This is the mechanistic premise on which the sour-milk therapy rested: that the large bowel harbours putrefactive microbes whose toxic products are absorbed and drive the degenerative changes of old age. Metchnikoff set out the argument in The Prolongation of Life (1908) and in the earlier The Nature of Man (1903). The claim is classified as refuted: the intestinal-autointoxication theory was rejected in the early 20th century, with Walter C. Alvarez among its most vehement critics, on the ground that putrefactive substances are not absorbed from the gut in the harmful quantities the theory required (Bested et al, 2013). The same review notes that some elements of the theory were abandoned without ever being thoroughly tested rather than formally disproven; what is established is that the specific absorption mechanism this claim asserts does not operate. Modern microbiome science investigates gut-host interactions on a different and separately evidenced basis; it does not vindicate the autointoxication framework as Metchnikoff stated it (Mackowiak, 2013).