142 FOODS 



The unit of measurement in all researches of this sort, very necessary 

 to be well understood, is the calorie or, formerly, the millecalorie (-nnnF 

 part of the calorie proper). Sometimes one unit and sometimes the 

 other is used, but they are so unlike in size that confusion can scarcely 

 arise. A calorie is the quantity of energy, expressed as heat, necessary 

 to raise the temperature of one kilo (1000 gm., 1000 c.c., or one liter) 

 of pure water 1 C. A millecalorie is the energy in the form of heat 

 needful to raise 1 c.c. of water 1 C. The absolute combustion- or 

 heat-value of any nutrient is, then, the number of calories of energy 

 liberated by its complete union with oxygen. It matters not a bit 

 whether it be in a furnace of the laboratory or in the circulation and 

 tissues of an animal. To be exact, however, oxidation in the organism 

 might not extend to every particle of any ingested mass of food even if all 

 were absorbed, and all of any food is seldom wholly absorbed. There 

 is invariably waste of one sort and another, as the composition of the 

 feces shows. Still, in general terms, for theory's sake, we may say that 

 any given quantity of food gives out as much energy when consumed 

 by the body as when burned in an ignition-tube. It has been one of 

 the tasks of calorimetrists to ascertain by experiment the "combustion- 

 equivalents" of all important articles of diet, the differences between 

 their laboratory combustion- values and their intra-organic energy- values 

 being so small as to be negligible. A complete table of such deter- 

 minations has less interest and physiological value, however, than it 

 would have were the conditions of organic usefulness in oxidation as 

 simple and as certain as those methods by which these tables are 

 derived. 



A few examples will suffice. The number of calories of heat liberated 

 in the burning of 1 gm. of average dried bacon is 8.86, that is, in the 

 union of oxygen with 1 gm. of bacon just enough heat is liberated to 

 raise the temperature of 8.86 liters of water 1 C. Similarly, the 

 combustion-equivalent of fat mutton is 4.03 calories; of fat beef, 3.27; 

 of white bread, 2.74; of eggs, 1.59; of lean beef, 0.98; of potatoes, also 

 0.98; of milk, 0.70; of apples, 0.74; while lettuce yields only 0.20 calories 

 of heat per gram. Thus, bacon is adapted to be wisely eaten on an 

 active day in winter, and lettuce, apples, and milk on lazy days in 

 summer. Experience inherited as instinct very early taught many 

 animals this dietetic principle, and today among human beings it has 

 become settled in universal custom. Thus, the natives of the tropics 

 live largely on juicy fruits and vegetables, while the inhabitants far 

 north and south of the equator eat great amounts of fatty meat and 

 even, it is said, oils and fat in quantities which would be nauseating to 

 the majority of mankind nearer the equator. 



Just as every actual article of food has its combustion-equivalent, so 

 have the alimentary proximate principles, proteid, fat, and carbohy- 

 drate. Rubner found by actual ealorimetric experiments on the tissues 

 (so avoiding the more uncertain methods of calculation), allowing for 

 energy still in the excreted urea, etc., that the following three food -com- 



