‘Underweight have higher dementia risk’

The findings from the largest-ever study to examine the links between body-mass index (BMI) and dementia risk also show that middle-aged obese people are nearly 30 per cent less likely to develop dementia than people of a healthy weight, contradicting findings from some previous research.

Researchers based at the London School of Hygiene & Tropical Medicine, and OXON Epidemiology, both in London, examined information recorded during routine GP examiniations over nearly two decades covering around one in 11 of the population.

They analysed the medical records of nearly two million people with an average age of 55 at the start of the study, and an average BMI of 26.5, just within the range usually classed as overweight.

More than 45,000 went on to develop dementia.

They found people who were underweight in middle age, with a BMI of less than 20, were a third more likely to be diagnosed with dementia than those of a healthy weight, and this increased risk of dementia persisted even 15 years after the underweight was recorded.

As their BMI in middle age increased, the risk of dementia reduced, with very obese people with a BMI of 40, 29 per cent less likely to get dementia than people in the normal weight range.

An increase in BMI was associated with a substantial steadily decreasing risk of dementia for BMI of up to 25.

Above a BMI of 25, dementia risk decreased more gradually, and this trend continued up to a BMI of 35 or higher, the study in The Lancet Diabetes & Endocrinology journal revealed.

Study author Prof Stuart Pocock, of the London School of Hygiene & Tropical Medicine, said: “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.

“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 Nawab Quizilbash, of OXON Epidemiology, said: “The reasons why a high BMI might be associated with a reduced risk of dementia aren’t clear, and further work is needed to understand why this might be the case.

“Many different issues related to diet, exercise, frailty, genetic factors, and weight change could play a part.”