First of all, there is no such thing as a 'negative calorie' - as a calorie is just a unit of heat and it cannot be negative. So when ever people talk about negative calorie foods, it just means a food where the calorie level is so low that it will take more energy to eat and digest it than the food contains.
For example, if you would consider water to be a food, then cold water is defiantly a negative calorie food. It doesn't contains any calories at all, and the body has to use energy to bring it up to blood temperature. So every time we drink a glass of nice cold water, we burn up a few calories and lose a little weight! But, there are very few foods like this. There's a list of foods below, but most foods on the list don't really use up more calories than they add, or if they do, the effect is negligible...until you remember that at the same time, the body uses energy just to keep us alive and breathing. For example, celery doesn't give us enough calories to cover both the energy that your body uses to digest it, plus the energy that we expend even when we're doing nothing. So this is why people call this a negative calorie food.
Negative calorie foods are very attractive to anorexics. A negative calorie diet will obviously result in starvation in the long term and nutritional deficiencies can start occurring very quickly. The calories burnt usually come from muscle mass, so the result is debility and wasting.
A much more healthy way to use a negative calorie diet is as a form of fasting or detox. It's something you can do for one to three days right after the Christmas season or some other time when you have been eating a over-rich diet. Of course provided that you're otherwise healthy, it can be a good way to clean out your system. However, it's still best to take medical advice before you try this out.
If you do decide to use a negative calorie detox, keep these points in mind:
1. It's never a good idea to eat a huge quantities of one type of food, especially fruits. A lot of foods contain substances that can be damaging if they are consumed to excess. For example, the acid in grapefruit and pineapple can damage your stomach lining. Other foods can put an excessive burden on the liver or may cause diarrhea. So try to use different foods in small quantities.
2. Don't spend all day eating. You'll just end up bloated and get sick. If you're only doing this for a few days, you shouldn't feel too hungry. Plan four or five salad meals each day, and let your digestive system rest at other times.
3. You'll feel more satisfied if you eat slow and chew your food thoroughly. You'll also use more calories this way. If the foods can be eaten raw, they will be more effective than cooking them. Foods like carrots and beets will also be more satisfying if you eat them grated.
4. Schedule times when you don't have any important commitments. Just like with fasting or any kind of detox, you might suffer some uncomfortable symptoms like headaches, tiredness, depression and irritability. If these become severe, stop the diet and see a doctor before you try this again.
5. When you end the detox, plan a gradual return to normal eating.
If all the above points are checked and passed, let's move on to the list of foods.
apples, asparagus, beets, blueberries, broccoli, cantaloupes, carrot, cauliflower, celery, cranberries, cucumbers, dandelion, eggplant, endive, garden cress, garlic, grapefruit, green beans, green cabbage, hot chili peppers, lamb's lettuce, lemon mango, lettuce, onions, orange, papayas, pineapples, prunes, radishes, raspberries, spinach, strawberries, tomatoes, tangerine, turnip, zucchini.
Use this list of negative calorie foods wisely.
Now, if you want to try a truly negative calorie 'food,' you will have to be eating cardboard. This is 100% cellulose and since we lack the enzymes to break cellulose down, it would give us zero calories. The process of moving the cardboard through the body would use up a small but measurable amount of energy. The result? A truly negative calorie food. Of course this is one that we would not recommend.