COMPOSITION OF FOOD AND ACTION OF ENZYMES. 



739 



energy. The chemical changes of metabolism or nutrition are, 

 in the long run, mainly exothermic, that is, they are attended 

 by the production of heat. Some of the chemical or internal energy 

 that held the complex molecules together assumes the form of heat 

 when these molecules are broken down by oxidative changes to sim- 

 pler, more stable structures, such as water, carbon dioxid, and urea. 

 Proteins, fats, and carbohydrates form materials that the tissue 

 cells are adjusted to act upon after they have undergone certain 

 changes during digestion. Other complex organic compounds con- 

 taining chemical energy are either injurious to the tissues, or they 

 have a structure such that the tissues cannot act upon them. 

 Such substances cannot be considered as foods in the scientific 

 sense. When, therefore, we desire to know the food value of any 

 animal or vegetable product, we analyze it to determine its compo- 

 sition as regards water, salts, proteins, fats, and carbohydrates. 

 The following table compiled by Munk from the analyses given by 

 Konig * may be taken as an indication of the average composition 

 of the most commonly used foods: 



COMPOSITION OF FOODS. 



An examination of this table shows that the animal foods, par- 

 ticularly the meats, are characterized by their small percentage in 

 carbohydrate and by a relatively large amount of protein or of 

 protein and fat. With regard to the last two foodstuffs, meats differ 



*See Konig, "Die menschlichen Nahrungs und Genussmittel " ; and 

 Atwater and Bryant, " The Chemical Composition of American Food Mate- 

 rials," Bulletin 28, United States Department of Agriculture, 1899. 



