Doctors have confirmed that a special high-fat diet can help children seriously afflicted with epilepsy.
Among children given the diet, seizures dropped by more than a third over a three-month period.
At the same time, children not given the diet experienced a more than 33 per cent rise in seizures.
The ketogenic diet has been successfully used to treat children with drug-resistant epilepsy since the 1920s. However, this was the first time it had been tested in a randomised trial.
The diet consists of large amounts of fat, little carbohydrate and controlled amounts of protein.
It is thought to mimic the biochemical response to starvation, when ketone bodies fuel the brain rather than sugar.
Ketone bodies, used as an energy source for the heart and brain, are compounds produced when fatty acids are broken down in the liver and kidneys. They provide a vital energy lifeline when fasting.
The trial, based at the Institute of Child Health at Great Ormond Street Hospital in London, involved 145 children aged two to 16, who suffered epilepsy fits at least one a day or more than seven times a week.
Around half the children were assigned a ketogenic diet immediately. The other half were given the diet after a delay of three months.
In the diet group, the number of seizures dropped by more than a third. Children who had to wait before having a ketogenic diet saw the number of seizures they suffered rise by about the same amount.
A total of 28 children in the diet group saw a more than 50% seizure reduction. Five children on the ketogenic diet had their seizures cut by more than 90% compared with none in the comparison group.
The findings are reported in the journal The Lancet Neurology.
Study authors Professor Helen Cross from the Institute of Child Health, and research dietician Dr Liz Neal wrote: "The ketogenic diet has not had the same kudos as medication, because it has been claimed there is no evidence base, and it is thought to be difficult to adhere to. In fact, this trial shows it is as effective as any new anti-convulsant drug. If this was a new drug we had tried, regulators would be making it freely available.
"In view of this we believe that the diet should be more widely available as a treatment on the NHS, for children with epilepsy who have failed to respond to anti-convulsant medication. We stress this is a diet which should only be undertaken on medical advice and under medical and dietetic supervision." The most frequent side effects associated with the diet were constipation, vomiting, lack of energy and hunger.
In an accompanying commentary, Dr Max Wiznitzer, from the Rainbow Children's Hospital in Cleveland, Ohio, US, said more information was needed about the long-term effects of the ketogenic diet.
He added: "Better identification of epilepsies that benefit from starting early on the ketogenic diet and comparisons between the choices of ketogenic diet are needed."