Dieters have long sworn that grapefruit helps them lose weight.
Now, scientists are beginning to believe them.
A study has found that drinking grapefruit juice when eating fatty food lowers the amount of weight put on by up to a fifth.
The research also suggested that grapefruit could be as good as prescription drugs at keeping blood sugar levels under control – a key part of managing diabetes.
The experiments were conducted on mice – but researchers say the results justify studies on humans.
Professor Joseph Napoli, of the University of California, Berkeley, said: ‘We see all sorts of scams about nutrition.
But these results, based on controlled experiments, warrant further study of the potential health-promoting properties of grapefruit juice.’
The Grapefruit Diet, also called the Hollywood Diet, dates back to the 1930s and has a host of celebrity fans including singer Kylie Minogue.
It involves having grapefruit or grapefruit juice with every meal while cutting back on calories.
The researchers found that when the mice were fed fatty food for three months, those given grapefruit juice to drink gained up to 18 per cent less weight than those given water.
They also had lower blood sugar and insulin levels – despite eating the same number of calories and doing the same amount of exercise as the mice who drank water.
In fact, grapefruit juice was as good at controlling insulin as the widely used diabetes drug metformin, the journal PLOS ONE reports.
However, the fruit juice only had an effect on weight when the animals ate fatty food.
The researchers said they did not know how grapefruit stops the pounds from piling on.
The British Dietetic Association said the fruit now needs to be thoroughly tested in humans to see if it could help with weight loss and stem the rise of obesity and diabetes.
Spokesman Mariette Abrahams, a dietician, said until then it is too early for people to try grapefruit diets.
‘Grapefruit should be part of a healthy balanced diet, but it shouldn’t be the focus of the diet,’ she said.