Self-cannibalism

Self-cannibalism is a type of body modification. Bodily functions self-cannibalize by consuming dead cells, as well as to break down complex substances into simple ones. Commonplace in the digestive system are self-cannibalistic processes, to name a few are metabolism, catabolism, and anabolism.

Cell cycle regulation
Cyclins are a group of proteins that activate kinases involved in cell division. The degradation of cyclins is the key step that governs the exit from mitosis and progress into the next cell cycle. Cyclins accumulate in the course the cell cycle, then abruptly disappear just before the anaphase of mitosis. The cyclins are removed via a ubiquitin-mediated proteolytic pathway.

You will surely Die / Aging-Death

Ouroboros
Further reading: Ouroboros

An extreme method is the ritualistic manner of drinking one's own blood, commonly known as autovampirism.

Other
The act of eating the placenta of their young after childbirth is also a form of self-cannablism known as Placentophagy.*