The free radical theory of aging (FRTA) states that organisms age because cells accumulate free radical damage over time. Denham Harman first proposed the free radical theory of aging in the 1950s. Free radicals that are thought to be involved in the process of aging include superoxide and nitric oxide. An increase in superoxide affects aging whereas a decrease in nitric oxide formation, or its bioavailability, does the same.

A free radical is any atom or molecule that has a single unpaired electron in an outer shell. Free radicals are highly reactive. Free radical damage is closely associated with oxidative damage. Damage occurs when the free radical encounters another molecule and seeks to find another electron to pair its unpaired electron, causing the affected molecule to become a free radical itself. The new free radical can then pull an electron off the next molecule, and a chemical chain reaction of radical production occurs. The free radicals produced in such reactions often terminate by removing an electron from a molecule which becomes changed or cannot function without it, especially in biology. Such an event causes damage to the molecule, and thus to the cell that contains it. The chain reaction caused by free radicals can lead to cross-linking of atomic structures. The DNA can become cross-linked. DNA cross-linking can in turn lead to various effects of aging, especially cancer. Cross-linking can occur between fat and protein molecules, which leads to wrinkles. Free radicals can oxidize LDL, and this is a key event in the formation of plaque in arteries, leading to heart disease and stroke. Antioxidants are reducing agents, and limit oxidative damage to biological structures by passivating them from free radicals.

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Journal of Aging and Geriatric Medicine

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