pounds variously designated as food
stuffs, proximate principles, nutritive ingredients or nutrients, which
are classified as _protein_, _fats_, _carbohydrates_ and _mineral
matters_. These have various functions in the nourishment of the body.

The _refuse_ commonly contains compounds similar to those in the food
from which it is derived, but since it cannot be eaten, it is usually
considered as a non-nutrient. It is of importance chiefly in a
consideration of the pecuniary economy of food. _Water_ is also
considered as a non-nutrient, because although it is a constituent of
all the tissues and fluids of the body, the body may obtain the water it
needs from that drunk; hence, that contained in the food materials is of
no special significance as a nutrient.

_Mineral matters_, such as sulphates, chlorides, phosphates and
carbonates of sodium, potassium, calcium, &c., are found in different
combinations and quantities in most food materials. These are used by
the body in the formation of the various tissues, especially the
skeletal and protective tissues, in digestion, and in metabolic
processes within the body. They yield little or no energy, unless
perhaps the very small amount involved in their chemical transformation.

Protein[1] is a term used to designate the whole group of nitrogenous
compounds of food except the nitrogenous fats. It includes the
albuminoids, as albumin of egg-white, and of blood serum, myosin of meat
(muscle), casein of milk, globulin of blood and of egg yolk, fibrin of
blood, gluten of flour; the gelatinoids, as gelatin and allied
substances of connective tissue, collagen of tendon, ossein of bone and
the so-called extractives (e.g. creatin) of meats; and the amids (e.g.
asparagin) and allied compounds of vegetables and fruits.

The albuminoids and gelatinoids, classed together as proteids, are the
most important constituents of food, because they alone can supply the
nitrogenous material necessary for the formation of the body tissues.
For this purpose, the albuminoids are most valuable. Both groups of
compounds, however, supply the body with energy, and the gelatinoids in
being thus utilized protect the albuminoids from consumption for this
purpose. When their supply in the food is in excess of the needs of the
body, the surplus proteids may be converted into body fat and stored.

The so-called extractives, which are the principal constituents of meat
extract, beef tea and the like, act pr