Your mother may have known something all those years ago when she went on a grapefruit diet to lose weight. People have eaten the fruit and drunk the juice for decades in the hope of dropping weight, and claims that grapefruit contained “something” that burned fat have sold many a diet book.

Now it seems there may be some basis for this enduring dietary craze. In fact, an active ingredient in grapefruit appears to lower glucose levels as effectively as a widely-prescribed diabetes drug.

Researchers at the University of California, Berkeley decided to test the link between grapefruit juice and weight loss using a controlled experiment. Previous studies on grapefruit juice were not rigorous, tended to be small and produced contradictory results.

A natural fruit drink lowered glucose levels as effectively as a prescription drug.

What they found surprised them. Mice that ate a high fat diet gained less weight when they drank grapefruit juice compared to mice that drank water. And the juice-drinking mice had better levels of glucose, insulin, and triglycerides, a type of fat, compared to those that drank water.

The results were so astonishing, “We even re-checked the calibration of our glucose sensors, and got the same results over and over again,” said Andreas Stahl, one of the researchers in a statement.

Different groups of mice were fed a variety of liquids. The control group was fed water with a small amount of glucose and saccharin added. The other groups drank different concentrations of slightly sweetened grapefruit juice and water. The calories were kept equal for all the mice. Along with the liquids, mice ate a diet that was either 60 percent fat or 10 percent fat for 100 days.

One group of mice was also given naringin, a bioactive compound found in grapefruit. Others were given metformin, a drug often prescribed for people with type 2 diabetes for lowering blood glucose.

The mice that ate a high fat diet and consumed grapefruit juice gained 18 percent less weight than the mouse that drank the sweetened water. Their blood glucose levels decreased by 13 to 17 percent, and they experienced a threefold drop in insulin levels.

Perhaps more surprising was that the grapefruit component, naringin, had a glucose-lowering effect that was just as effective as metformin. According to Joseph Napoli, professor and chair of nutritional sciences and toxicology at UC Berkeley, “That means a natural fruit drink lowered glucose levels as effectively as a prescription drug.”

The mice that were fed a high fat diet and given naringin did not experience weight loss, however, so the researchers suspect some other of the many active compounds in grapefruit juice must be beneficial in weight loss.

The mice that ate the low fat diet saw a lesser (two-fold) decrease in insulin levels and experienced no weight or other metabolic changes.

While the researchers were unable to explain why or how grapefruit juice affects weight gain, the findings are so encouraging that the team plans to continue their research.

The study, published in the peer-reviewed journal PLOS ONE, was funded by the California Grapefruit Growers Cooperative, but the group had no study input beyond providing funding and grapefruits.