e urine. The chief end-product in
man is urea. This substance represents from 82-88% of the total
nitrogen excreted by the kidneys. However, the less highly oxidized
products represent the incomplete products of protein metabolism and
thus indicate the changes through which these products must pass
before being changed into urea. If for any reason there is an
impairment of the liver through which they must pass and where the
change into urea is accomplished, there will be a rise of ammonia and
a corresponding decrease in the output of urea in the urine. Thus,
ammonia is formed at the expense of the urea. This occurs in fevers,
diabetes, and certain structural diseases of the liver. According to
Sherman:[60] "Normally about 2 to 6% of the total nitrogen eliminated
is in the form of ammonium salts, the amount depending largely upon
the relation between the acid-forming and base-forming elements in the
food."

~Acid-forming and Base-forming Foods.~--Mendel[61] states: "There are
foods which act as potential acids and others which function as bases
in the organism. When burned up either in the laboratory or in the
body cells, they have a preponderance of acid or base, as the case may
be, in their ash." In this respect potatoes, apples, raisins, and
cantaloupes, for example, are base-forming foods which depress the
output of ammonia and increase the solubility of uric acid in the
urine, whereas meal, cereals, and prunes (the latter with their
content of benzoic acid) furnish acids in predominance.

~Purin Bases.~--These compounds are formed in the body as cleavage
products of nucleoproteins or taken into the body in food. The chief
of these products are ~adenin~, ~guanin~, ~hypoxanthin~, ~xanthin~,
and ~uric acid~. The latter is the most highly oxidized of all the
purin bases and is the form in which they are chiefly eliminated in
the urine.

~Formation of Uric Acid.~--The formation of uric acid can in a measure
be controlled by attention to the diet, eliminating those foods known
to be purin-bearing. Normally from 1 to 3% of the nitrogen eliminated
will be in the form of uric acid. The normal human being oxidizes
about half of the purins eaten and excretes about half, mainly in the
form of uric acid. According to Mendel, the formation of uric acid
takes place throughout the body, and its partial destruction is
accomplished by the kidneys, muscles, and liver. The formation of
purins in the body and their elimination in t