Being underweight in mid-life could increase your risk of dementia , claim researchers . A new study shows you are a third more likely to be diagnosed with diseases such as Alzheimer 's than those of a healthy weight . Obese people appear to be protected against dementia , with a risk around 30 per cent lower than those of healthy weight . The findings contradict many previous studies which have suggested that being fat – with a high Body Mass Index ( BMI ) - could raise the risk . A new study shows underweight people are a third more likely to be diagnosed with diseases such as Alzheimer 's than those of a healthy weight ( file photo ) Study author , Professor Stuart Pocock , from the London School of Hygiene and Tropical Medicine , said doctors and policymakers may have to re-think how to best identify those at high risk of dementia . He said '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 . ' Other experts said more research was needed , and the way GPs used to diagnose dementia may have skewed the results . Researchers based at the London School of Hygiene and Tropical Medicine and Oxon Epidemiology , also in London , analysed patient information recorded during routine general practice over nearly two decades . Data on 1,958,191 patients aged 55 on average with a BMI of 26.5 on average - just within the range usually classed as overweight - were examined . BMI is assessed by comparing height to weight , in a calculation which divides weight in kilos by height in metres squared . A healthy score is between 18.5 and 24.99 – above is overweight and obese . The study regarded BMI of less than 20 as underweight . After nine years follow-up , 45,507 people were diagnosed with dementia . Researchers found people who were underweight in middle age were 34 per cent more likely to be diagnosed with dementia than those of a healthy weight . The findings contradict many previous studies which have suggested that being fat – with a high Body Mass Index ( BMI ) - could raise the risk ( file photo ) As people 's BMI increased , the risk of dementia fell , with very obese people having a BMI greater than 40 less likely to develop the condition than those of normal weight . The link was not affected by the decade in which patients were born , age at diagnosis , alcohol use or smoking . The new research was published in The Lancet Diabetes And Endocrinology journal ( must credit ) . Dr Nawab Qizilbash , from Oxon Epidemiology in London , said the reasons why high BMI might lead to a lower risk of dementia are not clear . 'Many different issues related to diet , exercise , frailty , genetic factors and weight change could play a part ' he said . Prof Tom Dening , Professor of Dementia Research , University of Nottingham , said limitations of the study included poor dementia diagnosis rates by GPs at the time . He said 'It is plausible that faced with a grossly obese patient , the average GP may have concentrated on the obvious medical risks and paid less attention to the cognitive issues than they might have done with a comparable patient of normal weight . 'Or maybe the GP did n't refer them to a memory clinic but sent them to see a gastroenterologist or diabetologist instead . 'In other words , the study may be telling us something about the diagnostic practices of GPs , as much about the natural history of metabolic disease and dementia . ' Dr Doug Brown , Director of Research and Development at Alzheimer 's Society , said 'Previous research has suggested that being overweight in midlife increases risk of developing the condition and yet this study suggests that it may actually be protective . 'This mixed picture highlights the difficulty of conducting studies into the complex lifestyle risk factors for dementia and reinforces the need for further research . Dr Simon Ridley , Head of Research at Alzheimer 's Research UK , said 'There is currently no sure-fire way to prevent dementia . 'In the meantime , evidence suggests that we can reduce the risk by eating a healthy , balanced diet , exercising regularly , not smoking , and keeping blood pressure in check .
Underweight people are also more likely to be diagnosed with Alzheimers . Obese person is 30 per cent less likely to get dementia than healthy person . This could help scientists develop new treatments , researchers have said .