The value of a protein we have seen is specified only when stated in terms of its amino-acid content. The protein itself is complex in chemical structure, but the units out of which it is built, the amino-acids, are relatively simple; and it is the latter that the body absorbs and assimilates.

The question now arises, Is what we have found for protein also true for carbohydrates and fats? Are these two substances also chemically complex, and is it probable that the body cannot absorb these substances as such, but rather as simpler units, the "building stones," out of which the more complicated material is synthesized? Have we, in other words, a something which is related to carbohydrate and another something which is reflated to fat in the way the amino-acid is related to protein?