What is a calorie?

Scientists originally defined a small calorie as the energy needed to heat one gram of water by one degree Celsius. Food labels actually use the kilocalorie, equal to 1,000 small calories, often written with a capital C. The body gets energy from three main nutrients: carbohydrates and protein provide about 4 kilocalories per gram, while fat provides about 9. Adults typically need around 2,000 to 2,500 kilocalories a day, depending on age, size and activity. Eating more energy than the body uses leads to weight gain, while eating less leads to weight loss.