Grapefruit, pineapple and celery are all nutritious, low-calorie foods that can be part of a healthy diet. They shouldn't be the only foods you rely on, however, because they don't contain all the necessary nutrients for good health. Check with your doctor before following this type of diet to make sure it would be safe for you.
Each cup of raw pineapple has 83 calories and provides 131 percent of the daily value for vitamin C and 77 percent of the DV for manganese. The same amount of raw celery has just 16 calories and contains 37 percent of the DV for vitamin K. A cup of raw grapefruit sections with juice contains 74 calories and has 10 percent of the DV for fiber, 43 percent of the DV for vitamin A and 132 percent of the DV for vitamin C.
If you don't eat other foods, your diet will be too low in fat and protein and not contain the essential vitamins B-12 and D. While grapefruit, celery and pineapple provide small amounts of many essential vitamins and minerals, it isn't likely such a restrictive diet would provide sufficient amounts of most of these nutrients.
A diet consisting of just these three foods is considered a fad diet. Fad diets can be dangerous to your health, potentially increasing your risk for medical conditions including osteoporosis, heart disease, high blood pressure and cancer. Any weight loss is likely to come back once you return to eating a regular diet.
People on certain medications may need to avoid grapefruit and grapefruit juice, which can interact with these medications and cause too little or too much of the medication to accumulate in your body at one time. This increases the risk for adverse effects. Some medications that may interact with grapefruit include statins, blood pressure medications, antihistamines, anti-anxiety medications and anti-arrhythmia medications.