"Some foods produce a negative thermogenic effect or a catabolic effect. A negative thermogenic effect results when the food contains fewer calories than the energy used to digest them during the metabolic process. This creates a negative value, resulting in negative thermogenesis. Some foods that create a negative calorie balance include apples, grapefruit, blueberries, lettuce, pineapple, oranges, carrots, limes, spinach, lemons, watermelon, sweet potato, zucchini and cooked leeks."
Great find. You should post this in the comments section of that article.