1. Carbohydrates

These are absorbed as monosaccharids transported to the liver, built up into glycogen, stored there for a time, then when they are needed, broken up into monosaccharids again, carried in the circulation to any tissues where active work is going on, especially to the muscle tissue, which is a part of the body that is always active. In the active tissue the sugar is oxidized to CO2, and H2O, the former carried into the lungs and thrown out in exhalations.

Fig. 6. - Showing Diagrammatically the Changes to which Foodstuffs are Subjected in the Body. Note that they enter the body at a high level and leave it at a low level, all or nearly all of the energy having been liberated through oxidation.

2. Fats

Fats are absorbed either in emulsion or as fatty acids in glycerin. The latter are reconstructed into fats and passed on along with the absorbed emulsion into the lacteals, where they exist in an exceedingly fine state of division, passed through the thoracic duct into the venous system, thence, by way of the circulation, to the active tissues of the body. Here they undergo oxidative changes, which release their latent energy and result in the formation of carbon-dioxid gas and water, which is excreted in the same way as similar substances arising from the oxidation of carbohydrates.

3. Proteins

These are absorbed as peptones, changed in the epithelium of the small intestine to serum albumin and serum globulin, and passed for the most part into the portal system. The proteins undergo no further anabolic change and circulate in the blood until used by the tissues. A certain amount of these proteins, as described in Part I, are used to repair or build tissues of the body. Any amount of protein over and above the amount needed for growth and repair is promptly oxidized in the active tissues, yielding the energy peculiar to the tissue in which it is oxidized. Products of oxidation - most of them nitrogenous, some containing also phosphorus and sulphur - are transported to the liver, where they are elaborated for excretion by way of the kidneys.