Grapes, celery and other low-calorie foods are sometimes said to be negative-calorie foods. A negative-calorie food is an item that provides fewer calories than your body uses to digest it. About 10 percent of your total daily energy expenditure goes to digest and store the nutrients in the food you eat, Dr. Donald Hensrud of the Mayo Clinic explains, but cautions that while it's "theoretically possible" that some foods are negative-calorie, scientific study hasn't confirmed this as of 2010. Hensrud concludes: "Many of the foods touted as negative-calorie foods are vegetables and fruits that can aid in losing weight as part of an overall healthy diet plan."

The catabolic diet, which focuses on eating likely negative-calorie foods, divides these foods into two categories: high-grade catabolic and low-grade catabolic. High-grade catabolic foods will burn a larger portion of the calories found in the food. High-grade catabolic fruits include oranges, lemons, grapefruits, pineapples, tangerines, strawberries, plums and blackberries, according to the HealthRecipes website. Examples of low-grade catabolic fruits are grapes, nectarines and honeydew melon. Grapes are included in a group of fruits that work as catabolic foods when consumed raw.

HealthRecipes lists more than a dozen negative-calorie vegetables. Celery is a low-calorie food that is fat-free and low in sodium and cholesterol. One 4-inch stalk of celery contains about 1 calorie, which means your body will expend more calories during digestion than the stick provides. For example, a 5-calorie piece of celery may take 100 calories to digest — a net loss of 95 calories from your body fat, HealthRecipes calculates. Other examples of negative-calorie veggies include broccoli, carrots, chili peppers, lettuce, turnips, beets, green cabbage, cauliflower, cucumbers, garlic, onions, zucchini, artichokes and green beans.

A wide variety of seafood is considered high-grade catabolic foods, HealthRecipes explains, including white fish such as cod and flounder, tuna, sea bass, shrimp, clams, oysters, mussels and crabs. Avoid high-calorie additions such as tartar sauce, cocktail sauce and other dressings that will greatly increase the calorie content of your meal.

Spinach is nutrient-dense, low in calories and helps to keep you satiated. A large spinach salad — about 2 cups of spinach — will provide only 14 calories, far fewer than your body will burn digesting that amount, HealthRecipes calculates. On the other hand, if your favorite dessert is 2 cups of ice cream, packing 400 calories, and your body only burns 200 calories during digestion, you will have gained 200 calories. More than likely, your body will store this calorie surplus as fat.

It's not healthy to build your diet completely on negative calorie foods, the Mayo Clinic's Hensrud cautions, as this may lead to malnutrition. Replace high-calorie foods in your diet with negative-calorie foods to enhance weight loss. Consult your doctor before beginning any new diet.