The Big Question: Does dieting really work, and will it endanger your health?

Why ask this now?

Fighting the flab may not only be pointless - it could be dangerous, too, according to one of the largest studies of the effects of dieting, published yesterday.

The review of 30 research papers involving thousands of slimmers found that though many of them succeed in shedding pounds while dieting, they pile the weight back on as soon as they stop, with most ending up heavier than they did to start with.

The University of California scientists who conducted the review, published in the journal American Psychologist, say this kind of yo-yo dieting may also be damaging to health. Research has shown repeated rapid weight gain and loss may increase the risk of heart disease, heart attacks and premature death.

Is dieting a waste of time, then?

Yes - unless you have serious amounts of will-power. One-in-four people in Britain are said to be dieting to lose weight at any one time, making resolutions they will certainly break within days, weeks or months. They expend large amounts of energy and huge amounts of money buying special foods, joining diet clubs and counting calories.

They will suffer from the constant hunger and the loss of one of life's great pleasures - eating - and when they fail, as most ultimately will, they will feel guilt and self-hatred. The Californian scientists found only a small minority successfully lost weight and kept it off over the five year duration of the study.

The moral is do not waste time deciding which diet to follow. Lo carb or no carb, high or low fat, protein rich or poor - it makes no difference. It is not the diet that determines how much weight you lose but the rigour with which you follow it.

What is the secret of losing excess weight?

Simple - eat less and exercise more. There is no other way of losing weight. All diets - and there are zillions of them - are designed to help dieters eat less. Some diets claim to alter metabolism so you burn calories faster or absorb them more slowly. But if there is any effect on metabolism - and it is disputed - the effect is minimal. The reason for the obesity explosion in Britain, where an estimated 27 million adults are obese or overweight, and across the western world is that an energy imbalance has built up as we consume more and do less. Over the past 30 years consumption of calorie-rich fast foods and sugary snacks has risen while energy expenditure has declined with the increase in sedentary work and car use. The result is seen in ever-expanding waistlines that can only be curbed by cutting calorie intake to match reduced energy spent.

How do diets work?

The unacknowledged principle behind most diets is to make eating difficult. As there is only one way to lose weight - by eating less - inventors of diets have had to come up with ways of getting people to do this without noticing. Some diets require elaborate preparation, or are extraordinarily unappetising or bound by rigid rules. The aim is the same - to reduce the amount of calories consumed.

Can following a diet be dangerous?

It may be - especially for the young. Teenage girls, driven to covet size zero fashions, are at greatest risk of taking dieting to extremes. One-in-100 girls aged 15 to 25 is said to suffer form the eating disorders anorexia and bulimia, which may have lifelong effects and in rare cases lead to death. Extreme dieting is dangerous for adults, too, depriving them of essential nutrients and leading to excessive consumption of certain foods in unhealthy amounts.

More than 45 million copies of the Atkins diet books have been sold since the 1970s making it the most successful diet of all time. The Atkins diet recommends unlimited consumption of butter, fatty meat and high fat dairy products while carbohydrate intake is restricted to 30 grams a day, equivalent to a small potato. Although research suggests it is safe and effective at promoting weight loss in the short term - calorie for calorie, protein appears to be more satisfying and thus better at curbing appetite, than fat or carbohydrate - this is a diet practically guaranteed to induce heart disease in the long term.

Research shows that people who have stuck with the diet for more than a year tend to suffer headaches, muscle cramps and diarrhoea caused by carbohydrate deficiency. The minimum daily requirement of carbohydrate for an adult is 150 grams a day but on the Atkins diet it is cut to one-fifth of that.

Is it worth trying to lose weight?

Possibly not. Despite the huge social and medical pressure on the overweight to shed their excess pounds there is scant evidence that it will improve their health. What we know is that the overweight are at greater risk of ill health - from heart disease, diabetes, some types of cancer and joint problems. Obesity is estimated to cause at least 30,000 excess deaths a year in the UK. But it does not follow that losing weight will cut the risk.

We simply do not know whether a person who loses 20 pounds will thereby acquire the reduced risk of a person who started out 20 pounds lighter. Yo-yo dieting increases the risks, as noted above.

The best way to protect health is to prevent children becoming obese in the first place, by eating healthily and exercising more.

It is far better to stop the weight going on than to find ways to get it off later.

Can dieting be good for health?

Yes. Fruit and vegetables to replace cakes and dairy foods, recommended in many diets to promote weight loss, also reduce cholesterol and the risk of heart disease and cancer and have been shown to cut the risk of Alzheimer's disease. Fibre in wholemeal bread and cereals helps fill the belly and curb appetite while also being good for the digestion and, according to some research, protecting against bowel and breast cancer. This is the Mediterranean diet, long known to be protective against a range of chronic diseases.

More controversially, recent research has shown that restricting food intake to half the normal calories in a range of species from worms to mice can extend their lifespan. Eating less might help you live to be 100 - but life would be a misery.

If you must lose weight, what is the best way to do it?

The ideal diet is one that contains 600 calories a day less than the individual burns in energy, according to the National Institute for Health and Clinical Excellence (Nice). This is equivalent to cutting out one Mars bar and one Danish pastry a day.

In its first official guidance on weight loss issued in December, Nice warned that crash dieting could be dangerous and recommended the 600 daily calorie reduction as the most likely to result in "sustainable weight loss". Wise advice - but unlikely to become a bestseller.

So should you go on a diet to shed those excess pounds?

Yes...

* People who are overweight or obese have an increased risk of heart disease, cancer and diabetes

* There are social pressures, which can even show up in reduced income, to lose weight

* If people feel unhappy with their body shape, they may be more confident and content if they slim down

No...

* Diets don't work in the long term as dieters tend to recover all the weight they lost and more

* Dieting can put health at risk by encouraging excess consumption of certain foods

* There is no evidence that losing weight reduces the health risks of being overweight