Folklore of late has exhorted the virtues of one of the larger members of the Citrus species, C. paradisi, also known as grapefruit. The tallest tale has centred upon the assertion that eating it can promote weight loss.
Preclinical investigations point to different grapefruit constituents modifying intestinal and liver metabolism related to lipid transport, the former possibly resulting in reduced lipid absorption.1,2 Additional in vitro work suggests that naringenin, an abundant flavonoid in grapefruit, can either enhance or impair insulin sensitivity.2,3 Numerous diet books have advocated a grapefruit-centric diet, but only recently has the fruit been put to the squeeze of a controlled clinical trial.
Dr Ken Fujioka from the Scripps Clinic in California led a clinical trial funded by the Florida Citrus Commission.4 One hundred obese subjects (average weight 98kg; body mass index 35.6kg/m2) were assigned to one of four intervention groups (consumed three times daily before meals) for 12 weeks: (a) 7oz apple juice plus placebo capsule; (b) 7oz apple juice plus whole grapefruit extract capsules (Grapefruit Solution by Diaeta); (c) 8oz of grapefruit juice plus placebo capsule; or (d) half of a fresh grapefruit plus placebo capsule. No other dietary modifications were required but the subjects were required to walk 20-30 minutes three to four times weekly.
After 12 weeks, 79 subjects completed the study, with only group D, the fresh grapefruit group, showing statistically significant weight loss compared to placebo (apple juice plus placebo capsules). Body composition (fat, fat-free mass, bone, water) was not assessed.
Significantly greater weight loss than placebo was seen in all of the grapefruit treatment groups among subjects who met the classical definition of metabolic syndrome — three or more of these characteristics: abdominal obesity, elevated blood triglycerides, reduced blood HDL cholesterol, elevated blood pressure or elevated fasting blood glucose.
About 30 per cent of the study population met this criteria, being rather evenly distributed among the four groups. Blood insulin excursions (at 12 weeks) during a two-hour glucose tolerance test, as a crude index of whole body insulin sensitivity, showed the fresh grapefruit group to have a significantly lower value compared to those receiving grapefruit capsules.
These findings indicate the promise of fresh grapefruit and leave open the possibility that grapefruit juice, or a powdered extract of whole grapefruit, may also exert some desirable effects, apparently of a lesser magnitude. They also suggest that one or more bioactives occurring in fresh grapefruit are either process-labile or are present in lesser quantities in the processed forms. Additional research may dispel the grapefruit myth and ripen the promise of grapefruit as a biofunctional food ingredient offering consumer promise.
1. Borradaile NM, et al. Inhibition of HepG2 cell apolipoprotein B secretion by the citrus flavonoid naringenin involves activation of phosphatidylinositol 3-kinase, independent of insulin receptor substrate-1 phosphorylation. Diabetes 2003; 52:2554-61.
2. Borradaile NM, et al. Hepatocyte apoB-containing lipoprotein secretion is decreased by the grapefruit flavonoid, naringenin, via inhibition of MTP-mediated microsomal triglyceride accumulation. Biochemistry 2003; 42: 1283-91.
3. Harmon AW and Patel YM. Naringenin inhibits phosphoinositide 3-kinase activity and glucose uptake in 3T3-L1 adipocytes. Biochem Biophys Res Comm 2003; 305:229-34.
4. Fujioka K, et al. The effects of grapefruit on weight and insulin resistance: relationship to the metabolic syndrome. Diabetes 2004; 53 (Suppl 2): A594.