A negative-calorie food is a food that requires more energy to be digested than it provides. While this concept is popular in dieting guides, there is no scientific evidence to support that any of the foods claimed as negative-calorie foods are such. Personally I think is more a placebo effect but as long as it tastes good and it feels me up with natural whole substances, I don't think what scientific studies show is relevant.
As a negative-calorie beverage, ice water has no calories but requires the body to expend a very small amount of energy to raise the liquid to body temperature. So it will help boost your metabolism. Same goes with hot water with lemon.
Foods that are negative in calories are mostly low-calorie fruits and vegetables such as:
