Antioxidants in Sport Nutrition: All the Same Effectiveness?

Excerpt

While oxygen is vital for life of an aerobic organism, the by-products of its metabolism can be harmful to cells. The very small not-to-water-reduced part of oxygen leads to the production of reactive oxygen intermediates, also known as ROS. This is happening ubiquitary but in particular in the working muscle during or after exercise (Alessio et al. 2000; Caillaud et al. 1999; Clarkson and Thompson 2000). ROS includes superoxide (), nitric oxide (NO^•) and hydroxyl radicals (HO^•) and also non-radicals such as singlet oxygen (¹O₂) or hydrogen peroxide (H₂O₂). Depending on the type of exercise, a number of potential mechanisms for the generation of ROS within the muscle have been proposed, such as (a) increased formation of in the mitochondrial respiratory chain, (b) xanthine oxidase (XO) catalysed degradation of AMP (adenosine monophosphate) during ischaemic muscular work leading to increased production of , (c) increased ROS formation in the oxidative-burst reaction due to activation of polymorphoneutrophils (PMNs) after exercise-induced muscle damage, (d) loss of calcium homeostasis in stressed muscles, (e) enhanced cytokine production and activation of nuclear factor kappa B (NF-κB), catecholamine autooxidation and many more (König et al. 2007; Niess et al. 1999; Vina et al. 2000). Owing to the unpaired electron in its outer orbit, ROS tend to extract electrons from other molecules to reach a chemically more stable state. However, the generation of ROS is per se not harmful and necessary for the proper functioning of metabolic processes, muscular contraction and immune defence. Muscle antioxidant defence systems are upregulated in response to exercise. NF-κB and mitogen-activated protein kinase are the two major oxidative-stress-sensitive signal transduction pathways that have been shown to activate the gene expression of a number of enzymes and proteins that play important roles in maintenance of intracellular oxidant–antioxidant homeostasis (Ji 2008).