May 12, 2006

Study to weigh in on grapefruit diet

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Heidi Silver, Ph.D., is studying the effects of grapefruit consumption on weight loss and appetite.
Photo by Dana Johnson

Study to weigh in on grapefruit diet

Vanderbilt University Medical Center investigators have launched a two-year study to see how consumption of grapefruit and grapefruit juice affects appetite, dietary intake, weight loss and body composition.

The origins of the grapefruit diet date back to the food fads of the 1970s, when it was popular to eat grapefruits with every meal or eat nothing but grapefruit for several days in a row. It was described by some nutrition professionals as a mysterious urban dieting legend — that grapefruit contained enzymes that burned fat away or sped up metabolism.

Despite the notion by many people that eating grapefruit leads to significant weight loss, to date there exists only one or two modest scientific studies supporting the theory.

“The myth of the grapefruit diet has been around for decades,” said Heidi Silver, Ph.D., research assistant professor at Vanderbilt's Center for Human Nutrition and the study's principal investigator. “If you search the Internet, you will find diet-related Web sites that offer menu plans focused on eating grapefruit for weight loss.

“But in all this time there has never really been any solid science to support this assumption, other than one study conducted by the Scripps Clinic in California which was completed in 2004. Notably, the Scripps study did show greater weight loss in the groups of participants who ate grapefruit or drank grapefruit juice for 12 weeks.”

Silver said the grapefruit diet became popular because the fruit is tasty, low in calories (between 66 and 84 calories per serving), inexpensive and grows in many varieties. It also is low in concentrated sugars but high in fiber, vitamins and minerals, making it a nutrient dense food.

“The concept of nutrient density is very important in nutrition research now. It indicates how much of a nutritional impact a particular food item has by volume or per gram weight of the food,” Silver said. “Other studies have found that people consistently eat the same volume of food day after day. It may be true that eating grapefruit or drinking grapefruit juice offers an early sensation of satiation, that feeling of fullness early into eating a meal that curbs the amount of food intake at meals.”

Silver's study consists of two separate phases. In phase I, the focus is on weight loss intervention, to determine whether eating grapefruit or drinking grapefruit juice can result in greater weight loss compared to placebo.

Participants will meet with registered dietitians weekly for individualized weight loss plans. Both dietary intake and physical activity will be measured carefully.

“In the animal studies that have been done, investigators observed that the animals had increased activity in their cages after they ate or smelled grapefruit, which increased their daily energy expenditure,” Silver said.

Phase II of the study will examine whether grapefruit and grapefruit juice actually play a role in curbing appetite through satiation or satiety. There will be approximately 90 participants recruited in phase I. About half of this original cohort is expected to move on to the study's second phase.

Silver explained, “The individuals who progress along to phase II will have their food and fluid intake weighed before and after meals and snacks to see if the consumption of grapefruit really can curb appetite.”

Funded by the Florida Department of Citrus, the study will use a total of 4,158 white marsh grapefruits and 893 bottles of grapefruit juice that are being shipped each month from Florida.

“For the purpose of this study, it is important that the grapefruit is all from the same growers' lot. This is for size and weight consistency, and also to make sure that if there turns out to be a metabolic effect, that it will be from the same maturity of fruit,” Silver said.

To be considered for this study, a prospective candidate must be age 21-50, have a body mass index of 30 to 39.9 (corresponding with NIH classifications for Obesity Class I and II), and must not smoke, drink alcohol or take prescription medications which have a known propensity to interact with certain bioactive compounds found in grapefruit or grapefruit juice.

“People who meet the NIH's classification of Obesity Class I and II are individuals who are at high, and very high, risk for the metabolic syndrome. The metabolic syndrome is characterized by a group of metabolic risk factors like being overweight, having high blood pressure, or having high cholesterol or triglycerides in the blood that increase the incidence of comorbidities such as type 2 diabetes and cardiovascular diseases like coronary heart disease,” Silver said. “It is estimated that at least 22 percent of adults in the U.S. have metabolic syndrome.”

Participants will receive individualized nutrition education, counseling and supervision from registered dietitians, including nutrition and diet assessment, development of a comprehensive reduced-calorie meal plan, blood analysis, a tailored plan for increasing physical activity and body composition analysis.

For more information about participation in the grapefruit study, contact Silver's research team at 936-0985.