This simple piece of advice is true, but it's one that many of us struggle to follow.
It is easy to blame a lack of willpower or a penchant for sugary, fat-laden snacks. And often weight does pile back on because people revert to their old way of eating too much of the wrong foods.
But researchers say the reason so many of us relapse and fail on diets is because we have unrealistic expectations.
And this is not our fault but that of experts, because the advice they give us is flawed.
Long slogMost people start dieting with the notion that they will start to see results fast.
Experts tell us that if we cut around 500 calories from our daily diet, or burn them off exercising, then we can expect to lose 1lb (0.5kg) in weight every week.
Continue reading the main story“Start QuoteStudies show that somewhere between 50% and 80% of dieters will put weight back on”End QuoteDr Kevin Hall of the US National Institutes for Health The British Dietetics Association, the NHS and the American Dietetic Association all say losing weight at this rate is "about right" and that if you stick at it for 12 months, for example, you will shed about 52 lb (26kg).But US researchers from the National Institutes for Health say this is a gross overestimation because the calculation used is flawed.
They say it takes much longer to lose the weight - around three years to be precise, according to their work published in The Lancet.
For example, a year of dieting will result in only half of the amount of weight loss that experts currently predict.
Dr Kevin Hall and colleagues say this explains why many of us give up within months, because we expect unrealistic results that cannot be achieved.
Studies of outpatient weight loss programmes show most dieters peak at six months with the pounds starting to creep back on after this.