The calorie on your cereal box is really a thousand calories in disguise
In physics a calorie warms one gram of water by one degree. The Calorie on an American food label is a thousand times bigger, a kilocalorie wearing a lowercase disguise. The mix-up is well over a century old, and the unit itself was officially replaced by the joule in 1948.
The word comes from the Latin for heat and dates from an era when heat was imagined as a fluid called caloric. Nicolas Clément introduced it in calorimetry lectures between 1819 and 1824, and his version was the large unit. In 1852 the chemist Pierre Antoine Favre and physicist Jean Thiébault Silbermann used the same word for the small one. Marcellin Berthelot tried to sort things out in 1879 by separating gram-calories from kilogram-calories and suggesting a capital C for the bigger unit.
That capital letter caught on with Wilbur Olin Atwater of Wesleyan University, whose 1887 article on the energy in food proved hugely influential. The physician Joseph Howard Raymond proposed the tidier word kilocalorie in an 1894 physiology textbook, but it took years to spread. By 1928 people were already complaining that relying on a capital letter to tell the two apart was asking for trouble, and in practice North American labels usually print a lowercase calorie anyway.
Pinning down the exact size was its own headache. The heat needed to warm water by a degree shifts with pressure and starting temperature, so several definitions arose, notably the 15-degree calorie and the thermochemical calorie. The modern convention fixes one small calorie at exactly 4.184 joules, chosen so the thermochemical unit kept the same value it had before 1948.
Regional habits still differ. European labels show both kilojoules and kilocalories, most nutritionists outside North America prefer kilojoules, and American nutritionists mostly stick with kilocalories while physiologists lean towards the joule. For food, fat supplies about 9 kilocalories per gram, alcohol 7, and carbohydrate and protein roughly 4 each. Chemists still sometimes quote reaction energies in kilocalories per mole, though the joule keeps gaining ground.
Source: Calorie
