What are Calories?
The calorie is a unit for measuring food energy. Carbohydrates, fiber, fats, proteins, organic acids, polyols, and ethanol all release energy during metabolism—this is often called 'food energy'. When nutrients react with oxygen in the cells of living things, energy is released. A small amount of energy is available through fermentation. Fats and ethanol have the greatest amount of food energy per mass, 38 and 30 kJ/g (9 and 7 kcal/g), respectively. Proteins and most carbohydrates have about 4 calories/g. Carbohydrates that are not easily absorbed, such as fiber or lactose in lactose-intolerant individuals, contribute less food energy. Polyols (including sugar alcohols) and organic acids have less than than 4 calories/g.
Recommended daily calorie intake
Recommendations in the United States are 2,700 and 2,100 calories for men and women (respectively) between 31 and 50, at a physical activity level equivalent to walking about 1.5 to 3 miles per day at 3 to 4 miles per hour on top of the light physical activity associated with typical day-to-day life, with French guidance suggesting roughly the same levels.
Children, those with sedentary lifestyles, and older people require less energy; physically active people more. In Australia, because different people require different daily energy intakes there is no single recommended intake instead there being a series of recommendations for each age and gender group although packaged food and fast food outlet menu labels refer to the average Australian daily energy intake of 2079 calories.
According to the Food and Agriculture Organization of the United Nations, the average minimum energy requirement per person per day is about 1,800 calories.
This definition may contain information from Wikipedia.