as fuel for generating heat and
energy. Carbohydrates form but a small proportion of the body-tissue,
less than one per cent. Starches, sugars, and the fibre of plants, or
cellulose, are included under this term. They serve the same purpose as
fat.

All dietitians are agreed that protein is the essential combined in
food. Deprivation of it quickly produces a starved physical condition.
The actual quantity required cannot be determined with perfect accuracy,
although estimates can be made approximately correct. The importance of
the other nutrient compounds is but secondary. But the system must have
all the nutrient compounds in correct proportions if it is to be
maintained in perfect health. These proportions differ slightly
according to the individual's physical constitution, temperament and
occupation.

Food replenishes waste caused by the continual wear and tear incidental
to daily life: the wear and tear of the muscles in all physical
exertion, of the brain in thinking, of the internal organs in the
digestion of food, in all the intricate processes of metabolism, in the
excretion of waste matter, and the secretion of vital fluids, etc. The
ideal diet is one which replenishes waste with the smallest amount of
suitable material, so that the system is kept in its normal condition of
health at a minimum of expense of energy. The value, therefore, of some
general knowledge of the chemical constituents of food is obvious. The
diet must be properly balanced, that is, the food eaten must provide
the nutrients the body requires, and not contain an excess of one
element or a deficiency of another. It is impossible to substitute
protein for fat, or _vice versa_, and get the same physiological result,
although the human organism is wonderfully tolerant of abuse, and
remarkably ingenious in its ability to adapt itself to abnormal
conditions.

It has been argued that it is essentially necessary for a well-balanced
dietary that the variety of food be large, or if the variety is to be
for any reason restricted, it must be chosen with great discretion.
Dietetic authorities are not agreed as to whether the variety should be
large or small, but there is a concensus of opinion that, be it large or
small, it should be selected with a view to supplying the proper
nutrients in proper proportions. The arguments, so far as the writer
understands them, for and against a large variety of foods, are as
follows:--

If the variety be large