A new study from UC Berkeley suggests that drinking grapefruit juice may stem weight gain — at least for those who eat a high-fat diet. 

According to the research, mice fed a high-fat diet gained 18 percent less weight when they drank clarified, no-pulp grapefruit juice compared with a control group of mice that drank water. The grapefruit juice-quaffing mice also showed improved levels of glucose, insulin and a type of fat called triacylglycerol compared with their water-drinking countermice.

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The study was published last Wednesday, Oct. 8, in the  journal PLOS ONE.

The link between grapefruit juice and weight loss has been suggested before, in Hollywood diets and elsewhere. However, earlier studies behind those claims were often small, not well-controlled and contradictory, according to Andreas Stahl and Joseph Napoli, the two UC Berkeley faculty members who led the new research.

Caveat: this study was funded by the California Grapefruit Growers Cooperative, but the UC Berkeley researchers emphasized that the funders had no control or influence over the study design or research findings. And both Stahl and Napoli said they went into the research with some skepticism.

“I was surprised by the findings,” said Stahl, associate 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 study authors randomly divided mice into six groups, including a control group that drank only water. Those drinking grapefruit juice got a mixture diluted with water at different concentrations, and sweetened slightly with saccharin to counteract grapefruit's bitterness. The researchers also added glucose and artificial sweeteners to the control group's water so that it would match the calorie and saccharin content of the grapefruit juice.

At the end of the study period, the mice that ate the high-fat diet and drank diluted grapefruit juice not only gained less weight than their control counterparts, they also had a 13 to 17 percent decrease in blood glucose levels and a threefold decrease in insulin levels, which reveals greater sensitivity to insulin. 

The researchers gave one group of mice naringin, a bioactive compound in grapefruit juice that has been identified as a key agent in weight loss, and another group metformin, a glucose-lowering drug often prescribed for those with Type 2 diabetes.

The mice were fed a diet that was either 60 percent fat or 10 percent fat for 100 days, and their metabolic health was monitored throughout the study.

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

The group of high-fat-diet mice that received naringin, the bioactive grapefruit component, had lower blood glucose levels than the control group, but there was no effect on weight, suggesting that some other ingredient in grapefruit juice is also beneficial.

The impacts were not as great on mice that ate a low-fat diet. Those that drank the grapefruit juice saw a two-fold decrease in insulin levels, but there was no significant change in weight or other metabolic variables.

The researchers said they ruled out the typical explanations for weight loss in their study. It wasn't the amount of food consumed, since the ingested calories among the different groups were about the same. The level of activity and body temperatures were comparable, and the authors even checked the calories eliminated in the feces of the mice to check for problems with the body's absorption of nutrients.

“Basically, we couldn't see a smoking gun that could explain why or how grapefruit juice affects weight gain,” Stahl said.

The researchers said they hope to continue the investigation into grapefruit juice. “Obesity and insulin resistance are such huge problems in our society, ”  Stahl said. “These data provide impetus to carry out more studies.”

One big question is whether grapefruit juice sweetened with sugar, such as typically found in supermarkets, would have these same beneficial effects. Drinking unsweetened grapefruit juice wouldn't be an option for most people, no matter how desperate they are to lose weight. 

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