Medicinal Plants And Antioxidants: Why They Are Not Always Beneficial?

Oxidative stress is defined as an imbalance between stress and the available protective elements suggested playing a crucial role in induction of diseases. Medicinal plants mostly combat these complications with their antioxidant activities. Several preclinical and epidemiological studies have found an inverse association between the consumption of fruits, vegetables and/or grains which generally are high in antioxidants and induction of diseases in humans. Antioxidants are substances that remove , prevent or delay oxidative damage to a target molecule. Therefore, an antioxidant may act to control the level of free radicals to counteract oxi-dative damage (1). There is a lot of evidence on protective and curative effects of medicinal plants on various complications. Some of these effects include antimicrobial (2, 3), anti-cancer (4), anti-diabetic (5, 6), anti-atherosclerosis (7, 8), immune-modulatory (9), and even renoprotection or hepa-to-protective effects (10–14). Medicinal plants are rich sources of polyphenols with antioxidant activities and these beneficial effects have been attributed to their antioxidant mechanisms. There has also been a linear correlation between oxygen radical absorbance capacity values and total phe-nolic contents in several medicinal plants. In a study the equivalent antioxidant capacity values of several forms of garlic extracts were correlated well with their total phenolic, flavonoid and fla-vonol contents (15). Although medicinal plants effects in prevention and treatment of disorders have been widely attributed to their antioxidants activities, however, there is increasing evidence pointing to their pro-oxidant hazardous effects, too. This property of medicinal plants is mostly contributed to their for-mulae preparation. Some of polyphenols have shown to be readily oxidized in preparation of beverages like green tea. The prescription formu-lae preparation in traditional medicine usually involves a long decoction process with water and medicinal plants for several hours. Polyphenols in the medicinal plants in this process or storage may be oxidized. This oxidation critically reduces the beneficial properties of herbal medicine products (16). Pro-oxidant however, is a chemical agent that induces oxidative stress, either by generating reac-tive oxygen species (ROS) or by inhibiting antiox-idant systems. The oxidative stress induced by this agent can damage cells and tissues. Polyphe-nols in medicinal plants can act as either antioxi-dants or pro-oxidants, depending on conditions. Concentration of the extract and whether oxygen or transition metals are present or not are of these conditions. Transition metals such as iron, copper and manganese are present in most cases of pro-oxidant activities of agents (1). Adulteration and contamination of medicinal plants …