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The Free Radical Theory of Aging
The theory that free radicals are toxic agents was first proposed by Rebeca Gerschman and colleagues.

In 1956, Denham Harman proposed the free radical theory of aging and even demonstrated that free radical reactions contribute to the degradation of biological systems.

Oxidative damage of many types accumulate with age, the free radical theory of aging argues that aging results from the damage generated by reactive oxygen species (ROS). ROS are small, highly reactive, oxygen-containing molecules that can damage a complex of cellular components such as fat, proteins, or DNA, they are naturally generated in small amounts during the body's metabolic reactions. These conditions become more common as we age, including diseases related to aging, such as dementia, cancer and heart disease. The Free Radical Theory of Aging is not accepted by everyone because we cannot determine if the free-radicals or the aging came first and if it is why the human body ages. Reasonably, the damage done by free radical may only begin after the aging process does. Future research must be conducted to see if one of the solutions to stabilize the free-radials may postpone aging.

The DNA Damage Theory of Aging
DNA damage has been one of the many causes in diseases related to aging. The stability of the genome is defined by the cells machinery of repair, damage tolerance, and checkpoint pathways that counteracts DNA damage. One hypothesis proposed by Failla in 1958 is that damage accumulation to the DNA causes aging. Soon after developed by physicist Leo Szilard. This theory has changed over the years as new research has discovered new types of DNA damage and mutations, and several theories of aging argue that DNA damage with or without mutations causes aging. ( If DNA damage and other stressors can be conserved, the anticancer and survival responses from cells may boost defenses that maintain the integrity of the cell. This may improve health and extend the lifespan. The importance of DNA damage and the genome maintenance in relation to aging can be investigated further more for treatment.