- The term "calorie" was first used in the early 19th century during a series of lectures on the use of heat engines. Originally, "calorie" referred to numerous kinds of heat measurements, not just food calorie counting. Although the term is popular with the general public, the United Nations Food and Agricultural Organization calls the calorie "not a coherent unit of thermochemical energy" while at the same time realizing "many scientists, non-scientists and consumers still find it difficult to abandon the use of calories."
- The calorie is a unit of measurement for energy. The "small calorie" is defined as the amount of energy (heat) needed to raise the temperature of one gram of water from 14.5 degrees centigrade to 15.5 degrees centigrade at a constant pressure of one atmosphere (sea level). The "large calorie" is the kilogram calorie, which has been shortened to kilocalorie, and shortened further to kcal. A kilocalorie (one nutritional label calorie) is the amount of energy needed to heat 1,000 grams of water (about 1 liter of water) one degree centigrade from 14.5 C to 15.5 C at one atmosphere.
- The term "calorie" helps consumers know how much energy is in a particular product. In Europe or certain other locations across the world, food calories may be listed as kilocalories (kcal) or kilojoules (kJ). The term kilocalorie is synonymous with calorie. The joule (J) is the unit of energy in the International System of Units (SI). To convert between kilocalories and kilojoules, 1kJ = 0.239 kcal and 1 kcal = 4.184 kJ.
- Whether called a kilocalorie or calorie when discussing food, neither term provides the total number of calories in food. In the late 19th century, scientists with the United States Department of Agriculture published factor tables for selected foods based on composition, digestibility, metabolism and other factors. Using these tables and others for guidance, a product's digestible parts determine energy values (calories) assigned to a total recipe or product.
- The component parts of food that can be digested yield the following standard table amounts of food energy (calories) per gram of alcohol, carbohydrates, fats, protein and other constituents. The food component with the most calories per gram is fat, at 9 calories. Next is alcohol, sometimes referred to as ethanol. Alcohol at 100 percent is also referred to as 200 proof. This is 7 calories of energy density per gram. Proteins and carbohydrates each provide 4 calories per gram, organic acids yield 3 and polyols (sugar free sweeteners) bring with them 2.4 calories.