Underweight middle-aged people have higher risk of dementia

Middle-aged people who are underweight are more likely to develop dementia than people of similar age with a healthy body mass index (BMI), according to research.

The findings, which come from the largest ever study to examine the statistical association between BMI and dementia risk, also show that middle-aged obese people (BMI greater than 30 kg/m2) are nearly 30% less likely to develop dementia than people of a healthy weight. This contradicts findings from previous research, which suggested that obesity leads to an increased risk of dementia.

Researchers based at the London School of Hygiene & Tropical Medicine (LSHTM) and OXON Epidemiology analysed the medical records of nearly two million people with an average (median) age of 55 years and an average (median) BMI of 26.5 kg/m2 at the start of the study period. This is just within the range usually classed as overweight. During an average (median) of nine years follow-up, nearly 50,000 people were diagnosed with dementia.

The research, published in The Lancet Diabetes & Endocrinology journal, found that people who were underweight in middle age were a third (34%) 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 participants’ BMI at middle age increased, the risk of dementia reduced, with very obese people (BMI greater than 40 kg/m2) 29% less likely to get dementia than people in the normal weight range. 

LSHTM professor and study author Stuart Pocock said: ‘Our results suggest that doctors, public health scientists and policy makers 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.’