The study's lead researcher said the results are a 'surprise', but  it's no excuse to pile on the pounds . For years we have been told that the first step to staying healthy is to keep to a healthy weight - that means making sure your Body Mass Index (BMI) is within the normal range of 18 to 24. Few people are advised to gain weight, unless their BMI is below 18, but could that change? A study published last week suggests that overweight people are less likely to develop dementia. The researchers, drawn mostly from the London School of Hygiene and Tropical Medicine, analysed the medical records of nearly two million people for up to ten years and found that those who were overweight (with a BMI of 25 to 29) had an 18 per cent lower risk of dementia compared to those with a healthy BMI. Remarkably the obese (with a BMI of 30 plus) did even better; they were 24 per cent less likely to develop dementia. Being underweight increased the dementia risk by 39 per cent. Dr Nawab Qizilbash, a clinical epidemiologist and the lead researcher, called the results a 'surprise', but said it was no excuse to pile on the pounds: 'This is a robust and reliable study that opens up an avenue in the search for factors that protect against Alzheimer's.' But these results are not that extraordinary. Three years ago, an even larger study, published in the Journal of the American Medical Association, reported that overweight people tended to live longer than those of normal weight. The study analysed 100 trials involving three million people. It found that being overweight (a BMI between 25 and 29) reduced the risk of premature death by six per cent compared with someone of a normal BMI. More surprising was that even if someone was at the lower end of obese (BMI of 30 to 34.5) their risk of premature death didn't go up. Being obese started to reduce lifespan only when your BMI went above 35 - here the risk of dying earlier climbed to 29 per cent greater than those with a normal BMI, according to the study, headed by Dr Katherine Flegal, of the U.S. government Centres for Disease Control. So could being cuddly fat, rather than hugely overweight, protect you against dying early? Being overweight (a BMI between 25 and 29) reduced the risk of premature death by six per cent . Professor David Haslam, a GP and chairman of the National Obesity Forum, says something called the obesity paradox is at work: 'Obesity increases your chances of developing disorders such as heart disease and type 2 diabetes but once you've fallen ill, being a bit overweight seems to have a protective effect. 'The obesity paradox had been called 'surreal and unsettling', but it does seem to be real. We need to understand it.' Malcolm Kendrick, a GP and author with an interest in obesity, claimed in his recent book Doctoring Data that being overweight was beneficial. He is suspicious of the word 'paradox' when used with a medical hypothesis: 'It is usually a way of describing something that contradicts your theory. For instance, people who believe high cholesterol causes heart disease use the phrase 'cholesterol paradox' to explain why countries with high levels of cholesterol have low levels of heart disease. They should admit their theory is wrong.' He says that what counts as a normal or 'healthy' BMI has been miscalculated: 'It would make sense if normal was what is now called overweight.' Millions would then not be pushed into losing weight when they did not need to or when it might be harmful. The number of Britons who go on a diet in any given year . These 'paradoxical' results suggest that BMI is too crude a tool on its own to make decisions about what a healthy weight is. It can misclassify athletes packed with muscle as overweight or even obese. Equally, current BMI categories give the wrong picture for what might be called the 'fit fat'; people who may have a lot of fat but also eat well and exercise regularly. Their BMI may classify them as obese, but other markers (blood sugar, fat levels in their blood and blood pressure) can all be in the super healthy range. Apparently healthy-weight people with little muscle mass because they rarely exercise also make BMI an unreliable indicator of health because rather than storing their fat under the skin, they may carry it in their abdomen, wrapped around their organs and pumping out chemical messengers and hormones linked to diabetes, heart disease and cancer. A tape measure is said to be one way to spot if you have this dangerous fat, but leading obesity expert Professor Arya Sharma, chair in obesity research and management at the University of Alberta, disagrees: 'A tape measure still won't tell you whether your stomach fat is unhealthy. Symptoms such as hypertension, diabetes or sleep apnoea are far more informative.' Current BMI categories give the wrong picture for what might be called the 'fit fat' All this suggests it is not a good idea to advise someone to lose weight just on their BMI. Professor Haslam says: 'I'd never do that; I'd need to know about their diet, how much they exercise, what their blood pressure and blood glucose levels were.' He welcomes the latest Alzheimer's and obesity research because it should make people more aware of the complexities of the fat debate: 'We've got too stuck in the idea that when it comes to weight, one size fits all.' Why fatter people should be better protected from Alzheimer's is guesswork, but evidence suggests having more fat reserves when you fall ill is associated with better recovery. 'Heavier people have better survival rates,' says Dr Flegal. 'If you get pneumonia and lose 15 lb, it helps to have 15 lb spare'. Fat is a vital source of the energy needed for recovery and also helps the immune system. Professor Haslam says: 'Those who are elderly or suffering from long-term conditions should be advised to become fitter, but not to lose weight.' One of Dr Flegal's fiercest critics is Professor Walter Willett, chair of nutrition at Harvard School of Public Health. He, too, is concerned that BMI does not provide the whole picture of someone's health but has described Dr Flegal's research as 'rubbish'. He is particularly worried her claims could 'undermine public policies to curb rising obesity rates'. He says they could be 'hijacked by special interest groups such as soft drink manufacturers and food lobbies'. There is no doubt that being obese, and to a lesser extent overweight, is linked to a raised risk of cancers, diabetes, and other chronic disorders. While you might not die prematurely, at the very least you are likely to have a poor quality of life; diabetes complications can include blindness, ulcers and amputations. Nonetheless Professor Haslam believes the new findings cannot be ignored: 'We need to use them to improve the advice and support we give to patients. It's not right to try to discredit them for political reasons.'
A study suggests overweight people are less likely to develop dementia .
Those who had a BMI of 25 to 29 had an 18 per cent lower risk .
Having more fat when you fall ill is associated with better recovery .
Remarkably the obese (with a BMI of 30 plus) did even better .