people actually have a lower risk of dementia than those of a healthy weight. Conversely, being underweight in middle and old age apparently increases the risk. These surprising findings, which contradict current health advice, call for further investigation into this complex relationship and perhaps even a reassessment of the risk factors for dementia. The study by the London School of Hygiene & Tropical Medicine has been published in The Lancet Diabetes & Endocrinology.

Much like obesity, dementia is a growing public health concern. It currently affects almost 50 million people worldwide, but this number is expected to more than triple by 2050. Although the precise cause is unknown, scientists have identified numerous factors that raise the likelihood of developing dementia, such as age, genetics, poor diet and a lack of exercise. Additionally, a few recent studies have identified associations between dementia and being overweight or obese, but the mechanisms for such an association are so poorly understood that the relationship has remained controversial.

With the hope of gaining a more comprehensive picture, scientists in the U.K. designed a study aimed at investigating the association between body mass index (BMI), which is a measure of body fat, and the risk of dementia. This involved following close to 2 million individuals in the U.K., aged 40 years or older, for an average of 9 years, but up to 20 years.

After taking into account things like smoking and alcohol consumption, the researchers found that, compared with people of a healthy weight (here defined as a BMI of between 20 and 25), underweight individuals (BMI <20) had a 34% higher risk of dementia. Under normal classification, this would also include some individuals in the healthy category, as underweight is usually defined as a BMI of below 18.5.

Interestingly, they also found that the incidence of dementia decreased as BMI increased, with very obese people (BMI>40) having a 29% lower dementia risk than people of a healthy weight, and overweight people having an 18% reduction.

Although this clearly contradicts previous studies, the researchers point out that this study is much stronger in terms of size. However, the researchers are currently unable to offer any explanation for the observed protective effect, and the study cannot prove being overweight is causing the reduced risk, although they note that many issues related to diet, exercise, genetic factors and weight change could play a role. Some studies have also hinted that vitamin E or E deficiencies could raise the risk for dementia, which could be less common in those who eat more.

The researchers also stress that this study does not mean that it is OK to be obese or overweight, as even if this does reduce the risk of dementia, individuals likely won’t survive long enough to see the benefits.

[Via Lancet Diabetes & Endocrinology, BBC News, Medical News Today and The Guardian]