Middle-aged people who are underweight (with a Bmi [BMI] under 20 kg/m2 ) are a third more likely to develop dementia than people of comparable age with a healthy BMI, according to new information published in?The Lancet Diabetes & Endocrinology journal.
The findings, that can come in the largest ever study to examine the statistical association between BMI and dementia risk, also show that middle-aged obese people (BMI more than 30 kg/m2) are nearly 30% not as likely to develop dementia than people of a healthy weight, contradicting findings from some previous research, which suggested that obesity results in an increased risk of dementia.
Researchers based at the London School of Hygiene & Tropical Medicine, and OXON Epidemiology, both in London, UK, analysed data from the Clinical Practice Research Datalink (CPRD), a large database of patient information recorded during routine general practice over nearly 20 years, representing around 9% of the UK population.
The researchers analysed the medical records of nearly 2 million (1,958,191) people with a typical (median) age of 55 years at the start of the study period, and an average (median) BMI of 26.5 kg/m2, just within the range usually classed as overweight. Throughout an average (median) of nine years follow-up, nearly fifty thousand (45,507) everyone was identified as having dementia.
People who have been underweight in mid-life 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 Fifteen years after the underweight was recorded.
As participants’ BMI at middle age increased, the chance of dementia reduced, with very obese people (BMI more than 40 kg/m2) 29% not as likely to obtain dementia than individuals the normal weight range. A rise in BMI was associated with a substantial steadily decreasing risk of dementia for BMI as high as 25 kg/m2 (classed like a healthy weight). Above a BMI of 25 kg/m2 (classed as obese or overweight), dementia risk decreased more gradually, which trend continued up to a BMI of 35 kg/m2 or higher.
The association between BMI and dementia risk wasn’t affected by the decade in which the participants were born, nor by their age at diagnosis. Adjusting for confounding factors known to increase the chance of dementia, for example alcohol consumption or smoking, made little difference to the results.
According to review author Professor Stuart Pocock in the London School of Hygiene & Tropical Medicine, “Our results suggest that doctors, public health scientists, and policy makers have to re-think how to best identify who is at high-risk of dementia. We also have to pay focus on the reasons and public health consequences of the link between underweight and increased dementia risk which our studies have established. However, our results also open an intriguing new avenue in the search for protective factors for dementia if we can realise why people with a higher BMI possess a reduced chance of dementia, it’s entirely possible that further down the line, researchers could possibly begin using these insights to develop new treating dementia.” 
“Why a higher BMI may be of a reduced chance of dementia aren’t clear, and further jobs are needed to realise why this can be the situation,” adds Dr Nawab Qizilbash from OXON Epidemiology working in london, UK and Madrid, Spain, the study’s lead author. “If increased weight in mid-life is protective against dementia, the reasons with this inverse association are unclear at present. Many different the process of diet, exercise, frailty, genetic factors, and weight change could play a part.” 
Writing in a linked Comment, Professor Deborah Gustafson from SUNY Downstate Medical Center in New York, USA, says, “The published literature about BMI and dementia is equivocal. Some studies report an optimistic association between high mid-life BMI and dementia, whereas others do not… Many considerations are essential in the assessment of the epidemiology of the association between BMI and late-onset dementia, out of the box the case for a lot of recorded associations involving late-life disorders. To understand the association between BMI and late-onset dementia should sober us as to the complexity of identifying risk and protective factors for dementia. The report by Qizilbash and colleagues is not the final word on this controversial topic.”