It is common knowledge that a diet high in fat can lead to weight gain. But could drinking grapefruit juice reduce this effect? A new study by researchers at the University of California-Berkeley claims it did for mice fed a high-fat diet, and it even lowered blood glucose levels and improved insulin tolerance.

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Grapefruit juice reduced weight gain in mice fed a high-fat diet, according to researchers.

The research team, led by Joseph Napoli and Andreas Stahl, both of the Department of Nutritional Sciences and Toxicology at the university, publish their findings in the journal PLOS ONE.

Grapefruit has been hailed for its weight-loss effects since the 1930s, forming a part of the famous Hollywood Diet. Studies claimed that grapefruit consists of a fat-burning enzyme that promotes rapid weight loss.

But Napoli and Stahl say the validity of such studies can be questioned. “Relatively few human studies have examined the effects of grapefruit or grapefruit juice consumption per se on metabolism in well-controlled experiments, and these have produced intriguing but contradictory results,” they note.

In this study, the team set out to improve understanding of the metabolic effects of grapefruit juice consumption.

The researchers tested the effects of clarified, pulp-free grapefruit juice diluted with water at different concentrations on five groups of mice fed either a high- or low-fat diet for 100 days. The grapefruit juice was sweetened with saccharin to make it less bitter.

These effects were compared with one group of control mice, which were fed a high-fat diet but were given water to replace grapefruit juice. The team added glucose and artificial sweeteners to the water so it had the same calorie and saccharin content as the grapefruit juice.

At the end of the study, the researchers found that mice fed a high-fat diet that drank grapefruit juice gained 18% less weight than mice on a high-fat diet that drank water. As well as greater weight loss, grapefruit-drinking mice fed a high-fat diet also showed a 13-17% reduction in blood glucose levels and a three-fold reduction in insulin levels.

Grapefruit juice had no effect on weight for mice fed a low-fat diet, although these mice did show a two-fold reduction in insulin levels.

The team tested the effects of a compound found in grapefruit juice – naringin – on one group of mice fed a high-fat diet. Naringin has previously been linked to weight loss. Another group of mice fed a high-fat diet were given metformin – a drug used for lowering glucose levels in people with type 2 diabetes.

The researchers found that naringin lowered blood glucose levels in the mice just as much as metformin. “That means a natural fruit drink lowered glucose levels as effectively as a prescription drug,” says Napoli.

The researchers note, however, that naringin alone appeared to have no effect on the weight of mice fed a high-fat diet, meaning there is another compound in grapefruit that promotes weight loss.

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

The researchers admit that they are unable to say why grapefruit juice appears to halt weight gain.

They note that all mice had similar calorie intake and exercise levels, so these can be ruled out as explanations. They say it cannot even be put down to an issue with nutrient absorption, as they checked the calories that had been eliminated in feces.

“Basically, we couldn’t see a smoking gun that could explain why or how grapefruit juice affects weight gain,” says Stahl. However, this is something they plan to investigate in future research.

Commenting on their overall findings, the researchers say:

We have provided new evidence for potential health-promoting properties of grapefruit juice in murine high-fat diet-driven obesity and non-obesity models. These results justify additional studies in animal models and humans to assess the mechanisms and scope of grapefruit juice action.”

The research was funded by the California Grapefruit Growers Cooperative. The investigators stress that the organization had no influence or control over the design or findings of the study.

The benefits of grapefruit may not be limited to weight loss. In 2012, Medical News Today reported on a study that claimed grapefruit juice can improve the effectiveness of an anti-cancer drug, while another study earlier this year suggested that grapefruit could prevent the formation of kidney cysts.

Read our Knowledge Center article on the health benefits of grapefruit to learn about other ways the fruit could be good for you.

Grapefruit consumption, however, should be avoided when using certain medications – such as some statins and antihistamines. The fruit can increase drug levels in the blood, increasing the likelihood of severe side effects.

A 2012 study published in the Canadian Medical Association Journal (CMAJ) claimed that 43 medications can have harmful side effects if they interact with grapefruit, as the fruit blocks the CYP3A4 enzyme that reduces the effects of around 50% of all medications.