THE RGLE OF CELLULOSE IN NUTRITION

Summary. The question of the rôle which cellulose plays in nutrition is of particular importance to the agriculturist, since fibre constitutes a very significant part of the normal rations of farm animals. The fibre in plants and feeding stuffs consists of cellulose, admixed with varying amounts of lignocellulose. In the young plant, the cell walls are composed almost wholly of cellulose. The latter is digested in the ruminant tract almost to the same extent as is the carbohydrate component. As the plant matures, however, the cell walls become toughened by the deposition of the incrusting substance, lignocellulose. The latter is entirely indigestible. The process of lignification is accompanied, therefore, by a diminution in the digestibility of the fibre of the crop. In other words, the digestibility of fibre is inversely proportional to its content of lignocellulose. For reasons made clear in the text, the digestibility of the other food nutrients, such as protein, oil and carbohydrate, also displays a corresponding running‐off during lignification. Ruminant animals have the greatest power of digesting the fibre in feeding stuffs. Although swine are able to assimilate the cellulose of non‐lignified feeding stuffs almost as completely as can sheep and cattle, they are unable to make such good use of lignified fibre. Horses can digest the fibre in their rations, but not with the degree of efficiency displayed by ruminants. In the human alimentary tract, the less resistant forms of cellulose are digested quite readily, while the power of certain invertebrates to subsist on cellulose is very marked. Birds are able to utilise the fibre in their food to a limited extent. Our knowledge of the value of cellulose in the nutrition of farm animals is derived from the results of Kellner's investigations. Kellner has shown that if 1 lb. of pure, finely divided cellulose be added to the maintenance ration of a bullock, as much fat, namely ½ lb., will be formed in the body of the animal as would be produced by the addition of 1 lb. of starch or other digestible polysaccharide. The starch equivalent of digestible cellulose is unity. Under ordinary conditions of feeding, however, when the cellulose is embedded in indigestible fibre, the net starch equivalent derived from 1 lb. of digestible fibre is less than unity, since a deduction must be made on account of the energy used up by the animal in masticating the indigestible portion and in separating the cellulose from the incrusta. The presence of cyto‐hydrolytic enzymes has been demonstrated in seeds and other plant tissues, in the hepato‐pancreatic secretions of several species of land snails and in the saliva of certain insects. The breakdown of cellulose in the digestive tracts of higher animals, however, is not the work of a cytase present in any of the digestive secretions, although it is conceivable that the cytases present in certain types of plant food may become active under the favourable conditions provided by the alimentary canal. Digestion of cellulose in the animal tract is mainly brought about by the fermentative activity of cellulose‐splitting bacteria, the digestion taking place in the parts of the tract where the food stagnates, as, for example, the rumen in sheep and cattle, and the caecum and colon in the case of horses and pigs. Digestion of cellulose by enzymes gives rise to the sugars, cellobiose and glucose. The bacterial fermentation of cellulose in artificial media, on the other hand, gives rise to organic acids, mainly acetic acid and a small amount of butyric acid, and gaseous products, such as carbon dioxide and either methane or hydrogen. Ethyl alcohol may also be formed in the fermentation. Experiments carried out on ruminants in respiration chambers have demonstrated that the methane fermentation is the main cellulose fermentation which occurs in the animal. A consideration of the physiological value of these breakdown products, however, leads to the following conclusion: Although the organic acids arising from bacterial fermentation of cellulose may, by virtue of their heat value, be useful in the maintenance of bodily warmth, and indeed, in the case of the under‐fed animal, may spare the oxidation of body fat for production of heat, yet they will have no value for fat production in the body of the productively fed animal. The accepted hypothesis that cellulose is rendered available to the animal solely by destructive bacterial fermentation is not compatible, therefore, with Kellner's finding that digestible cellulose and digestible starch possess equal values for fattening. The following scheme of digestion of cellulose and starch is put forward to account for the equal productive values of these constituents: Evidence in support of this theory of bacterial digestion of cellulose to glucose in the animal is provided by the results of recent work on the conversion of cellulose into glucose, in artificial media, by controlling the activity of cellulose‐splitting micro‐organisms. The full significance of this evidence is discussed in the text. Enzymatic digestion: