The findings, published in PLOS ONE, investigated whether there is a scientific basis to the inclusion of grapefruit in many ‘fad diets’.
Led by Andreas Stahl and Joseph Napoli from the University of California – Berkeley, and funded by the California Grapefruit Growers Cooperative, the animal data investigated whether consumption of grapefruit juice had a beneficial effect on weight gain seen during high-fat feeding.
The team reported that mice fed a high-fat diet gained 18% less weight when they drank clarified, no-pulp grapefruit juice compared with a control group of mice that drank water. The juice-drinking mice also showed improved levels of glucose, insulin and a type of fat called triacylglycerol compared with their water-drinking counterparts, said the team.
The Berkeley researchers emphasised that the funders had no control or influence over the study design or research findings. Indeed, both Stahl and Napoli said they went into this research with some scepticism.
"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."
"We see all sorts of scams about nutrition,” Napoli commented. “But these results, based on controlled experiments, warrant further study of the potential health-promoting properties of grapefruit juice."
The researchers randomly divided mice into six groups, including a control group that drank only water. The mice were fed a diet that was either 60% fat or 10% fat for 100 days, and their metabolic health was monitored throughout the study.
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, the team noted.
At the end of the study period, the mice that ate a high-fat diet and drank diluted grapefruit juice not only gained less weight than their control counterparts, they also had a 13 to 17% decrease in blood glucose levels and a threefold decrease in insulin levels, which reveals greater sensitivity to insulin, said Stahl and Napoli.
The team also 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 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."
The authors also noted that 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.
High-fat diet vs. low-fat diet
The team revealed that they did not find as big an impact on mice that ate a low-fat diet. They noted that mice 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," said 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."
The added that they had ruled out the typical explanations for weight loss in their study – noting that it was not due to the amount of food consumed, since the ingested calories among the different groups were about the same.
They added that the level of activity and body temperatures were comparable, and the authors even checked the calories eliminated in the faeces 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," said Stahl.
Source: PLOS ONE
Published online ahead of print, doi: 10.1371/journal.pone.0108408
"Consumption of Clarified Grapefruit Juice Ameliorates High-Fat Diet Induced Insulin Resistance and Weight Gain in Mice"
Authors: Rostislav Chudnovskiy, Airlia Thompson, Kevin Tharp, Marc Hellerstein, Joseph L. Napoli, Andreas Stahl