Antioxidant

Antioxidants are compounds that can prevent or slow down the oxidation of other molecules. Oxidation is a chemical reaction that can produce free radicals, leading to chain reactions that may damage cells. Antioxidants terminate these chain reactions by removing free radical intermediates and inhibiting other oxidation reactions. They do this by being oxidized themselves, making them crucial in the body's defense against oxidative stress.^[1]

Known dietary antioxidants are vitamins A, C, and E, but the term antioxidant has also been applied to numerous other dietary compounds that only have antioxidant properties in vitro, with little evidence for antioxidant properties in vivo.^[2] Dietary supplements marketed as antioxidants have not been shown to maintain health or prevent disease in humans.^[2]

Types of Antioxidants

There are several types of antioxidants, each playing a distinct role in combating oxidative stress:

Enzymatic Antioxidants: These include enzymes such as superoxide dismutase (SOD), catalase, and glutathione peroxidase, which directly neutralize ROS.
Non-enzymatic Antioxidants: This group includes Vitamin C, Vitamin E, and glutathione. These small molecule antioxidants scavenge free radicals and contribute to the repair and regeneration of oxidized antioxidants.
Metal-Binding Proteins: Proteins such as ferritin and ceruloplasmin that sequester free iron and copper ions, which can catalyze the production of free radicals, thus reducing oxidative stress.

Dietary Sources of Antioxidants

Diet is a significant source of antioxidants. Foods high in antioxidants include:

Fruits and Vegetables: Berries, citrus fruits, and leafy greens are high in vitamins C and E, as well as various phytochemicals.
Nuts and Seeds: These are sources of vitamin E and selenium, another important antioxidant.
Whole Grains: Whole grains contain antioxidants like vitamin E and phytochemicals.
Tea and Coffee: Rich in flavonoids, these beverages contribute significantly to the total antioxidant intake.

Antioxidant Supplements

Antioxidant supplements have gained popularity as potential aids in combating oxidative stress and its related aging processes. These supplements are concentrated forms of antioxidants that can be taken in addition to a regular diet. However, antioxidant supplements have not shown preventive effects and may be harmful with unwanted consequences to our health. The optimal source of antioxidants seems to come from our diet, not from antioxidant supplements.^[3]

Older observational studies and some randomized clinical trials with high risks of systematic errors ('bias') have suggested that antioxidant supplements may improve health and prolong life. A number of randomized clinical trials with adequate methodologies observed neutral or negative results of antioxidant supplements. Recently completed large randomized clinical trials with low risks of bias and systematic reviews of randomized clinical trials taking systematic errors ('bias') and risks of random errors ('play of chance') into account have shown that antioxidant supplements do not seem to prevent cancer, cardiovascular diseases, or death. Even more, beta-carotene, vitamin A, and vitamin E may increase mortality.^[4] Some recent large observational studies now support these findings. According to recent dietary guidelines, there is no evidence to support the use of antioxidant supplements in the primary prevention of chronic diseases or mortality.^[5]^[3]

List of Antioxidant Supplements

Here's a list of some widely used antioxidant supplements:

Vitamin C (Ascorbic Acid): A water-soluble vitamin known for its role in boosting the immune system and scavenging free radicals.
Vitamin E (Tocopherol): A fat-soluble vitamin that protects cell membranes from oxidative damage, especially in lipid-rich areas of the body.
Beta-Carotene: A precursor to vitamin A and a powerful antioxidant, particularly effective against singlet oxygen.
Selenium: A trace mineral that is a component of antioxidant enzymes like glutathione peroxidase.
Zinc: Essential for the functioning of several antioxidant enzymes and immune function.
Coenzyme Q10 (CoQ10): An important molecule in energy production and also acts as an antioxidant in cell membranes and lipoproteins.
Flavonoids: A diverse group of phytonutrients found in many fruits and vegetables, known for their antioxidant properties.
Lycopene: A carotenoid found in tomatoes, watermelon, and other red fruits and vegetables, known for its antioxidant capacity.
Resveratrol: Found in the skin of red grapes, berries, and peanuts, resveratrol is studied for its antioxidant and anti-aging properties.
Curcumin: The active ingredient in turmeric, curcumin possesses strong antioxidant and anti-inflammatory properties.
Alpha-Lipoic Acid: A compound that works with mitochondria and the body's natural antioxidant defenses.
Green Tea Extract: Rich in catechins, green tea extract is renowned for its antioxidant properties.

