It is not within the scope of this book to deal with the science of nutrition; but a few general principles may be given which concern the effect upon the system of the different classes of food. Animal food requires a considerable quantity of oxygen for its complete combustion. Meat in general has a more stimulating effect upon the system and is more strengthening than vegetable food. There is, however, a tendency to eat too much meat, and when its effects are not counter-balanced by free outdoor exercise, it causes biliousness and sometimes gout and other troubles. Albuminous foods can be eaten longer alone without exciting loathing than can fats, sugars or starches. A carbonaceous diet taxes the excretory organs less than animal food. Meat is not necessary to life. Nitrogenous food man must have, but it need not be in the form of meat. The estimate commonly given is, that meat should occupy one-fourth and vegetable food three-fourths of a mixed diet, but in many cases the meat eaten is much in excess of this allowance. The proper association of different foods always keeps healthy people in better condition; there are times, however, when it may be necessary to abstain from certain articles of diet. It may be well to bear in mind, that the protein compounds can do the work of the carbohydrates and fats in being consumed for fuel, but the carbohydrates and fats cannot do the work of protein in building and repairing the tissues of the body. As already stated, a mixed diet is the only rational one for man. An exclusively vegetable diet, while it may maintain a condition of health for a time, eventually results in a loss of strength and power to resist disease. Therefore it is necessary to understand the approximate value of each class of food in arranging the daily dietary.