136 STORRS AGRICULTURAL EXPERIMENT STATION. 
more easily if it is put in terms of “flesh formers” and “fuel 
values” than if he must consider protein, fats, and carbohydrates 
in the ways which have become so generally current. 
Food has two chief functions—to build tissue and serve as 
fuel. Inthe building of tissue from food and in the constant 
breaking up and repair of tissue we have to do with the metabol- 
ism of matter. In the service of food and of tissue material as 
fuel, to yield heat and muscular power, we have to do with the 
metabolism of energy. The protein compounds are the tissue 
formers. The fats and the carbohydrates are the chief fuel 
ingredients; but protein compounds also serve as fuel. In this 
service as fuel the nutrients replace each other in proportion 
to their potential energy. The economy of food in nutrition 
requires sufficient protein for the formation of tissue and sufficient 
energy for supplying heat and strength. The fuel value of food 
is thus one of the two chief factors of its value for nutriment. 
The fat of the food is stored as fat in the body. The protein 
and carbohydrates of the food are transformed into body fat. 
The fuel value of the food thus becomes an indication of its 
capacity for fat formation. 
The fat of the body is its reserve store of fuel. The fuel value 
of the fats is more than twice as great, weight for weight, as that 
of the protein or carbohydrates. Fat is body fuel in its most 
concentrated form. ‘Therein lies the economy of nature in the 
storage of fat in the body. 
Of course the whole doctrine of nutrition is not as simple a 
matter as these statements would imply, but they do represent its 
fundamental principles. One important part of the problem is 
the source of intellectual energy. Doubtless this comes, in some 
way, from the food, but what it is and how it is produced and 
what food materials are most efficient for its production are 
questions that the experimental science of to-day are far from 
answering. 
In the metabolism of matter and energy we have the foundation 
of the theory of nutrition, the starting point of experimental inquiry, 
and the means of simplifying the theories which we have to teach. 
It is thus clear that the experimenter who will study thoroughly 
the laws of nutrition and the uses and the nutritive values of food 
must have a means for determining the potential energy of the 
ingredients of the food and of the compounds of the body which 
are formed from them. Since this potential energy is measured 


