But patients who were extremely obese had significantly reduced risk of dementia, the researchers found.
The medical data of almost 2m UK people was analysed by researchers at the London School of Hygiene & Tropical Medicine and OXON Epidemiology and published in the Lancet Diabetes & Endocrinology.
Patients with a BMI of 20 or below were found to have a 34% lower risk of dementia. The research also found that as the participant’s BMI increased, the risk of dementia lessened. Those with a BMI over 40 were almost 30% less likely to develop dementia than those in the normal weight band.
The study’s author, 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 rethink how best to 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.’
Obesity finding 'startling'
Dr Andrew Green, chairman of the GPC prescribing and clinical subcommittee, said he wasn’t surprised that extreme thinness leads to an increased risk of dementia. He added that although the reduced risk of dementia with obesity is ‘startling’, it is important to remember that ‘association does not prove causality’.
‘Certainly the other health benefits that arise from avoiding being significantly overweight are so important that this [research] should not change current advice,’ he said.
In the study, rates were adjusted for ‘confounding factors’ such as sex, smoking, alcohol, use of recent anti-hypertensive drugs and statins. All participants were 40 years old or above and cases were monitored for an average of nine years. Dementia occurred in 45,507 cases.
The researchers say they are unsure why there is a link between being overweight in middle age and a reduced risk of developing dementia.
Lead author, Dr Nawab Qizilbash from 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.
‘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.’