The class of carbohydrates have much in common with the fats. They serve the same purpose of checking albuminous waste ; like them, they are resolved by combustion within the body, ultimately, into carbonic acid and water, and so, like the fats, are capable of yielding heat and mechanical work. Unlike the fats and the albuminates, however, they do not appear to enter into the structure of the tissues, although they are found in some of the fluids and organs of the body. 

All the carbohydrates are converted into glucose, or grape sugar (or maltose), before they are absorbed, and in this form they are much more readily metabolised than the fats or albuminates. 

It is believed by many, and the weight of evidence, as will be seen, is in favour of the conclusion, that carbohydrates can be converted into fat within the organism. Bauer,* however, is indisposed to accept this view. Basing his opinions on the experiments of Pettenkofer and Voit, who showed that carbohydrates, even when administered in great excess, are almost completely destroyed within the body, he maintains that although the carbohydrates, when given together with albumen and fat, favour an increase of the constituents of the body, and especially of fat, yet it is not because they are themselves converted into fat, but because, owing to the facility with which they are metabolised, they protect the other food-stuffs from destruction. 

"When fat and carbohydrates co-exist in the food, the latter are always the first to be consumed; and when they are present in sufficient amount, the consumption of fat in the body may be completely suspended." And he explains in a similar manner the fact that a deposit of fat may be observed to take place when the diet consists of albuminates and carbohydrates alone, without any fat; for in that case, he says, the fat, which "originates as a product of the splitting-up of albumen, is withdrawn from further metabolism in favour of the carbohydrates, and contributes to the gain." He also rejects the view that the ready decomposition of the carbohydrates in the body depends on their great affinity for oxygen; he considers it lies rather in the properties of the animal tissues, and he points out that the assumed equivalents of starch and fat, as 240 of the former to 100 of the latter, calculated on the quantity of oxygen required for their combustion, are incorrect; and that in the living organisms "175 parts of starch are in the material actions approximately equivalent to 100 of fat." 

Germain Sée begins by supporting the view taken by Bauer, and asserts that the principal function of the carbohydrates is the immediate development of heat and mechanical work ; that they are not annexed in any way directly or indirectly to the organisms ; and that the fat that is deposited in consequence of their use is derived from the splitting-up of albuminates. He urges the experiments of Boussingault, who found that when he fed ducks on a pure carbohydrate like rice, they grew thin; but on adding a small quantity of butter, they grew fat. The same experimenter also asserted that milch-cows only gave out the quantity of fat in their milk that was contained in their food. Sée also points out that the particular kinds of grain selected for fattening animals are always such as contain, like maize, a considerable quantity of fat. But, notwithstanding all this, he appears in the end to yield to the weight of evidence that fat may be, under certain circumstances, formed from carbohydrates. 

Dujardin-Beaumetz believes in the possibility of the transformation of glucose, the product of the digestion of carbohydrates, into fat. He sees a great analogy between the formula for glucose, C6H12O6, and that of glycerine, C3H8O3, and thinks that the latter may result from the splitting up of the former with the addition of hydrogen. He also shares, to a certain extent, Pavy's views, and considers that a portion of the glucose derived from the digestion of carbohydrates is deposited as " hepatic glycogen" in the liver, and thus furnishes the glucose necessary to the organism when the food does not contain any carbohydrates, Pavy maintains, as is well known, that saccharine matter, when absorbed, "on reaching the liver is transformed by that organ into amyloid sub- stance [glycogen], which is stored up in its cells for subsequent further change preliminary to being appropriated to the purposes of life." The occurrence of this "amyloid substance" in the liver, even when a purely animal diet has been taken, he accounts for by the supposition that the liver is the organ in which the splitting-up of the albuminates into urea and a non-nitrogenous substance occurs, and that the latter is metamorphosed by the liver into "glycogen." Pavy believes that carbohydrates are first converted into this "amyloid substance," and that this is afterwards converted into fat. But he points out what is doubtless a most important condition in the conversion of carbohydrates into fat, namely, "the co-operation of nitrogenous in conjunction with saline matter," for it is probably by the changes occurring during the metabolism of the albuminates that this transformation is excited. The presence of a small amount of fat with the carbohydrates would seem also to favour this conversion, for the rapid deposition of fat which sometimes occurs when animals are fed on such a mixture appears to be more than can be accounted for by the small quantities of fat ingested. Pavy dops not admit that any of the carbohydrates undergo direct oxidation in the system, or contribute strictly to force production.