COMPOSITION OF FOOD AND ACTION OF ENZYMES. 



655 



molecules into simpler compounds. The chemical changes of metab- 

 olism or nutrition are, on the whole, exothermic, that is, they are 

 attended by the production of heat. Some of the chemical or in- 

 ternal energy that held the complex molecules together assumes the 

 form of heat, or perhaps muscular work, after these molecules are 

 broken down to simpler, more stable structures, such as water, carbon 

 dioxid, and urea. Proteids, fats, and carbohydrates form materials 

 that the tissue cells are adjusted to act upon after they have under- 

 gone certain changes during digestion. Other complex organic 

 compounds containing chemical energy are either injurious to the 

 tissues or they have a structure such that the tissues can not act upon 

 them. Such substances can not be considered as foods in the scien- 

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

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

 position as regards water, salts, proteids, 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: 



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 proteid or of 

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



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

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

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



