A 12-week pilot study, led by Dr. Ken Fujioka, monitored weight and metabolic factors of the 100 men and women who participated in the Scripps Clinic “Grapefruit Diet” study. On average, participants who ate half a grapefruit with each meal lost 3.6 pounds, while those who drank a serving of grapefruit juice three times a day lost 3.3 pounds. However, many patients in the study lost more than 10 pounds.

The most important active ingredient in grapefruit that helps us to lose fat is naringin, a flavonoid compound that gives grapefruit its characteristic bitter flavor and blocks the uptake of fatty acids into cells to prevent our bodies from effectively using carbohydrates. This causes your body to look to dietary fat and body fat stores for energy.

But grapefruit also confers so many more health benefits. “Grapefruit packs in lots of nutritional goodies, supplying a heaping dose of vitamin C, folic acid, and potassium — all of which protect your heart,” says Dr. Barry Sears in his book The Top 100 Zone Foods: The Zone Food Science Ranking System “Pink grapefruit is relatively rich in anti-oxidants, and ruby red grapefruit provides an added bonus: lycopene, the phytochemical that helps prevent the ‘bad’ (LDL) cholesterol from oxidizing and damaging artery walls.”

Grapefruit also contains pectin, a form of soluble fiber that has been shown in animal studies to slow down the progression of atherosclerosis. Pigs fed a high-cholesterol diet plus grapefruit pectin had 24% narrowing of their arteries, while pigs fed the high-cholesterol diet without grapefruit pectin had 45% narrowing.

There’s a catch, though. To get all that fiber you have to eat the walls that separate the segments (It’s okay to skip the stringy white stuff that’s attached to the inside of the rind). That means peeling and eating your grapefruit like an orange, or digging out the walls with your spoon.

In humans, drinking three 6-ounce glasses of grapefruit juice a day was shown to reduce the activity of an enzyme that activates cancer-causing chemicals found in tobacco smoke. In rats whose colons were injected with carcinogens, grapefruit and its isolated active compounds (apigenin, hesperidin, limonin, naringin, naringenin, nobiletin) not only increased the suicide (apoptosis) of cancer cells, but also the production of normal colon cells.

Though this last study is encouraging, let’s not forget about our ever so important hormone responses to foods. Drinking all that juice will cause an insulin response from the sugar, and since juice is considered processed by Paleo standards, you are best (as always) to stick to eating your grapefruit whole.

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