Topic of the Month

The antioxidant activity of micronutrients

December 1, 2011

Metabolic processes that occur in the presence of oxygen, innate immune defense processes and external factors lead to the formation of so-called reactive oxygen species, (ROS), the ‘prooxidants’ that oxidize lipids, DNA and proteins and can impair their functioning. A sufficient intake of plant-typical ingredients with ‘antioxidant’ activity appears to play an important role in preventing degenerative diseases like cardiovascular disease and some kinds of cancer. Prominent among these substances, which the human organism cannot synthesize for itself, are the carotenoids, vitamins C and E, and the flavonoids.


In recent decades, the term ‘oxidative stress’ has undergone a fundamental change, which has affected dietary recommendations and recommendations regarding food supplementation with individual substances that demonstrate antioxidant activity. Today, a condition is only described as oxidative stress if the imbalance between prooxidants and antioxidants causes cellular damage or disrupts signaling pathways in such a way that this may give rise to diseases. This has led to a search for indicators of oxidative damage, so that the antioxidant activity of selected foodstuffs can be investigated, for example.


Oxidative stress

The term oxidative stress describes an imbalance between prooxidants and antioxidants and the effect of this imbalance on an organism. Reactive oxygen compounds, which include both radicals (superoxide radical anion, peroxyl radicals, the nitroxyl and the hydroxyl radical) and non-radicals (singlet oxygen, hydrogen peroxide or hydroperoxide), are scavenged by the body’s own antioxidant defense system. Some reactive oxygen compounds, in contrast, are purposely formed as part of defense processes, for instance to ward off pathogenic organisms. Micronutrients with antioxidant activity can help strengthen the defense system and must be consumed in the diet in sufficient quantities. If the body does not have adequate supplies of antioxidants available and the prooxidants dominate, this can lead to oxidative stress and thus to damage: oxidative changes to biomolecules like DNA, proteins and lipids can encourage the genesis of diverse diseases. Oxidative changes to DNA bases can lead to mutations, one of the first occurrences in the genesis of cancer; oxidized Low Density Lipoproteins (LDL) is involved in the formation of atherosclerotic plaques. According to the current definition, antioxidants are substances that delay, prevent or remove oxidative damage to a target molecule (1). Additionally they protect against disruption to redox-sensitive signaling pathways: studies have shown that cellular redox sensors control gene expression and subsequent protein modification and that the control processes are modulated by antioxidants (2). The significance of these processes for human nutrition and health is not yet clear.

In order to investigate harmful effects a search is underway for reliable indicators (biomarkers) of oxidative damage. Many studies have already been conducted with biomarkers for lipid, DNA or protein oxidation to investigate the antioxidant activity of selected foodstuffs or products. It has been assumed that the consumption of antioxidants is associated with a reduced concentration of biomarkers in the bloodstream, urine and tissue, as compared with a control group. Currently, the biological and analytical validity of individual biomarkers for oxidative damage is being discussed.


Bioavailability of antioxidants

Bioavailability refers to the portion of a substance which, after its release in the organism, reaches the bloodstream, is absorbed, distributed to tissue and eventually reaches the site of action. In parallel it is also metabolized and eliminated. Factors influencing the bioavailability of antioxidants include the composition of the food matrix in which the micronutrients are integrated, and their storage form. Treatment and preparation of foods, e.g. deep freezing, cooking, pureeing and industrial processing, bring about the chemical digestion of cell walls (of fruit and vegetables, for instance), which usually improves the bioavailability of antioxidants. Since most antioxidants are not sufficiently stable to withstand high temperatures and react sensitively to exposure to oxygen, careful preparation is necessary. In the case of the fat-soluble carotenoids and vitamin Elipids consumed with the food facilitate uptake in the intestine. The concentration of antioxidants in foods usually decreases during storage. Appropriate storage, i.e. in a dark place or at low temperatures, delays degradation of the substances. Antioxidants intended for food supplementation are labeled with an indication of their greater bioavailability, which is guaranteed by optimal production processes.

