calories

Calories are a unit of energy. In nutrition, the term is usually used to mean the kilocalorie (kcal), the amount of energy required to raise the temperature of 1 kilogram of water by 1 degree Celsius. Dietary labels express energy in kilocalories, commonly written as Calories with a capital C.

When the body metabolizes macronutrients, it releases usable energy. The energy yield is about 4 kcal per gram for carbohydrate, 4 kcal per gram for protein, 9 kcal per gram for fat, and 7 kcal per gram for alcohol. Fiber provides little digestible energy. The bomb calorimeter measures gross energy in foods, and the Atwater system provides general factors to estimate metabolizable or net energy for nutrition labeling.

Energy requirements depend on basal metabolic rate, the thermic effect of food, and physical activity. Net energy balance—more energy consumed than expended, or vice versa—drives changes in body energy stores. Because these factors vary by age, sex, weight, health, and activity level, daily calorie needs differ widely; many adults fall in the range of roughly 1,600 to 2,800 kcal per day, but individual needs vary.

In practice, calories are used for planning meals, tracking intake, and understanding labeled foods. Foods differ in energy density (calories per gram) and nutrient density (nutrients per calorie). A balanced approach emphasizes nutrient-rich foods within an appropriate calorie range to support health and weight management.