Most grapefruit diet versions average from 800 to 1,000 calories per day, which with or without grapefruit will cause weight loss. There are benefits to adding grapefruit to the diet, even if no weight or fat loss occurs. Grapefruit is high in fiber and a rich source of vitamin C, and has known cancer fighting properties. Grapefruit also has a high water content, which may help increase metabolism. Although the verdict is still out on the grapefruit diet, adding grapefruit to a few of your regular meals each day might bring the added benefit of a little weight loss.