Food ValueSince carbohydrate are composed of units consisting of a carbon atom and a molecule of water, one molecule of oxygen will be required to oxidize a unit to CO 2 and water: C – H2 O + O2 = C O2 + H O2. The molecular respiratory quotient O2 consumed / CO 2 produced is therefore 1.00. The combustions of 1 gram of glucose yield about 3.8 kcal, while 1 gm of starch yields 4.1 kcal. The difference arises from the smaller proportional of water in starch.

Sucrose and lactose cannot be utilized at all, and maltose only to a small extent (by the intervention of amylase), by the tissues. If they gain access to the body, therefore, they are quantitatively excreted in the urine, since the traverse, the glomerular filter freely and are not appreciably reabsorbed from the renal tubules.

The common starches of food and all glycogen are composed entirely of glucose. Both are hydrolyzed in the intestines by pancreatic amylase, the glucose from them being absorbed. Glucose from any source may enter cells where, if it is not utilized for other purposes, it is converted to glycogen. This can again be broken down when carbohydrate is required. The transformation of an enormous number of relatively small glucose molecules into one gigantic glycogen molecule enables to cells to store and to discharge sugar with a negligible disturbance of osmotic pressure.