Animal studies, and one human study, demonstrated the antidiabetic effects of grapefruit.78, 79
In a 12-week study on metabolic syndrome patients, eating half a fresh grapefruit 3 times per day, before each major meal, resulted in an extra weight loss of 1.6 kg (3½ pounds), and reduced insulin resistance, compared to a placebo group.78
No other studies that I’m aware of have examined the effects of grapefruit consumption on blood sugar control in humans, but various rodent studies confirm its antidiabetic effect.79
Grapefruit interacts with various medications, so be sure to get permission from your medical doctor before consuming any grapefruit foods/beverages/supplements etc or anything containing grapefruit constituents.