Keeping pounds off may be easier than losing weight once it is gained
Being severely obese is as hazardous to health as a lifetime of smoking, shortening life by a decade, a group of Oxford University experts has warned.
Even moderate obesity cuts life expectancy by about three years, says the Clinical Trial Service Unit.
The findings, published in The Lancet, come from data on almost a million people from around the world.
In the UK, a quarter of adults are now considered obese, with a body mass index (BMI) above 30.
BMI is useful for assessing the extent to which fatty tissue causes ill health.
It is calculated by dividing a person's weight in kilograms by the square of their height in metres.
A person 1.70m (5 ft 7") tall would be considered moderately obese if they weighed 90kg (14 stone) rather than the ideal 70kg (11 stone).
Each incremental rise in BMI above the healthy zone of 20-25 increased premature death risk, the Clinical Trial Service Unit concluded.
Severe obesity - a BMI of 40 to 50, which applies to about 2% of the UK population - reduced life expectancy by about 10 years.
Moderate obesity - a BMI of 30-39, which applies to one in four UK adults - reduced life expectancy by three years.
Much of the obesity-related risk is down to heart disease and stroke, and to a lesser extent cancer.
Amongst middle-aged people in the UK, as many as one in four deaths from heart attack or stroke and one in 16 cancer deaths are due to being overweight or obese, the researchers estimate.
Fat at 40
In adult life, it may be easier to avoid substantial weight gain than to lose that weight once it has been gained, they say.
And avoiding middle age spread could add years to life.
BODY MASS INDEX
Calculated by dividing weight in kilograms by height in metres squared
Normal: 18.5 - 24.9
Overweight: 25 - 29.9
Obese: Above 30
Professor Peter Weissberg of the British Heart Foundation, which supported the work, said: "This is the latest and most convincing demonstration of the close relationship between being overweight and poor heart health, and confirms that smoking is harmful regardless of your weight.
"We all have a role to play in maintaining a healthy weight ourselves, but this study emphasises the importance of public health measures, such as the recently launched Change 4 Life campaign, as part of a raft of Government initiatives that will be needed to reduce the nation's weight."
Epidemiologist Dr Gary Whitlock of Oxford University, who led the analysis, said: ''Excess weight shortens human lifespan.
"In countries like Britain and America, weighing a third more than the optimum shortens lifespan by about three years.
"For most people, a third more than the optimum means carrying 20 to 30kg of excess weight. If you are becoming overweight or obese, avoiding further weight gain could well add years to your life.''
Sara Hiom of Cancer Research UK said: "Moderate obesity is becoming worryingly common in the UK and these factors combined are great cause for concern.
"We can eat less and move more to reduce weight. But smoking remains the single most significant cause of cancer death - and stopping smoking works."
Professor Alan Maryon-Davis, president of the UK Faculty of Public Health, said: "Being obese not only shortens life, it also leads to chronic ill-health - diabetes, high blood pressure, gallstones, back and joint troubles. My advice is don't let it creep up on you.
"Cut down the fat in your food and use every opportunity to be more physical."