Middle-aged people who are underweight (have a low body mass index, or BMI) are one-third more likely to develop dementia than healthy weight people of a similar age, according to a new study published in The Lancet Diabetes & Endocrinology journal.
Furthermore, middle-aged obese people are nearly 30 percent less likely to develop dementia than people of a healthy weight. The current findings contradict those from previous research, which suggested that obesity leads to an increased risk of dementia.
The study is the largest ever to explore the statistical link between BMI and dementia risk. Researchers based at the London School of Hygiene & Tropical Medicine and OXON Epidemiology analyzed data from the Clinical Practice Research Datalink (CPRD), a large database of patient information taken during routine general practice for nearly 20 years, representing around nine percent of the UK population.
The researchers analyzed the medical records of nearly two million people with an average (median) age of 55 years at the beginning of the study, and an average (median) BMI of 26.5, just within the range usually classified as overweight. During an average (median) of nine years follow-up, nearly 50,000 people were diagnosed with dementia.
Middle-aged people who were underweight were 34 percent more likely to develop dementia compared to those of a healthy weight, and this increased risk of dementia remained another 15 years after the underweight was recorded.
As a person’s BMI at middle age increased, the risk of dementia dropped, with very obese people (BMI greater than 40) 29 percent less likely to get dementia than people of normal weight.
Below 25, an increase in BMI was tied to a substantial decreasing risk of dementia. Above a BMI of 25 (classified as overweight or obese), dementia risk decreased more gradually, and this trend continued up to a BMI of 35 or higher.
Neither the age of the participant nor age at diagnosis appeared to affect the link between BMI and dementia risk. Even adjusting for other factors known to increase the risk of dementia, such as alcohol use or smoking, made little difference to the results.
“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,” said study author Professor Stuart Pocock from the London School of Hygiene & Tropical Medicine.
“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.”
Source: The Lancet