It's incredibly frustrating and disheartening if you've embarked on an intensive (and probably expensive) diet but the only pounds you're losing are the ones in your wallet. Luckily researchers at Johns Hopkins University have now put weightloss plans under the microscope to find out which really deliver. According to the US study, only very few of the options out there will actually help you achieve long-term weightloss. Some are no more beneficial than people dieting independently, others are even harmful.

The study was kicked off by researchers reviewing 4,200 previous studies on diets, checking for evidence of effectiveness. Shockingly, only a few dozen deserved the scientific gold standard of reliability.

Among the goodies were Weight Watchers, Jenny Craig and NutriSystem. In the end, researchers looked at just 11 diet regimes in detail.

Atkins, SlimFast, and the internet-based Biggest Loser Club, eDiets and Lose It!, were reviewed as self-directed programmes. In the high-intensity group were Weight Watchers, Jenny Craig and NutriSystem. Finally, very-low-calorie meal replacement regimes HMR, Medifast and OPTIFAST were monitored. Prices ranged from free to £460 a month.

Only two plans showed gold-standard data proving dieters lost more weight on average in a year than they would have done independently: Weight Watchers and Jenny Craig.

But even where this was the case, weightloss was in the region of just three-five per cent more.

NutriSystem also showed promising results, but only data for up to three months on the diet was available. Low-calorie plans also showed more weightloss after six months but there are health worries associated with cutting down on food too heavily.

"We want people to experience the health benefits of weight loss - lower blood pressure, cholesterol and blood sugar, and lower risk of developing diseases like diabetes," said Jeanne Clark, professor of medicine.

"Those benefits are long-term goals - so losing weight for three months, then regaining it, has limited health benefits.

"That’s why it’s important to have studies that look at weight loss at 12 months and beyond."

It's hoped research like this will aid doctors when asked by overweight patients about the most effective diet plans.

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