Middle-aged people who are underweight may have an increased risk of going on to develop dementia, a new study has found.

However, overweight people may have a reduced risk.

UK researchers analysed the medical records of almost two million people, which had been collected over a 20-year period. The average age of those involved was 55, while the average body mass index (BMI) was 26.5, which is just within the overweight range.

The participants were followed up for an average of nine years and during that time, over 45,000 people were diagnosed with dementia.

The researchers found that people who were classed as underweight in middle age were 34% more likely to be diagnosed with dementia compared to those of a healthy weight.

Furthermore, this increased risk persisted even 15 years after the person's underweight was recorded.

Meanwhile, the study found that as people's BMI increased in middle age, their risk of dementia fell. In fact, those who were considered very obese in middle age - with a BMI of 40 or more - had a 29% reduced risk of developing dementia compared to people of a healthy weight.

This marks the largest study to examine the statistical link between BMI and the risk of dementia. The researchers found that this link did not appear to be affected by the decade in which the participants were born, or by their age at the time of their diagnosis.

The results also stood when other influencing factors were taken into account, such as smoking and alcohol use.

"Our results suggest that doctors, public health scientists and policy makers need to re-think 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," commented the study's author, Prof Stuart Pocock, of the London School of Hygiene & Tropical Medicine.

He said that these findings 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," he suggested.

The researchers said that it is unclear why a high BMI appears to offer a protective effect against dementia and more work is needed to figure this out.

"If increased weight in mid-life is protective against dementia, the reasons for this inverse association are unclear at present. Many different issues related to diet, exercise, frailty, genetic factors, and weight change could play a part," they added.

Details of these findings are published in the journal, The Lancet Diabetes & Endocrinology.

Alzheimer's disease is the most common type of dementia. For more on this, see our Alzheimer's Clinic here