FOODS AND THEIR PHYSIOLOGICAL VALUE. 315 



of muscular and heat energy is derived from the carbohy- 

 drates. They, too, form the bulk of our foods, there being 

 few dishes indeed of which either the starches or the sugars 

 do not form an integral part. In addition they are perhaps 

 the most digestible of all foods, and finally a reason not to 

 be neglected, they are possibly the cheapest of foods. It 

 seems a rather queer coincidence that the carbohydrates are 

 the foods best suited to the process of digestion, best suited 

 as sources of energy, best suited to the palate, and finally, 

 best suited to our expenses. These coincidences are, no 

 doubt, the result of dietary evolution. The close relation- 

 ship of the starches and sugars is evident from the ease with 

 which the starches are changed into sugars or sugars into 

 starches. 



4. Hydrocarbons. In the hydrocarbons are included 

 the fats and oils. As indicated by their name they contain 

 mainly hydrogen and carbon. A little oxygen is also in 

 combination, but the hydrocarbons differ essentially from 

 the carbohydrates in the fact that the hydrogen and the 

 oxygen are not present in the proportion of water. Com- 

 pared with the carbohydrates the hydrocarbons contain rela- 

 tively more carbon and hydrogen and less oxygen. For this 

 reason when they are burned they give rise to much more 

 energy. There would be quite a material difference in the 

 amount of heat liberated between the combustion of a bar- 

 rel of sugar and a barrel of oil. It is for such reasons that 

 the fats are peculiarly well suited as a diet in winter or in 

 colder climates, and the Esquimau who drinks his blubber 

 supplies himself with one of the best foods for the liberation 

 of heat. The distinction between fats and oils is a rather 

 arbitrary one. Hydrocarbons which at ordinary tempera- 

 tures are more or less solid, are spoken of as fats, those which 

 at these temperatures are in a liquid condition are called oils. 



It is well to bear in mind that fats contain no nitrogen, 

 and that therefore their physiological value in the body is 

 similar to that of the starches and sugars. It would be im- 

 possible for an animal to live long if its diet were limited 



