Middle-aged people who are underweight face a significantly increased risk of developing dementia, researchers warned today.
The findings are surprising as other studies have linked obesity to the development of dementia.
Researchers said underweight people are a third more likely to develop dementia than people of similar age with a healthy body mass index (BMI).
The largest ever study examine the statistical association between body mass index and dementia risk also shows that middle-aged obese people are nearly 30% less likely to develop dementia than people of a healthy weight.
The findings, from research carried out by London School of Hygiene & Tropical Medicine, and OXON Epidemiology, both in London, UK, contradicts conclusions reached in previous research, which suggested that obesity leads to an increased risk of dementia.
The research team say their results mean that health professionals should re-evaluate who is most at risk from dementia.
Data from nearly two million people whose medical records are included in the Clinical Practice Research Datalink (CPRD), a large database of patient information recorded during routine general practice over nearly 20 years, was analysed for the study.
Participants had an average age of 55 years at the start of the study period, and an average body mass index of 26.5 kg/m2, just within the range usually classed as overweight.
During an average of nine years follow-up, 45,507 people were diagnosed with dementia.
Those who were underweight in middle age were 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 their body mass index increased, the risk of dementia reduced, with very obese people 29% less likely to get dementia than people in the normal weight range, says the report, which is published in The Lancet Diabetes & Endocrinology.
Professor Stuart Pocock from 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.”
Lead author Dr Nawab Qizilbash from OXON Epidemiology in London, said further work was needed to establish why high body mass index is linked to a reduced risk of dementia aren’t clear.
“If increased weight in mid-life is protective against dementia, the reasons for this inverse association are unclear at present,” he said. “Many different issues related to diet, exercise, frailty, genetic factors, and weight change could play a part.”
Qizilbash N, Gregson J et al. BMI and risk of dementia in two million people over two decades: a retrospective cohort study. Lancet Diabetes Endocrinol. 10 April 2015; doi: 10.1016/S2213-8587(15)00033-9 [abstract]
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