This is the central life-extension claim of Metchnikoff’s programme, stated in The Prolongation of Life (1908) and reasoned from the analogy between lactic fermentation arresting putrefaction in food and the same fermentation arresting putrefaction in the digestive tube (p. 166). The endpoint is mechanistic and population-anecdotal rather than a measured clinical outcome: Metchnikoff cited animal experiments (Belonowsky’s mice, Herter’s dogs) and the reported longevity of Bulgarian milk-drinkers, not a controlled human trial of lifespan. The claim is classified as refuted because its enabling theory, intestinal autointoxication, was discredited in the early 20th century (Bested et al, 2013) and because modern reassessment finds no evidence that the Bulgarian bacillus colonizes the gut or extends life (Mackowiak, 2013). Metchnikoff himself died in 1916 at age 71.