The three simple sugars, on the other hand, immediately proceed to the liver as soon as they are absorbed by the blood. There in some unknown way they are all converted to one substance, glycogen or animal starch (which also belongs to the class of carbohydrates). All carbohydrate food ultimately finds its way to the liver where it is stored as glycogen. Then whenever the body needs to expend energy, the glycogen reserve is immediately called into play. Through another mysterious process, the glycogen is converted back to glucose - but to glucose exclusively - which in turn is ultimately broken down to carbon dioxide and water, at the same time liberating energy in the form of heat. This conversion of glucose into carbon dioxide and water is a "burning" or "oxidation" process brought about by the cells. This process, like the formation of glycogen and the conversion of the glycogen to glucose, is still very imperfectly understood.

The value of a protein must be judged by the value of its structural units, the amino-acids; is the value of a carbohydrate to be judged by its structural units, glucose, fruit sugar and galactose? Is one of them more essential than another? Among amino-acids we have found lysine and tryptophane to be much more important than glycocoll; can we say for example that glucose is more important than fruit sugar?

* The discarded amino-acids find their way into the liver where, through a process of " de-aminization," part of the amino-acids are finally oxidized to carbon dioxide and water, yielding energy, and the nitrogenous parts find their way into the urine chiefly in the shape of urea.

No definite answer to this question can be given; and this despite much clinical work on the subject. But it must be confessed that this phase of metabolism has not received as exhaustive a test as protein metabolism.