Studies on Lignin Metabolism

Food materialsof vegetable origin constitute a large proportion of the diet of man and of animals.In many of the human foods, but more especially in the various animal feedingstuffs, lignin is present in some cases to the extent of 20 to 25 per cent of the dry weight of the food material.It is therefore rather surprizing that such a common food ingredient has not been studied and subjected to metabolism experiments to a greater extent than we find recorded in the literature.The difficulties encountered in the quantitative estimation of lignin may have withheld many investigators from studying the metabolism of this substance.There is an abundance of evidence to support the claim that lignin, unlike cellulose, is cyclic in structure, the nature of the ring being either aromatic or hydroaromatic.In support of this, it is sufficient to mention the work of Cross and Bevan (1903), Hochfelder (1915), Melander (1919), Honig and Fuchs (1919, 1920), Erdmann (1921), Fischer, Schrader, and Friedrich (1921), Strupp (1924), and that of Strache-Lant (1924).Several investigators, among them Meissner and Shepard (1866), Harten (1867), Weiske (1876), and more recently Kijnig (1926), have suggested that lignin probably furnishes the phenyl radical involved in the formation of hippuric acid, but they all failed to prove their hypothesis experimentally.The last mentioned investigator working with Dietrich in 1871, as well as Stutzer (1875), has claimed, however, that lignin is not digested in the animal body.Recently, Paloheimo (1925) stated that lignin reappears quantitatively and unchanged in the feces.