14 
INTRODUCTION 
foods contain large amounts of starch. Starch is easily 
changed to glucose by the digestive fluids. It furnishes some 
of the energy of heat and motion for the body. 
Glucose. — Glucose is a nutrient that occurs in grapes and 
in most other fruits. It is slightly different in composition 
from cane sugar or beet sugar. Starch is always changed into 
glucose by the digestive fluids and never into cane sugar. 
The food value of both glucose and cane sugar is high. 
They are used by the body to furnish the energy of heat and 
motion. 
Enzymes. — In the preceding paragraph it was said that 
the digestive fluids change starch into glucose. For many 
years it was not known what substance in the digestive fluid 
causes it to do this work. Now it is known that certain 
substances called enzymes are the real active agents in diges¬ 
tion. The enzyme that changes starch to glucose is called 
diastase. Pepsin is an enzyme that digests protein. There 
are many other enzymes. An enzyme is not changed by 
digestion, so that a very small amount of it digests a large 
amount of food. 
Proteins are nutrients that are found in great variety in 
our food. The proteins differ from starch and sugar in 
having nitrogen in combination with other elements. The 
characteristic element of protein is nitrogen. Each kind 
of protein has its own name. The protein in wheat is gluten; 
in beans it is legumin. Casein is the protein in milk and 
myosin is the protein in meat. The proteins are necessary 
in building and repairing the cells of animal bodies. If more 
proteins are eaten than necessary for this work, the surplus 
may be oxidized to furnish the energy of heat and motion or 
stored as fat. 
Edible Fats. — The term edible fats is used to include 
both fats and oil. These nutrients are almost wholly energy 
producers. They are used chiefly to produce heat. As we 
go north or south from the equator, we find the native people 
