THE GENERAL NATURE OF DIET 139 



probably, as was seen in our first chapter, perhaps in the cyanogen- 

 combination, lies the vital nexus. Why animal organisms cannot take 

 this nitrogen from albuminoids or from salts containing nitrogen, science 

 at present cannot tell us. The hemoglobin-like bioplasm of the vegetal 

 kingdom seems to do this paramount work for all living organisms. 



The five food-components which are surely sources of energy in the 

 body (proteids, albuminoids, fats, carbohydrates, and mineral salts, with 

 the possible addition of wafer) produce this energy in a multiplicity of 

 ways of chemical reaction. These methods are mostly katabolic in trend, 

 but some are partly anabolic. Some, too, of the reactions doubtless occur 

 in other ways which could be classed technically as neither anabolic nor 

 katabolic, since the large majority of chemical processes are productive of 

 some degree of heat. In the case of fats and carbohydrates the general 

 reaction is doubtless oxidation, a good part of the energy produced chemi- 

 cally in animals being derived from this source. The katabolism of 

 proteids and of albuminoids is more complex than that of fats and car- 

 bohydrates, but especially in case of the tissue-proteids (as distinguished 

 from the circulating proteids, mostly serum albumin and serum globulin) , 

 is partly a process of oxidation also. One must not lose sight of the simple 

 parallelism between an organism and a steam-engine in that in both cases 

 the actual burning of fuel, largely carbonaceous, is necessary for the 

 production of energy. This is expended partly as work, " the overcoming 

 of resistance," and partly as heat, which latter is quite essential in the 

 organism, but not in the steam-engine. There are, besides these, other 

 modes of vital energy-expense, mentioned in the chapter on Nutrition. We 

 shall soon see (under Calorimetry, p. 140) how accurately the total energy 

 set free in an organism can now be measured, and how fully the com- 

 bustion-values, although not too strictly the organic energy-production, 

 of any nutrient can be estimated and allowed for in the study of diets. 



Most foods in actual use contain both energizers and tissue-producers, 

 but some important foods do not. No animal, no man, for instance, 

 could live on pure sugar, butter, or starch, for these contain nothing 

 which could replace the wasting protein muscles whereby his body and 

 its organs are moved. Man could not live on gelatin, for although it con- 

 tains the needful nitrogen, the latter is for some reason locked up in 

 unavailable combination. On simple bread, potateos, rice, corn, 

 oatmeal, or flesh (water being added in each case) a person could, if 

 necessary, live a long time, for each of these contains proteids, carbo- 

 hydrates, fat, and inorganic salts, although in very various proportions. 

 On eggs alone or on milk alone life endures in theory for an unlimited 

 time, for these are, of course, the sole diet of young birds before hatching 

 and of all young mammals. In practice, however, it is doubtful if a 

 normal human adult could stand the strain of living solely on either of 

 these for many years surely not in a well-nourished condition. Our 

 first requirement of an adequate diet is that it contains both tissue- 

 builders and sources of vital energy, but considerations other than this 

 are essential. 



