NUTRITION 



4317 



NUTRITION 



of lettuce. Or, expressed in a different way, 

 fuel enough for a man at moderate muscular 

 work could be obtained from about fourteen 

 ounces of butter, but it would require over 

 two and one-half pounds of bread, nine and 

 one-half pounds of milk, ten and one-half 

 pounds of apples, or thirty-five pounds of let- 

 tuce. These are but a few of the figures which 

 might be given, for the fuel value of practically 

 every known food material has been deter- 

 mined and is recorded in the publications of 

 Departments of Agriculture and in many text- 

 books. 



Fuel Value of Different Foods. The great dif- 

 ferences between the fuel values of foods are 

 to be accounted for in two ways: First, some 

 foods contain far more water than others, and 

 water, of course, cannot be burned. Lettuce, 

 for example, is nine-tenths water, while wheat 

 flour is only one-tenth water. Some foods, par- 

 ticularly those that are manufactured, like cane 

 sugar and salad oils, contain no water. Second, 

 the burnable material in different foods dif- 

 fers widely in character. It may be sugar, 

 starch, fat or protein, or a mixture of two or 

 more of these, in any proportions. Fat has 

 two and one-fourth times the energy value of 

 any of the others, and for this reason its pres- 

 ence in a food material contributes more to 

 fuel value than the presence of the same 

 amount of any other nutrient. 



A given amount of energy, whether that 

 needed by the manual laborer or the profes- 

 sional man, could of course be obtained from 

 one food material or from any one of an almost 

 unlimited number of combinations of materials 

 from bread alone or from bread and butter, 

 or from bread, butter and sugar. People in- 

 stinctively cat many different kinds of food, 

 however, and investigators are trying to dis- 

 r the reasons for this. They have noted 

 the effect of different kinds of diet on growth, 

 h :md working capacity. In addition, they 

 have studied the character of the substances 

 produced by the burning of various kinds of 

 foods in the body, with a view to getting a 

 to the character of the foods themselves. 



It is plain, of course, that matter must be 

 constantly passing off from the body, or even 

 an adult would increase very rapidly in weight, 

 for the average person takes into his body 

 several pounds of food, water and oxygen every 

 day. Investigation has shown that there pass 

 off daily through tho lungs and the skin very 

 large amounts of carbon dioxide and of water. 

 The former is always produced when substances 



containing carbon, and the latter when sub- 

 stances containing hydrogen, are burned. Much 

 of the water given off enters the body as such, 

 but the amount excreted is more than can be 

 accounted for in this way. Part of the water, 

 therefore, and all of the carbon dioxide ex- 

 creted are considered to be products of the 

 burning sugar, starch, fat and other compounds 

 containing carbon and hydrogen. 



Urine, another waste product, contains not 

 only a large amount of water but also many 

 solid substances in solution. The chief of 

 these, so far as amount is concerned, is urea, 

 which contains not only carbon, hydrogen and 

 oxygen, but also nitrogen, an element abso- 

 lutely indispensable to life. Urea is present 

 even in the urine of a fasting person, which 

 indicates that it must be produced by the de- 

 struction of the tissues and that nitrogen in 

 some form is a necessary ingredient of repair 

 material. Analysis of food materials shows that 

 nitrogen is not present in starch, fat or sugar, 

 but only in a class of compounds known as 

 proteins. These are found most abundantly, 

 as compared with other fuels, in milk, cheese, 

 eggs, flesh foods, legumes (beans, peas and len- 

 tils), and some of the nuts. 



The "Building Stones" of the Body. The 

 amount of fuel which should be in the form of 

 protein is still a matter of debate. A few facts 

 will serve to show the kind of problem in- 

 volved. The nutrients (fats, sugar, starch, pro- 

 tein and others) are extremely complex sub- 

 stances. They break up, however, during the 

 process of digestion, into much simpler com- 

 pounds. Some of these simpler compounds are 

 fit only to be burned and to provide energy. 

 Others are suitable to be absorbed into the 

 living tissues and to serve as part of the mech- 

 anism of the body. They are in time, to be 

 sure, themselves cast out and replaced by other 

 similar compounds, but in the meantime they 

 have fulfilled a most important mission. The 

 substances thus capable of becoming part of 

 the living organism and of contributing to its 

 strength are sometimes called, even in scien- 

 tific literature, the "building stones" of tin- 

 human body. They are many and varied, some 

 being suitable to enter into muscle tissue, others 

 into cartilage, bones, nerves, etc. The pro- 

 teins of the foods which supply many of the 

 most indispensable of tlu-e -building stones" 

 are also varied, thoso in meat, for example, 

 differing from those in eggs, milk, nuts and 

 flour just as the proteins of these, substances 

 differ among themselves. 



