The Grapefruit Diet, which is also known as the Hollywood Diet, has been making the rounds since the 1930s. Advocates swear it helps them lose weight, and claim that grapefruit has a fat-burning enzyme.
Most health experts have disagreed, calling the diet a fad, and saying there's no indication grapefruit burns fat. A new study, however, shows that not only does grapefruit help dieters lose weight, it may be as good as prescription drugs in controlling blood sugar levels.
Researchers at the University of California, Berkeley, found that mice that ate a high-fat diet for three months gained 18 percent less weight when they drank grapefruit juice than a control group that drank water. In addition, the mice in the grapefruit group had improved levels of glucose, insulin, and triacylglycerol, a type of fat.
Mice were divided into six groups. One group drank only water, and the other five groups drank grapefruit juice diluted with water at different concentrations with a bit of saccharin to counteract the grapefruit's bitterness. The water of the control group had glucose and saccharin added to match the calorie and saccharin content of the grapefruit groups.
At the end of the study, mice that drank grapefruit juice gained 18 percent less weight than mice in the control group, and their blood glucose levels decreased by 13 to 17 percent.
The scientists also tested a flavonoid in grapefruit called naringin, which experts believe is a key to weight loss. While one group of mice was given naringin, another group was given metformin, a drug often given to Type 2 diabetics to lower glucose levels. Members of the two groups were fed a diet that was either 60 percent fat or 10 percent fat for 100 days.
"The grapefruit juice lowered blood glucose to the same degree as metformin," said co-researcher 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."
Naringin's impact on glucose levels was much more apparent on mice given the high-fat diet, but it had little effect on weight. That suggests, researchers say, that another ingredient in grapefruit is responsible. "There are many active compounds in grapefruit juice, and we don't always understand how all those compounds work," said co-researcher Andreas Stahl, associate professor of nutritional sciences and toxicology
The researchers, whose study will be published in the journal PLOS ONE, were skeptical before they began the study and were surprised by its results. "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."
Stahl and co-researcher Napoli ruled out usual reasons for weight loss. Both groups consumed the same amount of food and engaged in the same amount of activity.
"We see all sorts of scams about nutrition," said Napoli. "But these results, based on controlled experiments, warrant further study of the potential health-promoting properties of grapefruit juice."
The Grapefruit diet has several versions, but most are a protein-rich, low-carb plan that calls for grapefruit juice at every meal, and promises quick results—as much as 10 pounds in 12 days. Many Hollywood celebrities, including Kylie Minogue and Brooke Shields, have praised the diet.
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