Could grapefruit juice be the new weapon against Type 2 diabetes?

Grapefruit diets have been around since the 1930s and are often dismissed as fads. But new research from the University of California, Berkeley, suggests that a closer look at grapefruit juice is warranted.

In a study published in the peer-reviewed journal PLOS ONE, researchers found that mice fed a high-fat diet gained 18% less weight when they drank clarified, pulp-free grapefruit juice compared with a control group of mice that drank water. Juice-drinking mice also showed improved levels of glucose, insulin and a type of fat called triacylglycerol compared with their water-drinking counterparts.

The study authors randomly divided mice into six groups, including a control group that drank only water. 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 13-17% decrease in blood glucose levels and a threefold decrease in insulin levels.

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% or 10% 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 Joseph 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 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.

“There are many active compounds in grapefruit juice, and we don’t always understand how all those compounds work,” said Stahl.

The study did not find as big an impact 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 effects were more subtle for the low-fat diet group,” explained Stahl. “Mice are incredibly healthy animals with naturally low levels of bad cholesterol. So if they are eating a healthy, low-fat diet, it will take more to see a significant effect on their health.”