Let us deal with the carbohydrates first. They are chemically a much simpler variety of substance than the proteins. Some of them, such as starch and cane sugar, are, however, still too complicated to be absorbed by the body without preliminary simplification. Others, on the other hand, are absorbed without any change. Among the latter may be mentioned glucose and fruit sugar.

The Three Simple Carbohydrates

Now no matter what carbohydrate we eat, provided it is digestible - cellulose is an example of a carbohydrate which is not - and provided it needs further simplification before absorption, the juices in the digestive tract will always change it to one of three simple substances. Two of them have already been mentioned - glucose and fruit sugar. The third, galactose, is a product derived almost exclusively from milk sugar.

All carbohydrates then are changed, before absorption, to one or more of three substances: glucose, fruit sugar and galactose.* Here you see some analogy between these three and the amino-acids. But remember that we have eighteen amino-acids derived from proteins, in contradistinction to three "simple sugars" derived from complex carbohydrates; which immediately conveys an idea of the greater complexity of the proteins.

But if the three simple sugars bear some resemblance to the amino-acids, their behavior subsequent to absorption shows no such resemblance. You will remember that the amino-acids are taken up by the blood stream and conveyed to the various cells. There the cells utilize their property of selective action by selecting the amino-acids they need for tissue up-building and repair, discarding the remainder.*

* Mannose and sugars known as the pentoses are sometimes present, but the occasions are rare. The pentoses are more abundant in the plant kingdom.