Some differences of opinion exist as to the relative value of foods of the same class. Albuminates, as has been seen, can be obtained from either the animal or vegetable kingdom ; they have a similar chemical composition, and they serve the same purposes in the body. It has, however, been suggested that they are probably utilised in a somewhat different manner, or with different degrees of rapidity, and that the man who feeds on meat, like carnivorous animals, "will be more active, and more able to exert a sudden violent effort, than the vegetarian or the herbivorous animal, whose food has an equal potential energy, but which is supposed to be less easily evolved." In support of this view it has been urged that the movements of carnivorous animals, especially in the pursuit of their prey, are far more active than those of herbivorous cattle ; that the form in which they take their food enables them to give out sudden spurts of energy of which the vegetable feeder is incapable. But this view has been questioned by others, who refer to the known activity and speed of the horse, the rapid movements of the wild antelope and cow, and even of the wild pig, all animals mostly herbivorous, as inconsistent with the conclusion that vegetable feeders cannot give forth energy as rapidly and continuously, or even more so, than the predaceous carnivora. It is further stated that with the human race also, the East Indian native, if well fed on corn, or even on rice and peas, shows, when in training, no inferiority in capacity for active physical exertion to the animal feeder. It has also been argued that the complicated alimentary canal of the herbivora pointed to a slower digestion and absorption of food; and with certain kinds of vegetable food this would certainly seem to be the case ; but it has again been contended that this is chiefly intended for the digestion of cellulose, and that the digestion and absorption of albuminates may be as rapid as in other animals. 

It must, we think, be admitted that all practical observations tend to prove that animal food is digested more rapidly than vegetable food, and it therefore seems highly probable that meat can replace the waste of the nitrogenous tissues more rapidly than meal of any kind, and it is probably true that there is a more active change of tissue in meat eaters than in vegetable feeders, and that the former require more frequent supplies of food. Apparent differences in nutritive value in different meals, as in wheatmeal and barley meal, probably depend on difference of digestibility. 

The difference in the nutritive value of different fats would seem to depend on the relative facility with which they are digested and absorbed. Animal fats appear to be more easily absorbed than vegetable. And even different animal fats differ much in digestibility, and, therefore, in nutritive value. This depends partly on chemical composition, and partly on mechanical aggregation or subdivision. Mutton-fat is generally found difficult of digestion, while pork-fat is easily digested. Butter can be readily digested by many persons who cannot digest other forms of fat and the ready digestibility of ccxl-liver oil is one of its chief advantages. 

The different carbohydrates are generally supposed to be of equal value in nutrition. Sugar, from its ready solubility, should be more easily absorbed and more quickly utilised than starch, but it is found that when both are procurable a mixture of the two is usually preferred.