Followers of the grapefruit diet eat half a grapefruit or a serving of grapefruit juice with every low-calorie, high-protein meal. The diet is based on the premise that grapefruit has an enzyme that burns fat, says Andrea N. Giancoli, a spokesperson for the American Dietetic Association, but that enzyme has never been pinpointed. “It’s just a low-calorie diet. That’s what’s causing the weight loss. It’s nothing in the grapefruit. ... Grapefruit’s been elevated to a magical state, but it’s just a fruit that’s good for you.”
Giancoli estimates that people on the diet eat about 800 to 1,000 calories a day, which is “not enough for anyone to sustain themselves on for any period of time.”
Nevertheless, scientists have studied the fruit’s properties. In 2006, researchers from the Scripps Clinic in La Jolla, California, published the results of a study on grapefruit’s relationship to the metabolism. People who consumed fresh grapefruit or juice three times a day before each meal lost about three pounds more than the placebo group after 12 weeks. The study stipulated that subjects maintain their daily eating habits and “slightly [enhance] their exercise routine.” It’s been well reported that the study was funded by the Florida Department of Citrus. The study concluded that “the mechanism of this weight loss is unknown [but] it would appear reasonable to include grapefruit in a weight reduction diet.”
Giancoli says that the increased exercise could be the reason for the weight loss, and notes: “One study does not make a solid scientific fact.” A second study was just completed in December 2008 by researchers at the Vanderbilt University Medical Center, but the findings haven’t been published yet.