If you've seen those weight-loss adverts on TV or in magazines promising dramatic weight loss in only days, you've probably wondered whether these diets really work.

False or misleading claims are common in weight-loss advertising, though a quick glance through magazines and television channels indicates that quick-fix weight-loss advertising is flourishing.

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In a four-month study of more than 300 weight-loss programme promotions in the USA, nearly 40% of the advertisements made at least one false claim, according to a report issued by the Federal Trade Commission at a press conference in Washington, DC.

More than half of the ads made claims that lacked substantiation of their promises, including testimonials by consumers that were false or impossible to verify, programmes excluding dieting and exercise, and before-and-after photographs where the head of an overweight person was superimposed on a slim body.

Impossible claims
At a time when obesity is reaching epidemic proportions in the Western world, false hopes are being raised in consumers desperate to lose weight.

But claims that seem too good to be true – such as losing 19kg in just three weeks or losing weight without dieting or exercise – usually are.

Despite what the ads say, the only way to lose fat is to consume fewer kilojoules per day than your body needs.

According to American Institute of Medicine dietary guidelines, an adult woman between the ages of 18 and 50 years, requires between 6 700 and 10 500 kJ per day, depending on her height and level of activity.

Men need between 8 000 and 13 800 kJ a day, depending on their height and level of activity.

Theoretically, to lose 500g per week, the body needs 2 000 kJ less per day. So to lose 1kg of fat, the body has to burn off 32 200 kJ.

To lose 19kg in three weeks, you would need to lose over 6kg a week, or almost 1kg per day. So the body would have to burn off 32 200 kJ per day. The only way to do that would be to eat nothing and run for six hours a day, or do high-impact aerobics for 13 hours a day, for 21 days.

Take claims with a pinch of salt
The report is not aimed at medically proven weight-loss programmes addressing lifestyle, behaviour, exercise and nutrition, but rather unsubstantiated claims of quick weight-loss programmes that say diet and exercise are not important.

"There's no such thing as a miracle pill for weight loss," American Surgeon General Richard Carmona told the press conference. "The surest and safest way to weight loss and healthier living is by combining healthy eating and exercising."

– (Charmaine Horne, updated June 2008)

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