THE INTAKE OF MATERIALS AND ENERGY 57 



a carbohydrate, and the energy derived from the oxidation of fat is 

 correspondingly greater. The energy value of fats in human metabolism 

 is about two and one-fourth times that of carbohydrates. Fats are stored 

 as such in many parts of the body, especially in the deeper layer of the 

 skin and around the intestines. 



Proteins contain nitrogen and sulphur and sometimes other elements 

 in addition to carbon, hydrogen, and oxygen. Proteins cannot pass through 

 cell membranes; under suitable conditions, however, they can be split 

 into amino acids and in this condition become available to the cell. The 

 amino acids are the building stones from which proteins are constructed. 

 They are nitrogenous derivatives of the fatty acids. Glycine, CH 2 .NH 2 .- 

 COOH, is an aminoacetic acid, and the simplest member of the amino- 

 acid group. At least 23 such acids are known, some of which are more 

 important in human physiology than others. The various amino acids 

 are combined in the cells to form the particular proteins required by each 

 cell. There is little storage of excess proteins in the body tissues, and new 

 supplies are therefore constantly needed. Proteins are essential for growth 

 and repair, but they may also be used for fuel. When they are oxidized, 

 the nitrogenous part is split off and excreted as a waste product by the 

 kidneys. 



Vitamins. The vitamins are a numerous and varied group of organic 

 substances essential to the maintenance and proper functioning of the 

 body but required in extremely small quantities. They are used in too 

 small amounts to be sources of energy; their importance lies in the fact 

 that they are catalytic agents, without which certain indispensable 

 reactions cannot occur. Their action is quantitative and markedly 

 specific. Substances are classed as vitamins largely as a matter of con- 

 venience; vitamin is a functional term and not a chemical classification. 



Recognition of the existence of vitamins has come from the study of 

 disease, or malfunctioning, associated with some type of diet and found 

 to be curable by some other type of diet. By chemically fractionating the 

 curative diet and determining which of the fractions has retained the 

 curative properties, the search has been gradually narrowed. Today many 

 vitamins have been isolated and their chemical formulas determined. 

 Much of the experimental work has been done with laboratory animals, 

 and the results have later been applied in medical practice. 



The very existence of these essential substances was unknown prior 

 to 1881, and most of our information about them has been gained since 

 1920. Although knowledge of vitamins is recent, certain human diseases 

 were long ago ascribed to deficiences in diet. Experience had shown that 

 rickets could be cured by liver or liver extract, beriberi by rice polishings 

 or by whole rice, and scurvy by fresh fruit and especially by the juice 

 of limes and lemons. Between 1907 and 1920, it was demonstrated that 



