The idea that grapefruit is a silver bullet for weight loss has been around for decades. The citrus fruit is one of fad dieting’s most enduring stars, prompting eye-rolling from nutritionists and doctors.

But grapefruit’s time to shine may have finally arrived.

New research from UC Berkeley suggests that grapefruit’s powers shouldn’t be dismissed so quickly.

A study published in the peer-reviewed journal PLOS ONE found that mice fed a high-fat diet gained 18 percent less weight when they drank clarified, pulp-free grapefruit juice compared with a control group of mice that drank water, according to a news release from the university.

Joseph Napoli and Andreas Stahl, two UC Berkeley faculty members who led the research, said they went into the study with some skepticism.

“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,” said Napoli.

“I was surprised by the findings,” said Stahl. “We even re-checked the calibration of our glucose sensors, and we got the same results over and over again.”

The study was funded by the California Grapefruit Growers Cooperative, but the researchers were emphatic in noting that the funders had no influence over the research findings. “We were very clear in telling them, you’re going to get the data we get,” Napoli said. “We can’t guarantee you’re going to like what you see. It might be nothing.”

But we're guessing the members liked what they saw. The study found that mice on a high-fat diet that were given grapefruit juice not only gained less weight than their water-drinking companions, but they also had a 13 to 17 percent decrease in blood glucose levels and a threefold decrease in insulin levels.

For the group of mice given metformin, a glucose-lowering drug used to treat Type 2 diabetes, the results were particularly surprising.

“The grapefruit juice lowered blood glucose to the same degree as metformin,” said Napoli. “That means a natural fruit drink lowered glucose levels as effectively as a prescription drug.”

What's grapefruit secret? That remains to be seen, but the bittersweet citrus fruit certainly seems to hold promise.

“There are many active compounds in grapefruit juice, and we don’t always understand how all those compounds work,” said Stahl. “Basically, we couldn’t see a smoking gun that could explain why or how grapefruit juice affects weight gain.”

Regardless, it appears that grapefruit is poised to begin its ascent from diet gimmick superstar to bona-fide superfood; and the researchers say they hope to continue the investigation.

“Obesity and insulin resistance are such huge problems in our society,” said Stahl. “These data provide impetus to carry out more studies.”

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