Can Grapefruit Juice Prevent Weight Gain, Increase Insulin Sensitivity?

Since the 1930’s, one fad diet after another has hit the scene touting grapefruit consumption as a way to shed those unwanted pounds. But according to new animal research from the University of California, Berkeley, there may be some science to back up these claims.

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(This is not the first time grapefruit consumption has been in the news: A 2012 study found that the fruit and its juice can dangerously interact with a wide variety of prescription drugs, including several likely to be taken by people with diabetes.)

Previous studies on the weight-loss effects of grapefruit juice have been small or not well controlled, and the findings have been contradictory, according to Joseph Napoli, PhD, and Andreas Stahl, PhD, lead authors of the current research. In order to increase knowledge about the metabolic effects of grapefruit juice consumption, the scientists randomly divided mice into six groups. All of the groups were fed either a high-fat (60% fat) or a low-fat (10% fat) diet, and two were provided with no-pulp grapefruit juice diluted with water at different concentrations and sweetened slightly with saccharin to counteract the bitterness of the juice.

A third group received water mixed with naringin, a compound in grapefruit juice that has been identified as a key factor in weight loss; a fourth group was provided with the diabetes medicine metformin mixed into water; and a fifth group received metformin mixed into sweetened grapefruit juice. A control group of mice was fed a high-fat diet and given water with glucose and saccharin added to match the calorie and artificial sweetener content of the grapefruit juice mixtures. For 100 days, the mice maintained the study diets and had their metabolic health measured.

At the end of the study period, the researchers found that the mice eating a high-fat diet and drinking grapefruit juice gained 18% less weight than the mice eating a high-fat diet and drinking water. The mice following the high-fat, grapefruit juice diet also had a 13% to 17% reduction in blood glucose levels and a threefold decrease in insulin levels compared to the mice in the control group, indicating increased insulin sensitivity. Additionally, “the grapefruit juice lowered blood glucose to the same degree as metformin,” noted Napoli.

The mice on the high-fat diet that had received naringin had lower blood glucose levels than the mice in the control group, but the compound had no effect on weight loss, indicating that another component of grapefruit juice is also beneficial. The research also did not find a large impact on the mice eating a low-fat diet: While those that drank the grapefruit juice had a twofold decrease in insulin levels, there was no significant change in their weight or other metabolic parameters. The researchers suggest that because mice are typically healthy animals, it takes a more significant effect to see a beneficial health impact on those that are already eating a healthful, low-fat diet.

After determining that all of the mice consumed similar amounts of calories, had comparable activity levels and body temperatures, and had roughly equal levels of nutrient absorption, the scientists were able to rule out all of the typical possible explanations for the increased rate of weight loss seen in the mice on the high-fat, grapefruit juice diet.

The study was funded by the California Grapefruit Growers Cooperative, but the researchers stress that the group had no influence or control over the study’s design or findings.

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

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

For more information, read the article “Grapefruit juice stems weight gain in mice fed a high-fat diet, study finds” or see the study in the journal PLoS ONE. And to learn more about increasing insulin sensitivity and controlling weight, read “Increasing Insulin Sensitivity,” by professor Sheri Colberg, and “Strategies for Weight Management,” by clinical psychologist Ann Goebel-Fabbri.

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