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Being overweight or underweight could knock four years off life expectancy, a study in a Lancet journal suggests.

The report, one of the largest of its kind, involved nearly 2 million people who were registered with doctors in the UK.

Researchers found that, from the age of 40, people at the higher end of the healthy Body Mass Index (BMI) range had the lowest risk of dying from disease.

But people at the top and bottom ends of the BMI risked having shorter lives.

BMI is calculated by dividing an adult's weight by the square of their height.

A "healthy" BMI score ranges from 18.5 to 25.

Most doctors say it is the best method they have of working out whether someone is obese because it is accurate and simple to measure.

'Optimal level'

The study, published in the Lancet Diabetes and Endocrinology, showed that life expectancy for obese men and women was 4.2 and 3.5 years shorter respectively than people in the entire healthy BMI weight range.

The difference for underweight men and women was 4.3 (men) and 4.5 (women) years.

BMI was associated with all causes of death categories, except transport-related accidents, including cancer, cardiovascular diseases and respiratory diseases.

However, not everybody in the healthy category is at the lowest risk of disease, according to report author Dr Krishnan Bhaskaran.

He told the BBC: "For most causes of death we found that there was an 'optimal' BMI level, with risk of death increasing both below and above that level.

"At BMIs below 21, we observed more deaths from most causes, compared with the optimum BMI levels. However, this might partly reflect the fact that low body weight can be a marker of underlying ill-health.

"For most causes of death, the bigger the weight difference, the bigger the association we observed with mortality risk.

"So a weight difference of half a stone would make a relatively small (but real) difference; we could detect these small effects because this was a very large study."

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Your age group Your BMI is [comparative] the average of [bmi_score] for a [gender_singular] in your age group ([user_age_group]) in [user_country]. About [percent]% of [gender_plural] in your age group in [user_country] are overweight, obese or very obese. 0% Underweight 0% Healthy 0% Overweight 0% Obese 0% Very obese Your part of the UK In all parts of the UK, the majority of the adult population is overweight, obese or very obese, according to the latest national surveys. In [region], the figure is about [percentage]% of [gender_plural]. North East 50% West Mids 50% Scotland 50% East Mids 50% Yorks & Humb 50% N. Ireland 50% North West 50% South East 50% South West 50% East 50% Wales 50% London 50% 50% What does this mean for you? The information you've given us indicates you could be underweight. There can be health risks associated with a low BMI such as anaemia, osteoporosis, a weakened immune system and fertility problems. This is not a medical diagnostic tool so don't panic if this isn't the result you were expecting to see. If you're concerned about your weight, or your health in general, speak to a healthcare professional such as your GP. You're in the healthy range which is great. Research shows that having a healthy BMI can reduce your risk of serious health problems, such as type 2 diabetes, heart disease, stroke and some cancers. But not all people with a BMI in this range have a lower risk. Other factors such as smoking, high blood cholesterol or high blood pressure will increase your risk. If you're of Asian descent you have a higher risk of heart disease and diabetes at a lower BMI and waist circumference. A healthy BMI for you would be 18.5-23. We're more likely to gain weight as we get older so to stay a healthy weight you may need to make small changes to your diet or your activity levels as you age. The information you've given us indicates you are overweight. Research shows that a BMI above the healthy range can increase your risk of serious health problems, such as type 2 diabetes, heart disease, stroke, and some cancers. A healthy BMI for a person of your height would be 18.5-24.9. If you're of Asian descent you have a higher risk of heart disease and diabetes at a lower BMI and waist circumference. A healthy BMI for you would be 18.5-23. Losing even a small amount of weight, if sustained, can have a big impact. For most people changing your diet is by far the best way to lose weight. Activity can help you maintain your target weight, and can have other health benefits, but increasing activity alone is not nearly as effective as diet at helping you shed the pounds. Even small changes like reducing portion sizes or choosing lower calorie snacks and drinks can help you lose weight or stop putting it on. The information you've given us indicates you're in the obese category. Research shows that having a BMI in this range will significantly increase your risk of serious health problems, such as type 2 diabetes, heart disease, stroke, and some cancers. A healthy BMI for a person of your height would be 18.5-24.9. If you're of Asian descent you have a higher risk of heart disease and diabetes at a lower BMI and waist circumference. A healthy BMI for you would be 18.5-23. Losing even a small amount of weight, if sustained, can have a big impact. For most people changing your diet is by far the best way to lose weight. Activity can help you maintain your target weight, and can have other health benefits, but increasing activity alone is not nearly as effective as diet at helping you shed the pounds. There's lots of support available to help you make changes, either to lose weight or to stop putting on weight. The information you've given us indicates you're in the very obese category. Research shows that having a BMI in this range will significantly increase your risk of serious health problems, such as type 2 diabetes, heart disease, stroke, and some cancers. A healthy BMI for a person of your height would be 18.5-24.9. If you're of Asian descent you have a higher risk of heart disease and diabetes at a lower BMI and waist circumference. A healthy BMI for you would be 18.5-23. Losing even a small amount of weight, if sustained, can have a big impact. For most people changing your diet is by far the best way to lose weight. Activity can help you maintain your target weight, and can have other health benefits, but increasing activity alone is not nearly as effective as diet at helping you shed the pounds. If you are concerned, or would like to find out more, speak to your doctor or GP. If you are ready to make lifestyle changes, there is lots of support available. Your waist size BMI is not the only way of measuring whether you are a healthy weight. Doctors say that carrying too much fat around your belly can increase your risk of health problems. Excess fat in this area can stress internal organs - even if your BMI is in the healthy range. Your waist size is [size] For [gender_plural], the NHS says a waist size of: 80cm (31.5 inches) or more means an increased risk of health problems 88cm (34 inches) or more means a very high risk of health problems 94cm (37 inches) or more means an increased risk of health problems 102cm (40 inches) or more means a very high risk of health problems People from non-white ethnic groups may be at risk at a lower waist size How to check your waist with just a piece of string

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Some experts have questioned whether BMI is an accurate way of analysing a person's health.

However, Dr Katarina Kos, senior lecturer in Diabetes and Obesity at the University of Exeter, believes it is.

"For the majority of people, BMI is a good measure," she told the BBC.

Dr Kos added that the report did not contain any surprises but added that overweight people who could lower their BMI may reap the health benefits.

"We know from the diabetes remission data how low-calorie diets and weight loss can improve diabetes, for example," she said.

"And we know weight loss can also help in improving risk so that would also then improve mortality rates."

The report suggested that a higher BMI in older people may not be as dangerous, because a bit of extra weight was "protective" for them.

But Dr Kos, who worked on a report on this topic in 60 to 69-year-olds last year, disagreed with the findings.

Her report, on what is known as the obesity risk paradox, did not "support acceptance" of the theory.