Oxygen Radical Absorbance Capacity (ORAC)

Measurement of polyphenol and carotenoid content in food is not a straightforward process, as antioxidants collectively are a diverse group of compounds with different reactivities to various reactive oxygen species. In food science analyses in vitro, the oxygen radical absorbance capacity (ORAC) was once an industry standard for estimating antioxidant strength of whole foods, juices and food additives, mainly from the presence of polyphenols.^[6]^[7] Earlier measurements and ratings by the United States Department of Agriculture were withdrawn in 2012 as biologically irrelevant to human health, referring to an absence of physiological evidence for polyphenols having antioxidant properties in vivo.^[8] Consequently, the ORAC method, derived only from in vitro experiments, is no longer considered relevant to human diets or biology, as of 2010.^[8]

Legal

Other than for dietary antioxidant vitamins—vitamin A, vitamin C and vitamin E—no food compounds have been proved with antioxidant efficacy in vivo. Accordingly, regulatory agencies such as the Food and Drug Administration of the United States and the European Food Safety Authority (EFSA) have published guidance forbidding food product labels to claim or imply an antioxidant benefit when no such physiological evidence exists.^[9]^[10] This guidance for the United States and European Union establishes it is illegal to imply potential health benefits on package labels of products with high ORAC.

References

↑ Sies H et al.: Oxidative Stress. Annu Rev Biochem 2017. (PMID 28441057) [PubMed] [DOI] Oxidative stress is two sided: Whereas excessive oxidant challenge causes damage to biomolecules, maintenance of a physiological level of oxidant challenge, termed oxidative eustress, is essential for governing life processes through redox signaling. Recent interest has focused on the intricate ways by which redox signaling integrates these converse properties. Redox balance is maintained by prevention, interception, and repair, and concomitantly the regulatory potential of molecular thiol-driven master switches such as Nrf2/Keap1 or NF-κB/IκB is used for system-wide oxidative stress response. Nonradical species such as hydrogen peroxide (H2O2) or singlet molecular oxygen, rather than free-radical species, perform major second messenger functions. Chemokine-controlled NADPH oxidases and metabolically controlled mitochondrial sources of H2O2 as well as glutathione- and thioredoxin-related pathways, with powerful enzymatic back-up systems, are responsible for fine-tuning physiological redox signaling. This makes for a rich research field spanning from biochemistry and cell biology into nutritional sciences, environmental medicine, and molecular knowledge-based redox medicine.
↑ ^2.0 ^2.1 Antioxidants: In Depth, https://www.nccih.nih.gov/health/antioxidants-in-depth
↑ ^3.0 ^3.1 Bjelakovic G et al.: Antioxidant supplements and mortality. Curr Opin Clin Nutr Metab Care 2014. (PMID 24241129) [PubMed] [DOI] PURPOSE OF REVIEW: Oxidative damage to cells and tissues is considered involved in the aging process and in the development of chronic diseases in humans, including cancer and cardiovascular diseases, the leading causes of death in high-income countries. This has stimulated interest in the preventive potential of antioxidant supplements. Today, more than one half of adults in high-income countries ingest antioxidant supplements hoping to improve their health, oppose unhealthy behaviors, and counteract the ravages of aging. RECENT FINDINGS: Older observational studies and some randomized clinical trials with high risks of systematic errors ('bias') have suggested that antioxidant supplements may improve health and prolong life. A number of randomized clinical trials with adequate methodologies observed neutral or negative results of antioxidant supplements. Recently completed large randomized clinical trials with low risks of bias and systematic reviews of randomized clinical trials taking systematic errors ('bias') and risks of random errors ('play of chance') into account have shown that antioxidant supplements do not seem to prevent cancer, cardiovascular diseases, or death. Even more, beta-carotene, vitamin A, and vitamin E may increase mortality. Some recent large observational studies now support these findings. According to recent dietary guidelines, there is no evidence to support the use of antioxidant supplements in the primary prevention of chronic diseases or mortality. SUMMARY: Antioxidant supplements do not possess preventive effects and may be harmful with unwanted consequences to our health, especially in well-nourished populations. The optimal source of antioxidants seems to come from our diet, not from antioxidant supplements in pills or tablets.
