| This fad diet regained popularity in the mid-1970s as a bit of Xeroxlore. It is occasionally attributed erroneously to the Mayo Clinic, which has expressed a decidedly negative opinion of the diet, considering it unbalanced and possibly dangerous. However, a 2004 study led by Dr. Ken Fujioka at the Nutrition and Metabolic Research Center at Scripps Clinic found in a 12-week pilot study that, on average, participants who ate half a grapefruit with each meal lost 3.6 pounds and those who drank a serving of grapefruit juice three times a day lost 3.3 pounds. Additionally, many patients in the study lost more than 10 pounds.
Dr. Fujioka found that the grapefruit diet appears to reduce insulin levels and, thus, affects blood sugar regulation. Bear in mind that the pancreas secretes insulin in response to the amount of carbohydrate ingested (also affected by the glycemic index and glycemic load of a food or meal) and that grapefruit, by itself, has a glycemic index of 25 (compared to 100 for glucose) and a glycemic load of 1.4[2]
Another theory is that the fruit's low glycemic index is able to help the body's metabolism burn fat.
Still another explanation for the weight loss in the Scripps Clinic study can be found in the report — participants "slightly enhanced their exercise regimens." Depending upon what "slightly enhanced" means, this might well account for the weight loss observed.  | 
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