26 NUTRITIONAL PHYSIOLOGY 



Starches 1 are of high molecular complexity, imperfectly 

 soluble, and tasteless. Sugars are cleavage products of 

 starches or, if they occur apart from previously existing 

 starches, closely resemble such cleavage products. They 

 are of definite and moderate molecular weight, they are 

 soluble, crystallizable, and sweet. They contain the same 

 elements which are found in fats carbon, hydrogen, and 

 oxygen but the percentage composition is wholly differ- 

 ent and the structure of the molecule also. While the 

 carbohydrates are energetic, their fuel value is less than 

 half that of fats. 



We have now named this list of substances as uniting 

 to form the body water, proteins, mineral matter, fats, 

 and carbohydrates the order suggesting their relative 

 abundance. We have said that in the diet water also 

 takes the first place, but that carbohydrates ordinarily 

 take the second. Either proteins or fats may stand third 

 among the constituents of the diet. Often the amounts 

 of the two are found to be about equal. A possible diet 

 comprises 100 grams of protein, 100 grams of fat, and 250 

 grams of carbohydrate, illustrating this equality. The 

 mineral matter in the ration is not likely to exceed 20 

 grams a day. Both the body and its income, of course, 

 contain in small amounts substances which do not fall 

 into any of the classes named. Such miscellaneous 

 organic compounds are conveniently grouped as extrac- 

 tives. Many of them are nitrogenous and represent dis- 

 integration products of proteins. Reference to their 

 significance will be made from time to time. 



At the very outset the double service of food to the 



1 The three elements in starch are "present in proportions repre- 

 sented by the formula C 6 H ]0 O 5 . But the number of atoms in the 

 molecule is not correctly indicated by this formula; it requires to 

 be multiplied throughout by an unknown, but considerable number. 

 Sugars are of two common classes: the disaccharids, with the 

 formula CpHnOu, and the monosaccharids or hexoses, with the 

 formula CeH 12 O 6 . Cane-sugar, malt-sugar, and milk-sugar are 

 disaccharids. The hexoses of direct interest in nutrition are glucose 

 (also called dextrose), fructose (or levulose), and galactose (a deriva- 

 tive of milk-sugar). 



