The Biological basis of the aging process

The basic chemical process underlying aging was first put forward
by the free radical theory of aging in 1956; the reaction of
active free radicals (normally produced within an organism itself)
with cellular constituents initiates the changes associated
with aging. The involvement of free radicals in aging is related
to their key role in the origin and evolution of life. The specific
composition of tissue macromolecules (proteins, nucleic acids,
lipids and carbohydrates) in long-lived animal species gives
them an intrinsically high resistance to modification that probably
contributes to the superior longevity of these species.
Long-lived species also show low rates of reactive oxygen
species (ROS) generation and oxidative damage to their mitochondria.
Dietary restriction further decreases mitochondrial
ROS production and oxidative molecular damage due to the
decreased intake of dietary proteins. These effects of protein
restriction seem to be specifically due to the lowered methionine
intake of protein and dietary restricted animals. Both a low
rate of generation of endogenous damage and an intrinsically
high resistance to the modification of tissue macromolecules
are key traits of animal longevity.