120 A STUDY OF FOODS AND DIETARIES 



manner protein has been shown to have about the same fuel value 

 as carbohydrates, i.e. 4 calories to a gram. 



Fats and oils have the highest energy value of all foods. But 

 because of their rather indigestible qualities and because one soon 

 tires of an excessive amount of fat in one's dietary, carbohydrates 

 are more used to release energ}^ Cereals, bread, potatoes, and 

 other starchy vegetables should, for this reason, be a part of our 

 daily dietary. 



Tissue Building and Repair of Waste. — But it is not sufficient 

 for man to '' count his calories." We are made of living matter, 

 protoplasm. Living cells may waste away, and need to be 

 repaired or replaced. New cells must be formed. We grow in 

 weight up to early manhood or womanhood, and after that the body 

 weight may rise or fall, depending on our health or constitution. 

 Evidently, then, the tissues use food for building purposes. 



We have already seen that carbohydrates, fats, and proteins 

 all contain the elements carbon, oxj^gen, and hydrogen, and that 

 proteins alone contain the element nitrogen. We have learned 

 also that this wonderful substance, protoplasm, is either a very 

 complex compound or a mixture composed of carbon, hydrogen, 

 ox^^gen, nitrogen, and ten or more other chemical elements which 

 are found in soil or rock.^ Therefore if living matter is to grow, 

 it must have the proper elements for building. Such of these ele- 

 ments as are not present in carbohydrates and fats are found in 

 proteins. Proteins, although they may be oxidized to release 

 energy, must be used to give the body its nitrogen from which, in 

 part, living protoplasm is manufactured. 



Not all Proteins are Good Tissue Builders. — Recent feeding 

 experiments have shown that not all proteins are capable of build- 

 ing tissues. It has been found that the complex chemical sub- 

 stance called protein may be broken by the chemist into simpler 

 proteins caUed am'ino-acids. Some of these amino-acids are useful 

 in tissue building, and others are not.^ If two rats are fed on diets 



^ The elements in protoplasm obtained by chemical analysis are carbon, hydro- 

 gen, oxygen, nitrogen, sulphur, phosphorus, fluorine, chlorine, silica, iron, potas- 

 sium, sodium, calcium, magnesium, and others. 



2 It is found that some proteins will form new tissue, others may be used in the 

 body only to maintain body weight, and still others contain no amino-acids capable 

 of being used in tissue building. 



