Calorie

Calorie is a unit of energy. In scientific contexts, the small calorie (cal) is defined as the amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius. In nutrition, the term calorie almost always refers to the kilocalorie (kcal), equal to 1,000 small calories and roughly 4,184 joules. Nutrition labels report energy in kilocalories.

The energy content of foods depends on macronutrients: fat provides about 9 kcal per gram, carbohydrates and protein about 4 kcal per gram, and alcohol about 7 kcal per gram. Body energy expenditure includes basal metabolic rate, thermic effect of food, and physical activity. Overall energy balance determines weight change over time.

Historically, the concept arose in 19th-century physiology. The Atwater system established standard energy values for macronutrients and remains a common reference in nutrition labeling. Modern science also uses the joule, with 1 kcal equal to 4,184 joules. In dietary contexts, the term calorie is typically used interchangeably with kilocalorie, though scientifically precise usage favors kcal.

Limitations include variability in digestion and absorption, food matrix effects, and individual metabolism; not all calories are created equal in their effects on satiety, hormones, or gut microbiota. Food labeling often reports both kilocalories and kilojoules, and practical guidance emphasizes overall dietary patterns rather than calorie counting alone.