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The term “antioxidant” is not always clearly defined in either popular or scientific literature. In the most general sense, a natural or synthetic antioxidant directly or indirectly functions to minimize damage to biomolecules (mostly proteins, lipids, and DNA) caused by reactive oxygen species (ROS) and/or reactive nitrogen oxide species (RNOS). Screening complex mixtures of organic molecules (e.g., a fruit juice) for their in vitro antioxidant capacities is popular, but the health-related significance of such measurements is questionable. An “antioxidant nutrient” can be either a precursor or cofactor for an antioxidant molecule or can be an antioxidant in its own right. For example, “selenium” is considered an “antioxidant nutrient” but dietary selenium, in the form of selenite or selenate, is not a functional antioxidant: selenite and selenate must convert to L-selenocysteine which can then get incorporated into glutathione peroxidase (GPX) which is a key antioxidant selenoenzyme. Gamma-tocopherol which is the primary dietary form of vitamin E is both an antioxidant nutrient as well as a functional antioxidant. This article will focus on physiologically significant antioxidants that have been studied either in humans, animal models, or relevant in vitro cellular models. The physiochemical and physiological properties of individual antioxidants are complex, and not all molecules that function as antioxidants are necessarily beneficial to human health. A key goal is to understand how antioxidants modulate acts in signal transduction pathways.

An increase in oxidative stress always accompanies inflammation which can be useful in killing pathogens, but chronic inflammation can be pathophysiological in many circumstances such as obesity, autoimmune diseases, diabetes, atherosclerosis, as well as cancer initiation and progression.[13][14]