oved that the energy for
external and internal work is produced largely from the glucose
brought by the blood and oxidized in the muscles.

When a surplus amount of carbohydrate food is eaten, over and above
the immediate needs of the body for fuel, it is stored in the liver
and muscles as glycogen, which can be readily reconverted into
glucose. When the supply of carbohydrate food is greatly in excess of
the body's needs, that is when the liver and muscles cease to store
glycogen, it is built up into adipose tissue and furnishes a readily
available source of emergency fuel.

~Fate of the Carbohydrates.~--After their oxidation the end products
of carbohydrates, that is, the substances which are no longer
available for use in the body, leave it in the form of carbon dioxide
and water by way of the kidneys (urine), the skin, the lungs, and the
intestines.

~Fate of the Fats.~--The fats upon absorption are taken up by the
lymph vessels instead of the capillaries and enter the blood with the
lymph. According to various investigators, the fat which causes the
turbidity of the blood plasma at the height of absorption will, as a
rule, disappear after a few hours, part of it being burned as fuel,
producing energy for the internal and external work of the body, and
at least a part of the fats eaten being rebuilt into body fat. The
end-products of fat metabolism, like those of the carbohydrates,
consist of carbon dioxide and water, and leave the body by the same
excretory channels.

When the normal oxidation of the fatty acids is interfered with or is
overtaxed, a different reaction from that which usually occurs may
take place, and this results in an excretion of acetone in the urine
(see Chapter on Diabetes).

~Protein metabolism~ is certainly more complex than that of either of
the other active organic food groups. The amino acids which are the
products of protein digestion are taken up by the capillary blood
vessels in the intestinal walls and are passed by them into the portal
vein, soon to become available for the needs of the body.

~Fate of the Proteins.~--After utilization in the body, the proteins,
like the other foods, leave certain waste products which indicate to a
greater or lesser extent the completeness with which the organism has
made use of the food materials. The end-products of protein metabolism
are: ~urea~, ~ammonium salts~, ~purin bodies~, and ~creatinin~. These
products leave the body chiefly in th