Living cells continually generate free radicals or reactive oxygen species (ROS) through the respiratory chain during energetic metabolism. ROS can either be harmful or play important physiological roles in our body. Besides being produced during normal cell metabolism there are numerous exogenous factors, such as irradiation by UV light, X-rays, gamma-rays, and atmospheric pollutants which may lead to generation of ROS. Human body has various intrinsic mechanisms to counteract oxidative stress by producing antioxidants, or through externally derived foods and/or supplements. However whenever there is excess of free radicals their accumulation in the body generates a phenomenon called oxidative stress. As we age, this oxidative and/or nitrosative damages elicit a number of late-onset diseases after ROS/RNS accumulate to certain levels. The ROS/RNS-mediated late-onset diseases can occur in any system of the body and may lead to clinical conditions such as cancer, arthritis arteriosclerosis, and neurodegenerative diseases. Oxidative stress is marked with expression of specific biomarkers whose specificity towards the various disease condition needs validation. In this review, we summarize the source, balance, maintenance and physiological functions of ROS, and its toxic mechanisms underlying a number of diseases and also the biomarkers implicated in selected human diseases.
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