units for the measurement of energy, including energy obtained from food. In the eyes of scientists the unit is now obsolete, but the word is part of everyday language, at least in English, and shows no sign of falling into disuse.
Strictly, a calorie is the amount of heat required to raise the temperature of 1 gram of water by 1 °C. This is a very small amount, and dietitians found it more convenient to use the kilocalorie (kcal), 1,000 calories, in their calculations. Unfortunately they called this a ‘Calorie’ with a capital C. The use of the capital was often misunderstood or omitted, causing much confusion. In popular usage 1 kcal is ‘a calorie’.
In the modern system of measurement the unit of energy is the joule (J), defined as the work done when a force of 1 newton is exerted over a distance of 1 metre (a newton is roughly the force of a mass of 100 grams resting on the earth's surface). This is also inconveniently small, so dietitians use kilojoules (kJ), 1,000 joules. One kilocalorie is equal to 4.18 kJ. Dietary information on food packaging normally gives energy values in both units.
All the following figures are approximate. A very active man uses 3,600 kcal (15,100 kJ) a day, a very active woman 2,500 kcal (10,500 kJ). A sedentary man needs 2,600 kcal (10,900 kJ), a sedentary woman 2,100 kcal (8,780 kJ). Most people's needs lie between these extremes. The amount of energy used by an adult per hour is: asleep, 70 kcal (295 kJ); sitting, 85 kcal (360 kJ); standing, 90 kcal (380 kJ), walking slowly, 185 kcal (780 kJ); running at a gentle sustainable pace, 500 kcal (2,100 kJ); and walking briskly upstairs, 1,000 kcal (4,200 kJ).
The 19th-century chemist who deserves credit for being behind the whole apparatus of ‘recommended daily intakes’ and ‘energy values’ for foods, Jacob Moleschott, is the subject of an interesting essay by Jane O'Hara-May (1984).
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
O'Hara-May, Jane (1984), ‘The Moleschott Figures’, PPC 16.
