People who are underweight in middle-age – or even on the low side of normal weight – run a significantly higher risk of dementia as they get older, according to new research that contradicts current thinking.

The results of the large study, involving health records from 2 million people in the UK, have surprised the authors and other experts. It has been wrongly claimed that obese people have a higher risk of dementia, say the authors from the London School of Hygiene and Tropical Medicine. In fact, the numbers appear to show that increased weight is protective.

At highest risk, says the study, are middle-aged people with a BMI [body mass index] lower than 20 – which includes many in the “normal weight” category, since underweight is usually classified as lower than a BMI of 18.5.

These people have a 34% higher chance of dementia as they age than those with a BMI of 20 to just below 25, which this study classes as healthy weight. The heavier people become, the more their risk declines. Very obese people, with a BMI over 40, were 29% less likely to get dementia 15 years later than those in the normal weight category.

Our results suggest that doctors and policymakers need to rethink how to best identify who is at high risk of dementia Prof Stuart Pocock

Prof Stuart Pocock, one of the authors, said: “Our results suggest that doctors, public health scientists, and policymakers need to rethink how to best identify who is at high risk of dementia. We also need to pay attention to the causes and public health consequences of the link between underweight and increased dementia risk which our research has established.



“However, our results also open up an intriguing new avenue in the search for protective factors for dementia – if we can understand why people with a high BMI have a reduced risk of dementia, it’s possible that further down the line, researchers might be able to use these insights to develop new treatments for dementia.”

Lead author Dr Nawab Qizilbash from Oxon Epidemiology told the Guardian that the message from the study was not that it was OK to be overweight or obese in middle-age. “Even if there were to be a protective effect in dementia, you may not live long enough to benefit because you are at higher risk from other conditions,” he said.

The study, published in the Lancet Diabetes and Endocrinology journal, looks only at data, correlating BMI with dementia diagnoses in general practice records and making allowances for anything that could skew the picture. “We haven’t been able to find an explanation,” said Qizilbash. “We are left with this finding which overshadows all the previous studies put together. The question is whether there is another explanation for it. In epidemiology, you are always left with the question of whether there is another factor.”

Many issues “related to diet, exercise, frailty, genetic factors, and weight change” could play a part, says the paper. There have been small studies that suggest a deficiency of vitamin E or vitamin D may play a part in dementia, but these are purely speculative, said Qizilbash.

But the study “opens up an avenue to look at the protective effects on dementia of diet, vitamins, weight change as well as frailty and potentially genetic influences”.

Others were cautious about the results, while acknowledging the scale of the study, which gives it greater power than previous pieces of research.

This study doesn’t tell us that being underweight causes dementia, or that being overweight will prevent the condition Dr Simon Ridley

Dr Simon Ridley, from Alzheimer’s Research UK, said further work is needed. “This study doesn’t tell us that being underweight causes dementia, or that being overweight will prevent the condition,” he said.

“Many other studies have shown an association between obesity and an increased risk of dementia. These findings demonstrate the complexity of research into risk factors for dementia and it is important to note that BMI is a crude measure – not necessarily an indicator of health. It’s also not clear whether other factors could have affected these results.”

The best protection against dementia, he added, is “eating a healthy, balanced diet, exercising regularly, not smoking, and keeping blood pressure in check”.

Prof Deborah Gustafson from SUNY Downstate Medical Center in New York, USA, in a commentary with the study in the journal, writes: “To understand the association between BMI and late-onset dementia should sober us as to the complexity of identifying risk and protective factors for dementia. The report by Qizilbash and colleagues is not the final word on this controversial topic.”

