In a 12-week pilot study, led by Dr Ken Fujioka from the clinic's Nutrition and Metabolic Research Center, researchers found that people eating a serving of the citrus fruit with each meal led to an average weight loss of 3.6 pounds.
About 100 male and female participants, who had an average weight of 218 pounds, were divided into three groups. One ate half a grapefruit with each meal, a second was instructed to drink a serving of grapefruit juice three times a day, and a third acted as a control group. The study participants maintained their daily eating habits and slightly enhanced their exercise routine.
Those eating the fruit three times daily lost on average 3.6 pounds over the study period, while those on the juice regime lost 3.3 pounds. The control group only saw an average decrease of 0.5 pounds.
Some of the patients taking grapefruit lost more than 10 pounds, added the researchers.
"For years people have talked about the grapefruit diet, and some even swear by it, but now, we have data that grapefruit helps weight loss," said Dr Fujioka.
The study also indicates a physiological link between grapefruit and insulin. The researchers speculate that the chemical properties of the fruit reduces insulin, seen in measurements of the participants' glucose levels.
Insulin is known to help regulate fat metabolism and also plays a key role in diabetes. The finding means it could also protect obese people from developing type 2 diabetes. However lowering insulin levels also makes people feel less hungry and this could also explain its link to weight loss.
Grapefruit is already known to affect the metabolism of some drugs, by impacting the enzymes needed to break down common medications.
The researchers are planning a much larger study to see if the results can be confirmed.