The Physics
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Energy Density of Protein

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Schraer, William D. and Stoltze, Herbert J. Biology: The Study of Life. New York: Prentice Hall, 1999: 151. "With the use of the calorimeter, it has been determined that the amount of heat given off by 1 gram of protein is about 4 kilocalories." 16.8 MJ/kg
Esminger, Audrey H. and Esminger, M.E. The Concise Encyclopedia of Foods and Nutrition. Boca Raton, FL: CRC, 1995: 388. "However, energy needs of the body have a higher priority. As a source of energy, protein yields about 4 kcal/g." 16.8 MJ/kg
Veterinary Diet Manual. Ohio State University Veterinary Hospital Nutrition Support Service. 29 Nov 2001. "To multiply the % of kcal as protein or carbohydrate, multiply the grams by 4; for fat multiply by 9." 16.8 MJ/kg
American Meat Institute Foundation. The Science of Meat and Meat Products. San Francisco: Reinhold, 1960: 60. "Where energy values are given, they have been calculated on the basis of 4 calories per gram for protein and carbohydrate, and 9 calories per gram for fat." 16.8 MJ/kg
Ahrens, Richard A. Protein. World Book Online Americas Edition. 14 May 2003. "Proteins produce about 1,800 calories per pound (4 calories per gram), the same amount provided by the carbohydrates." 16.8 MJ/kg

A nice, tasty tenderloin steak lying on a plate just waiting for you is just what you want after a long day of hard work. At the same time you have your eyes on that juicy piece of meat, you still want to look good and maintain the minimal weight that you "always" see on your scale. Unfortunately, a person's desire for a tasty food, that is usually not the healthiest, is fulfilled and the person's health suffers. Well, in order to maintain a healthy body, a person must always consider the energy density of the foods they eat on a daily basis. The energy density of a food is the amount of energy provided per mass. In order for the body to function, it uses up this energy, and whatever energy is left over will be seen when the person looks in the mirror. This is so because foods that have a higher energy density are rich in fat. Also,the water content and the presence of fiber also influence energy density. The quantity of water in a food increases the weight of the food, but it doesn't increase its calorie value.

Foods like carbohydrates, proteins, and fats that provide energy to the body have an energy density. The energy density of proteins and carbohydrates is 4 cal/g, while the energy density of fat is 9 cal/g. The kilocalorie is the unit that is commonly used in the US to measure the energy content of food. Though, the joule is the unit of energy in the International System of Units or Le Systeme International d' Unites (SI). One food calorie equals 4200 joules. Also, the kilogram is the SI unit for mass. In SI units the energy density of protein and carbohydrates is 16.8 MJ/kg and of fats is 37.8 MJ/kg.

If a person maintains the usual volume or energy density of the food they eat, yet lower the calories in proportion, they'll consume fewer calories and feel just as full. Basically this means that if a person eats a pasta salad with less pasta and more vegetables, they will still be just as full as if they ate a pasta salad with more pasta and less vegetables. The only difference between the two salads would be the amount of calories per salad, the salad with more vegetables would of course be lower in calories and it would in effect have a lower energy density. This type of strategy can decrease the calorie content of meals by 30%.

David Dukhan -- 2003