Grapefruit is low in calories, with just 41 calories per half, and high in vitamin C, with 73 percent of the daily value per half. There is some preliminary evidence that grapefruit may help with weight loss in different ways, depending on when you eat it.
A study published in the "Journal of Medicinal Food" in 2006 found that people who ate half of a grapefruit before each meal lost more weight and had improvements in insulin resistance compared to those who didn't eat grapefruit. Another study, published in "Nutrition & Metabolism" in 2011, found that drinking water before meals had the same beneficial effect as eating grapefruit, although those in the grapefruit group had greater improvements in cholesterol levels. Weight loss due to adding grapefruit, when it occurred, was minimal, however -- about 4 pounds over the course of 12 weeks.
The main benefit of eating grapefruit after meals occurs if you eat grapefruit instead of a sweet treat for dessert. This allows you to cut calories, making weight loss easier. For example, a chocolate chip cookie has about 53 calories, a slice of chocolate cake has about 249 calories and a brownie has about 112 calories. Eating half of a grapefruit instead of one of these treats saves 11 to 208 calories, which can add up to a significant amount of weight loss over time.
Citrus fruits, such as grapefruit, may help lower your risk for heart disease, cancer, cataracts and anemia, according to the Food and Agriculture Organization of the United Nations. People who ate half of a grapefruit with each meal lost weight, decreased their waist circumference and lowered their blood pressure after six weeks in a study published in "Metabolism" in July 2012. These results are preliminary, however, and larger studies are needed to verify effects and their potential to help prevent obesity and heart disease.
Grapefruit can interact with certain medications, so don't add it to your diet without first consulting your doctor. It can cause your body to absorb more or less of the medication and potentially increase your risk for side effects. Affected medications include certain statins, blood pressure medications, depression medications, organ transplant medications, the arrhythmia medication amiodarone, the erectile dysfunction medication sildenafil, the allergy medication fexofenadine and the HIV medication saquinavir. Pink grapefruit isn't safe for those with heart muscle disorders, as it may affect normal heart rhythms, according to Drugs.com.