"There is no clear evidence that Atkins-style diets are better than any others for helping people stay slim, despite the popularity and apparent success of the Atkins diet," said Professor Arne Astrup of RVA University in Copenhagen, Denmark.

Low-carb diets have been around since the 1860s according to Astrup and his colleagues but the Atkins approach, which includes a restricted intake of carbohydrates and high fat foods, has made it popular with millions of people trying to lose weight.

"In the short-term (6 months) there is good evidence that the Atkins diet is producing weight loss. It is not just fluid loss, it is actually loss of body fat," Astrup said in an interview.

"Along with the weight loss there are also beneficial changes in all the risk factors ... for cardiovascular disease and type 2 diabetes," he added.

But in a review in The Lancet medical journal, Astrup and his team who studied three trials that looked at the longer-term effects found that after six months, people started to regain weight.

Those on the Atkins diet for a year also started to complain of headaches, muscle weakness, cramps and diarrhoea, which Astrup said could be explained by the reduced levels of fruits, vegetables and whole-grain cereals and bread that are consumed.

"These symptoms are clearly consistent with a kind of carbohydrate deficiency," he said. "People simply do not get the 150 grams of carbohydrates a day which is the minimal requirement for supplying your brain and muscles with the glucose from carbohydrates that are necessary for the organs' normal function."

He added that low-carb diets are good for short-term weight loss but after several months normal food recommendations, such as five or six portions of fruits and vegetables a day, low fat consumption and whole grain rice, bread and pasta products, should be followed and coupled with exercise.