From Ouroboros: "In addition the 23 chromosome nuclear genome most of us are familiar with, mitochondria contain their own, distinct genome. Each mitochondrion contains several copies of this genome, and most cell types contain hundreds of mitochondria per cell. ... [Researchers] measured the number of abasic sites [or AP sites] in the mitochondrial genome of young and aged rat brain. AP sites are positions along the DNA backbone where no adenine, guanine, cytosine, or thymine is attached; they are among the most frequent damages to DNA. The authors showed that in normally feeding animals, the number of AP sites increases with age - but calorie restricted (CR) mice did not show such an increase." Since accumulating mitochondrial DNA damage is a root cause of much age-related biochemical damage throughout the body, and since calorie restriction extends healthy longevity in laboratory animals, it makes sense that CR reduces the rate at which this damage accumulates.