The food analysis consisted in the following determinations: The nitrogen content, from which the protein was calculated by multiplying the value found by 6.25, on the assumption that all proteins contained approximately 16 per cent of nitrogen; the fat, by extracting the food with some solvent, usually ether, and weighing the amount of substance which was extracted from a known amount of food; the "crude fiber," or cellulose, by treating a sample of the food with a solution of acid and then with one of alkali sufficiently strong to dissolve all components except the more resistant forms of cellulose, and a small amount of mineral matter which this cellulose retained; the ash, or mineral matter, by burning a weighed sample and subsequently weighing the residue left after incineration. The analysis was made on a sample of food which was carefully dried to constant weight in order to free it from moisture. The sum of the protein, fat, ash and fiber, was then subtracted from 100 to give a figure which was designated "nitrogen-free extract." This was assumed in calculations used in estimating the amount of food necessary to meet the requirements of an individual as being essentially a measure of the amount of utilizable carbohydrate contained in the food. Such in outline is the chemical analysis of a food. It will be seen later that the biological method of analysis is very essential to complete our knowledge of the nutritive value of foods.

By 1900 very careful determinations had been made of the amount of heat liberated when a definite weight of the food is burned. This was known as the calorific or fuel value. Numerous determinations had been made of the amount of energy required for a man of ordinary size, per day, when at rest and when doing several kinds of work representative of moderate and more severe kinds of labor. The illustration most widely used by writers on nutrition, was that of comparing the body to an engine, in which fuel (food) was burned and by which mechanical work was performed, with the liberation of a certain amount of waste energy in the form of heat. The adjustment of the supply of calories, or energy units of food, to the needs of the individual, and the amount of protein necessary to make good the "wear and tear" of the tissues, which is an unavoidable result of the life processes, were considered as the two factors of prime importance in the planning of the diet.