In the human body, free radicals produced by the oxidation process disrupt cell membranes and other components, such as cellular proteins, lipids, and DNA. When oxygen is metabolized, unstable chemicals known as “free radicals” are produced. These molecules steal electrons from other molecules, damaging DNA and other cells.

Oxidative stress is generated by excess free radical generation in the body and a deficiency of antioxidants to battle this oxidation. The body can withstand some free radicals and need them for optimal function. However, over time, the damage caused by excess free radicals may become irreversible and contribute to certain diseases like heart and liver conditions and certain cancers like oral, stomach, and bowel cancers. Free radicals are produced when the body turns glucose into energy, but this is not the only route. Stress, cigarette smoking, alcohol, UV radiation, pollution, and other factors can also promote oxidation in the body. 

Source - Foods to Eat to Protect the Body from Oxidative Stress