Roughly equating to around 500 fewer calories a day, it is seen as a tough but manageable rule.
But now, a leading expert claims this rule is incorrect and people need to eat even less if they want a slimmer waistline.
Dr Kevin Hall is a mathematician at the National Institutes of Health, whose lab runs experiments on the effects of different diets on the body's metabolism.
He specialises in creating mathematical models for weight loss.
He told Runner's World the 3,500-calorie rule is accurate only if a pound of human fat is burned in a lab.
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