This term is used to include all those foods whose principal constituent is carbon. The carbonaceous foods are the fuel foods. All of these foods are composed of carbon, hydrogen, and oxygen. The sugars and starches are usually grouped together and called carbohydrates. The reason for this grouping is shown in the fact that hydrogen and oxygen of sugars and starches always exist in the proportion of two atoms of hydrogen for one atom of oxygen, that is, in the proportion to form water. Whenever the molecule is oxidized the amount of oxygen required to oxidize any carbohydrate molecule is therefore the amount required to oxidize the carbon of that molecule. The grape sugar or dextrose molecule has the formula C6H42O6. Two atoms of oxygen are required to oxidize each atom of carbon, therefore one molecule of dextrose or grape sugar requires twelve atoms of oxygen to oxidize it. Such oxidation releases six molecules of carbon-dioxid gas. A molecule of cane sugar (C12H22O11) requires twenty-four atoms of oxygen for its oxidation, and releases twelve molecules of CO2 and eleven molecules of water. In a similar way the oxidation of starch requires two atoms of oxygen for each atom of carbon, and releases five molecules of water for every six molecules of carbon-dioxid gas, the formula for starch being C6H40O5, taken n times, or (C6H40O5)n.

The fats form another group of carbonaceous foods, and differ radically from the carbohydrates in the grouping of the carbon, hydrogen and oxygen atoms within the molecules. The common food fats are usually mixtures of palmitin, stearin, and olein, olein being a liquid at all usual temperatures, the other two being solid at all usual temperatures. Olive oil is nearly pure olein; beef tallow has a large proportion of stearin. The quantitative chemical formulas for these fats are as follows:

Palmitin ...................... C3H5(C16H31O2)3

Stearin ........................ C3H5(C18H35O2)3

Olein.......................... C3H5(C18H33O2)3

Note from these formulas that the hydrogen and oxygen do not exist in the proportion to form water, there being only a very small amount of oxygen in each molecule, so that after the carbon is oxidized there still remains a large amount of hydrogen to be oxidized.* The oxidation of this hydrogen yields a great deal of heat. It must then be evident that an ounce of fat will yield far more energy when oxidized than an ounce of sugar or of starch. As a matter of fact, it yields about two and a half times as much.

It is not to be understood that the carbonaceous foods to be here enumerated contain only sugar, starches, and fats, but many of these foods, particularly the vegetables and fruits, contain admixtures of proteins, as well as of salts and water. However, these admixtures of protein, salts, and water are negligible in quantity. The classification, then, depends on the principal organic foodstuff present, which in every case is carbonaceous.