In addition to the composition of foodstuffs containing antioxidants, the behavioral patterns of consumers – eating habits, smoking, and alcohol consumption, etc. – influence the bioavailability of antioxidants, as do individual nutritional status, age, sex and diseases. Hence the concentrations of vitamins C and E and beta-carotene in the blood of smokers are lower than those of non-smokers (3). The observed effect is likely to be due to increased utilization by prooxidants. Sex-specific differences in plasma levels have been measured for carotenoids. Diseases linked to impaired lipid absorption are also often associated with reduced uptake of fat-soluble antioxidants (4). Individual differences in the bioavailability of antioxidant micronutrients are probably due to genetic variants (polymorphisms) in proteins which are directly or indirectly involved in the absorption, distribution or metabolism of the substances. Several polymorphisms have been described for oxygenases which catalyze the splitting of beta-carotene to vitamin A (retinal). Furthermore, it has been shown that polymorphisms in genes involved in lipid metabolism influence plasma concentrations of carotenoids and vitamin E (5).

It is assumed that in the long term, imbalanced consumption of high doses of individual antioxidants will be detrimental. The antioxidant network is a multi-component system and requires a balanced mixture. In-vitro it has been shown that high concentrations of antioxidants have a prooxidant effect. The significance of prooxidant effects for a complex organism has not yet been established. In general, intervention studies with individual antioxidants or mixtures of them are needed to provide evidence of bioavailability. As a rule this is done by measuring the increase in the micronutrient in plasma. However, in many cases just the by and large undifferentiated change in overall antioxidant capacity of blood or plasma has been measured as evidence of antioxidant activity.


Indicators of oxidative damage

To provide evidence of oxidative damage by reactive oxygen species (ROS), various methods have been developed that are designed to demonstrate chemical changes to oxidatively modified cell structures. The target molecules are usually lipids, DNA or proteins. Biomarkers of oxidative damage are also used to determine the effect of an intervention with antioxidants (6). This assumes that after consumption of antioxidant substances damage is reduced compared to controls. Various end degradation products that are formed when membrane or plasma lipids react with ROS can be measured as an indicator of lipid damage. For determination of oxidative DNA damage, often modified DNA bases are quantified using single-cell gel electrophoresis and DNA damage identified in this way. With protein oxidation, amino acid side-chains (e.g. histidine, arginine or lysine) are modified by introducing carbonyl groups and this can then be measured (7).

The standard for demonstrating evidence of the antioxidant effects of foods, functional foods, extracts, food supplements or single ingredients are clinical studies with sufficient statistical validity and valid biomarkers (8). The usefulness and validity of the biomarkers used to evaluate oxidative damage are, however, the subject of controversial debate. Ideally, it should still be possible to relate the biomarkers to the corresponding clinical parameter values, such as increased lipid peroxidation with atherosclerotic vascular disease or DNA damage with mutations as a trigger for cancer. Here too, the interrelationships are unclear and further research is needed. But despite these limitations the use of biomarkers is helpful.


Effects of antioxidants

Processes associated with the formation of reactive oxygen species (ROS) play an important role in communications within the cell and between individual cells. The ROS (e. g. superoxide anion and hydrogen peroxide) are themselves signaling molecules and trigger cell responses, or they lead to the disruption of the redox balance of a cell, in consequence of which redox-sensitive proteins (redox sensors) are modified, giving rise to a signal. These two signaling transmission pathways may partially overlap and influence each other. In this way, cell growth, differentiation, apoptosis, angiogenesis or energy metabolic processes are modulated via intracellular signaling pathways (9).

The primary function of antioxidant micronutrients as components of the diet is to act as cofactors of enzymesVitamin C, for instance, is essential to the biosynthesis of collagen, carnitine and adrenaline, as a coenzyme of proline and lysine hydroylases. Beta-carotene is of central importance in the photosynthesis apparatus of plants and also has the task of preventing photooxidative damage there, an activity that is also observed in human beings. But beta-carotene is primarily important as a source of vitamin A (10). Irrespective of dietary habits, beta-carotene and other provitamin A compounds make a substantial contribution to the supply of vitamin A. As far as we know, mechanisms of vitamin A activity do not react in a directly sensitive way to oxidative stress.

Recently flavonoids, most of which only demonstrate antioxidant activity in vitro, have been a focus of particular interest. Their antioxidant activity in an organism, in contrast, is usually slight. Other physiological and pharmacological properties that are not related to antioxidant activity are predominantly responsible for the effectiveness of some flavonoids – e.g. epicatechin, which is found in cocoa, green tea and apples, among others, is thought to have a vasodilatory action. This indicates a possible protective effect against cardiovascular disease (11).


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