January 10th, 2018
Role of anti-oxidants in nutrition and health – Limitation of activity and toxicity
Studies have further evaluated factors that can limit the potency of antioxidant compounds. One such study evaluated the effect of various dietary factors – iron, ascorbate, meat and casein – that have the ability to interact with tea extract polyphenols (Alexandropoulou, Komaitis & Kapsokefalou, 2006). To conduct the study an in vitro digestion environment was modeled to resemble digestive processes that occur in the gastrointestinal tract (Alexandropoulou, 2006, p. 360). For instance, samples were subjected to an incubation process at 37 oC for 4.5 hours at varying pHs and in presence of the enzymes (e.g. pepsin and pancreatin) that operate in the gastrointestinal tract GIT (Alexandropoulou, 2006, p. 360). Antioxidant activity was evaluated using FRAP assay – a method that uses reducing power of samples – with increases in absorbance being noted after 4 minutes following addition of FRAP reagent. A modification of ferozine method (a method for serum iron concentration measurement) was used to measure iron concentration whereas the polyphenol concentration was evaluated via the Folin-Ciocalteu method; which is a method commonly used in evaluating total phenol content in various fluids (Alexandropoulou, 2006, p. 360). The results indicated that various dietary factors could alter the antioxidant activity in different directions. Whereas meat and casein reduced the antioxidant capabilities of green tea, ascorbic acid increased such capabilities (Alexandropoulou, 2006, p. 360). Similarly iron curtailed the antioxidant potential of green tea digests as well as reducing its polyphenol concentration (Alexandropoulou, 2006, p. 360).
To further evaluate effects of iron, Kapsokefalou, Zhu and Miller (2006) set out to find out what effect addition of iron to green tea could have on the latter’s antioxidant capacity using rat subjects. The study used 32 rats which were randomly divided into 4 groups with subjects in the same group being infused, by gavage, with one of the four substances – green tea, iron citrate; green tea- iron citrate mixture, and water – used (Kapsokefalou, et al., 2006). Blood was drawn at intervals of 10, 30, 45, and 60 minutes after gavage to provide samples for analysis (Kapsokefalou, et al., 2006). The antioxidant activity in the plasma was evaluated through “ferric-reducing ability of plasma and total radical trapping antioxidant parameter assays in red blood cells (diacetyldichlorofluorescein assay)” (Kapsokefalou, et al., 2006). Further the plasma polyphenol concentration was evaluated through the Folin-Ciocalteau assay as in the previous study reviewed (Kapsokefalou, et al., 2006). The total radical trapping evaluations provided the most evident influence of iron addition in diet with antioxidant capacity in iron supplemented infusions and water infusions not showing an increase noted for green tea supplemented infusions at the 30th minute evaluation (Kapsokefalou, et al., 2006). The ferric plasma reducing measures however did not produce such clear evidence of iron effects (Kapsokefalou, et al., 2006). The study concluded that iron may have a reducing effect on the antioxidant activity of some compounds probably by forming complexes with such compounds thus reducing their ability to form complexes with oxidizing agents (Kapsokefalou, et al., 2006).
Apart from dietary limitations other studies have evaluated impacts of lipid (fatty acid) composition on antioxidant activity. One study has for instance evaluated how unsaturation (double or triple bonds) in fatty acid chains affects antioxidant’s potency of ß-carotene (exhibits vitamin A activity) and α-tocopherol (exhibits vitamin E activity) in hexane solutions (Palozza, Luberto & Bartoli, 1995). The study incubated fatty acid solutions with appropriate concentrations of the two antioxidants with 9 mM AIBN (azobisisobutyronitrile) – a MDA formation inducer – for 90 minutes at 37°C in the dark under air conditions with the generated MDA being extracted and measured at 535 nm (Palozza, et al., 1995, p. 945). ß-carotene and α-tocopherol assays were conducted using high-performance liquid chromatography – HPLC (Palozza, et al., 1995). The antioxidants were used to break the chain of MDA formation that has been shown to otherwise be positively associated with fatty acid unsaturation in homogenous solutions (Palozza, et al., 1995). The results indicated that unsaturation in fatty acids affected ß-carotene and α-tocopherol antioxidant properties in different ways. For α-tocopherol, two phases of MDA generation (lipid peroxidation) – an induction phase with a lag time of 30 minute and a propagation phase – were identified (Palozza, et al., 1995). The propagation phase was not influenced, as was the induction phase, and proceeded in a similar fashion as when no antioxidant is added with the rate being determined by extent of unsaturation (Palozza, et al., 1995). For ß-carotene; the antioxidant supplemented solution and the control solution do not have varying lag phases but the propagation phase is largely inhibited in the antioxidant (ß-carotene) supplemented solution in relation to the extent of unsaturation (Palozza, et al., 1995). Such findings indicated different antioxidant roles for the two compounds with ß-carotene effectiveness in unsaturated membranes being suggested (Palozza, et al., 1995).
On the flip side of antioxidants ingestion, Rietjens, et al. (2002) give an overview of the “pro-oxidative chemistry and toxicity of well-known natural antioxidants including vitamin C and E, carotenoids and flavonoids” (Rietjens, et al., 2002, p. 322). The overview based on a an intensive review of literature disabuses the notion that many compounds present in most natural foods present beneficial reducing capabilities and shows that in some conditions antioxidants such as Vitamin C could act as pro-oxidants (Rietjens, et al., 2002). For instance, in the presence of elements such as Fe3+, Vitamin C could reduce it to Fe2+ making it susceptible to superoxide producing reactions with H2O2 (Rietjens, et al., 2002). Superoxides are part of a group of ROS, a free radical, that can initiate cell damaging reactions Similarly the overview demystifies the notion that intakes of increasing amounts of antioxidants are beneficial, demonstrating that after some levels these could also be subject to toxicity. Such literature provides caution on the unabated use of food supplements containing antioxidants. go to the concluding part.