Drinking unsweetened grapefruit juice may help you lose weight, according to a study in the Spring 2006 issue of "Journal of Medicinal Food" and the October 2009 issue of "Diabetes." Both studies cite separate reasons for the weight loss. One credits a flavonoid present in grapefruit, while the 2009 study credits grapefruit’s insulin-lowering properties.
The 2009 study in "Diabetes" involved fattening up mice on a lard and sugar diet and later adding a grapefruit flavonoid to the rodents' diet. The mice lost weight without giving up the sugar and fat in their meals. Mulvihill, the lead researcher, attributed the weight loss to the flavonoid. Murray Huff, a member of Mulvihill's research team, told The Toronto Star that it would take between 6 and 8 cups of unsweetened grapefruit juice for people to get comparable weight loss benefits.
Possible Weight Gain
If you drank 8 cups of grapefruit juice daily, the calories might offset the fat-burning properties of the flavonoid. You could gain rather than lose weight. Unsweetened grapefruit juice contains approximately 96 calories in 1 cup and approximately 768 calories in 8 cups. If you add an extra 768 calories to your daily diet, you are likely to gain weight. You could gain 1 pound every five days, or more than 70 pounds a year, by adding 8 cups of grapefruit juice to your diet, based on the formula that 3,500 calories equals 1 pound of fat.
According to the 2006 "Journal of Medicinal Food" study, led by K. Fujioka, it is possible to add 3 cups of unsweetened grapefruit juice, which would amount to 288 extra calories daily, to your daily diet and still lose weight. In the study, 91 obese people consumed half a grapefruit, 1 cup of unsweetened grapefruit juice, a grapefruit powder capsule or a placebo before breakfast, lunch and dinner for 12 weeks. Without any other changes to their diets, those who ate half a grapefruit before meals lost about 3.5 pounds. Those who drank 1 cup of unsweetened grapefruit juice before meals lost about 3.3 pounds. Those who consumed grapefruit powder capsules lost about 2.4 pounds or about 3 ounces per week. The placebo group lost just over 1 ounce per week.
Insulin and Weight Loss
The participants in Fujioka’s study who lost weight also lowered their resistance to insulin. Insulin helps your body transport sugar to your cells to be used as energy. If your body doesn't respond properly to insulin, sugar gets stored as fat rather than burned as energy. Fujioka found that properties in grapefruit, unsweetened grapefruit juice and grapefruit powder capsules helped make insulin more efficient, normalizing insulin and blood glucose concentrations. If you've gained weight because of an imbalance in your insulin and blood glucose concentrations, you may lose weight when the balance is restored.
Speak to your doctor before making grapefruit juice a part of your diet, especially if you are on any type of medication. According to the National Institutes of Health, grapefruit juice interacts with several medications and may alter their effectiveness or cause several side effects.