Dietitian Valerie Agyeman, R.D., is a dietitian and host of women’s health podcast, Flourish Heights. A half of a grapefruit contains about 64.7 kcal; 1.19 g protein, 0.216 g fat, 16.4 g carbohydrates, 2.46 g fiber, and 10.6 g sugar. Grapefruit contains an enzyme that may block the breakdown of medications.