700 PHYSIOLOGY 







differ in different races and be larger in northern than in southern races. 

 Thus the Japanese soldier is content with 20 grm. of fat daily, while in our 

 country the average man would not be content with less than 75 grm. of 

 fat per day. Seventy-five grammes of fat has a Calorie value of about 680 

 i. e. nearly 25 per cent, of the daily requirements, and we may conclude 

 as a .general rule that one-fourth of the total energy of the diet should be 

 supplied in the form of fat. 



There are probably three reasons for this craving for fat. In the first 

 place, fat is easily assimilated and is almost entirely absorbed in the 

 alimentary canal; but whereas the greater part of the carbohydrate food 

 is absorbed three hours after food has been eaten, the chief absorption 

 of fat occurs between five and six hours after a meal. On this account 

 a meal lacking in fat is deficient in staying power, and individuals 

 deprived of fat get hungry sometimes before the next meal and their work 

 and efficiency diminishes. The lumbermen of Canada satisfy their huge 

 needs in Calories 6000 to 8000 a day by a diet in which there is a large 

 proportion of fat pork and in which the fat represents 35 to 40 per cent, 

 of the Calories. 



In the second place, the bulk of food must be of considerable importance, 

 especially when the total food required by the energy needs of the body is 

 very large. Weight for weight fat has more than double the Calorie value 

 of starch and sugar. But the difference in bulk is still greater, since fat is 

 taken without admixture in a pure form, whereas the other foods are all 

 mixed with a considerable proportion of water. The proteins in meat 

 form only 15 to 20 per cent, of the total bulk. Starch cannot be taken except 

 mixed with large quantities of saliva. When ordinarily cooked it is swollen 

 up with probably five to ten times its amount of water. Even after absorp- 

 tion the same necessity for increasing the bulk of carbohydrates with water 

 persists. Even glycogen does not occur to a larger amount in the liver 

 than 12 per cent., a figure which may be largely surpassed by fat, and in 

 adipose tissue there may be as much as 80 per cent, of fat. When carbo- 

 hydrate goes into the circulation it is changed into sugar, and as sugar it 

 needs twenty times its weight of water to carry it. It acts therefore to 

 some extent in the same way as common salt. Just as an extensive diet 

 of salt may produce dropsy, so a diet of carbohydrate increases in the first 

 place the water content of the body, and this factor, when associated with 

 inanition and fat shortage, may itself produce actual dropsy. The question 

 of bulk is probably one of the most important factors in determining the 

 need for fat. The human alimentary canal, at any rate in the races 

 of Western Europe, has been developed so as to cope with a diet in which 

 20 to 25 per cent, of the energy is presented in the form of fat. In order to 

 get the same energy from carbohydrates the alimentary canal would have to 

 be much larger. Theoretically then the absence of fat can be made up by 

 an increased supply of carbohydrates. But this can be carried out only 

 in a certain number of individuals and under certain conditions. The 

 ordinary individual deprived of fat diminishes his total intake of food and 



