Image caption Weighing and measuring yourself regularly helps track your weight-loss progress

Eat less and you will lose weight.

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 slog

Most 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.

Studies show that somewhere between 50% and 80% of dieters will put weight back on Dr 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.

Counting calories The recommended daily calorie intake for men is 2,500 and for women is 2,000

The lowest intake per day recommended for men is 1,500 and for women is 1,200 calories, unless they are in a medically supervised setting

Some, incorrectly, attribute this to the body getting used to having less food and metabolism slowing down. The dieter then feels that the regime is no longer working and often gives up altogether.

Alternatively, as the slimmer begins to see the weight falling off in the early months they are so pleased with their achievement that they begin to relax and the diet slips. But because weight loss is slow there is a lag phase where weight continues to drop even though the person is now eating more. The dieter then mistakenly concludes that they don't need to be so rigid with their diet in order to lose weight.

But eventually, the weight will catch up with them and they may well find they are now heavier than they were when they first started the diet.

Dr Hall explains: "The slow timescale for weight change is responsible for the gradual weight regain over many years despite the fact that the original lifestyle was resumed within the first year.

"Studies show that somewhere between 50% and 80% of dieters will put weight back on."

He says professionals need to change the advice that they give to dieters so they don't fall into this trap.

We all recommend it - it's what we are taught. But I don't know what the scientific evidence for it is Helen Bond from the British Dietetic Association

"If you can give a realistic picture, that can inform people and help them make choices."

Dr Hall says the error occurs because the "500 calorie-cut a day" sum fails to take account of how metabolism changes as we diet.

The mathematic equation relies on the assumption that one pound of fat contains 3,500 calories, so to lose one pound a week a person should consume approximately 3,500 fewer calories a week, or 500 fewer calories a day.

But in fact, weight loss is not this steady.

Natural fluctuations

Using knowledge about how the human body responds to changes of diet and physical activity, Dr Hall's team have created a computer program that they say gives a more realistic and reliable prediction of weight loss.

Their calculations reflect the fact that one person may lose weight faster or slower than another, even when they eat the same diet and do the same exercise.

For example, heavier people can expect greater weight change with the same change in diet, but it will take them longer to reach a stable body weight than people carrying less fat.

Plus the body adapts rapidly to a reduced calorie diet, regardless of the type of food eliminated to get this reduction.

This means that all diets with similarly reduced energy content will have the same effect in the short term, whether the food cut out is fat or sugary carbohydrates.

Image caption Habits can be hard to break

Dr Hall said: "We tested it on about 100 people and it gave a good fit. It was pretty accurate, whereas the old rule does not fall anywhere near.

"This means we can use it to make realistic predictions.

"The rough rule of thumb to go by is 10 calories per day per pound. And it takes a year to lose half of the excess weight and three years to get to 95%."

It's not clear why the advice was adopted in the first place.

Helen Bond, from the British Dietetic Association, admitted: "We all recommend it - it's what we are taught. But I don't know what the scientific evidence for it is.

"It stems from how much energy it takes to burn fat. A lot of diets are not proven by science."

She said some dieters might find it depressing to be told that it takes far longer to get weight down than previously thought.

"It's not very motivating to tell someone that if they cut their intake by 10 calories a day every day for the next three years they will lose a pound of weight.

"But saying 'cut out your daily habit of a 250-calorie chocolate bar and you will lose about 25 pounds and, if you stick at it, the weight will stay off' - that is."

However, Dr Hall says the computer model also shows how people can achieve more rapid weight loss if that is what they desire.

For example, someone could follow a very strict diet for the first year to get rid of a large bulk of their excess weight and then switch to a less restrictive diet to continue and maintain the weight loss. Adding in extra exercise will also have an impact.

At the end of the day, it still boils down to willpower. There is no quick fix to dieting and if you want it to work you need to stick at it, says Dr Hall.

A healthy diet is for life, not just post-Christmas.