Diets based on grapefruit or grapefruit juice, dismissed by some as a fad diet, might just warrant another look, say researchers who saw mice slimming down on a regimen of the citrus.

After two groups of mice were put on a high-fat diet, one group drinking no-pulp grapefruit juice put on 18 percent less weight compared with a control group of mice drinking just water, a study led by University of California, Berkeley, researchers found.

The mice on grapefruit juice also had better insulin, glucose and triacylglycerol levels than the ones drinking water, the researchers reported in the journal PLOS One.

Previous studies had suggested benefits to be had from a grapefruit diet but such studies were sometimes small, with poor scientific controls, and were often contradictory to each other, say Berkeley researchers Joseph Napoli and Andreas Stahl.

Although the California Grapefruit Growers Cooperative funded the study, Stahl and Napoli emphasized the group had no influence or control over the design of the study or its findings.

Stahl and Napoli both admitted they embarked on the research with considerable skepticism.

"I was surprised by the findings," says Stahl, a professor of nutritional sciences and toxicology. "We even re-checked the calibration of our glucose sensors, and we got the same results over and over again."

The grapefruit juice proved able to lower blood glucose to the same degree as metformin, a drug that is often  prescribed to people suffering from type 2 diabetes, the researchers said.

 "That means a natural fruit drink lowered glucose levels as effectively as a prescription drug," Napoli says.

The weight loss effect of the grapefruit was most evidence in mice on a high-fat diet, the researchers found; mice on a low-fat diet given grapefruit juice did see a two-fold decrease in levels of insulin but displayed no significant weight loss.

The exact ingredient or ingredients in grapefruit juice responsible for weight loss is not clear, the researchers said, and it deserves more research.

"There are many active compounds in grapefruit juice, and we don't always understand how all those compounds work," says Stahl.

Still, the researchers say, with obesity and insulin resistance such pervasive health problems in society, the findings suggest potential rewards from more study.

"We see all sorts of scams about nutrition," Napoli says. "But these results, based on controlled experiments, warrant further study of the potential health-promoting properties of grapefruit juice."