↑ Bjelakovic G et al.: Mortality in randomized trials of antioxidant supplements for primary and secondary prevention: systematic review and meta-analysis. JAMA 2007. (PMID 17327526) [PubMed] [DOI] CONTEXT: Antioxidant supplements are used for prevention of several diseases. OBJECTIVE: To assess the effect of antioxidant supplements on mortality in randomized primary and secondary prevention trials. DATA SOURCES AND TRIAL SELECTION: We searched electronic databases and bibliographies published by October 2005. All randomized trials involving adults comparing beta carotene, vitamin A, vitamin C (ascorbic acid), vitamin E, and selenium either singly or combined vs placebo or vs no intervention were included in our analysis. Randomization, blinding, and follow-up were considered markers of bias in the included trials. The effect of antioxidant supplements on all-cause mortality was analyzed with random-effects meta-analyses and reported as relative risk (RR) with 95% confidence intervals (CIs). Meta-regression was used to assess the effect of covariates across the trials. DATA EXTRACTION: We included 68 randomized trials with 232 606 participants (385 publications). DATA SYNTHESIS: When all low- and high-bias risk trials of antioxidant supplements were pooled together there was no significant effect on mortality (RR, 1.02; 95% CI, 0.98-1.06). Multivariate meta-regression analyses showed that low-bias risk trials (RR, 1.16; 95% CI, 1.04[corrected]-1.29) and selenium (RR, 0.998; 95% CI, 0.997-0.9995) were significantly associated with mortality. In 47 low-bias trials with 180 938 participants, the antioxidant supplements significantly increased mortality (RR, 1.05; 95% CI, 1.02-1.08). In low-bias risk trials, after exclusion of selenium trials, beta carotene (RR, 1.07; 95% CI, 1.02-1.11), vitamin A (RR, 1.16; 95% CI, 1.10-1.24), and vitamin E (RR, 1.04; 95% CI, 1.01-1.07), singly or combined, significantly increased mortality. Vitamin C and selenium had no significant effect on mortality. CONCLUSIONS: Treatment with beta carotene, vitamin A, and vitamin E may increase mortality. The potential roles of vitamin C and selenium on mortality need further study.
↑ Bjelakovic G et al.: Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database Syst Rev 2012. (PMID 22419320) [PubMed] [DOI] [Full text] BACKGROUND: Our systematic review has demonstrated that antioxidant supplements may increase mortality. We have now updated this review. OBJECTIVES: To assess the beneficial and harmful effects of antioxidant supplements for prevention of mortality in adults. SEARCH METHODS: We searched The Cochrane Library, MEDLINE, EMBASE, LILACS, the Science Citation Index Expanded, and Conference Proceedings Citation Index-Science to February 2011. We scanned bibliographies of relevant publications and asked pharmaceutical companies for additional trials. SELECTION CRITERIA: We included all primary and secondary prevention randomised clinical trials on antioxidant supplements (beta-carotene, vitamin A, vitamin C, vitamin E, and selenium) versus placebo or no intervention. DATA COLLECTION AND ANALYSIS: Three authors extracted data. Random-effects and fixed-effect model meta-analyses were conducted. Risk of bias was considered in order to minimise the risk of systematic errors. Trial sequential analyses were conducted to minimise the risk of random errors. Random-effects model meta-regression analyses were performed to assess sources of intertrial heterogeneity. MAIN RESULTS: Seventy-eight randomised trials with 296,707 participants were included. Fifty-six trials including 244,056 participants had low risk of bias. Twenty-six trials included 215,900 healthy participants. Fifty-two trials included 80,807 participants with various diseases in a stable phase. The mean age was 63 years (range 18 to 103 years). The mean proportion of women was 46%. Of the 78 trials, 46 used the parallel-group design, 30 the factorial design, and 2 the cross-over design. All antioxidants were administered orally, either alone or in combination with vitamins, minerals, or other interventions. The duration of supplementation varied from 28 days to 12 years (mean duration 3 years; median duration 2 years). Overall, the antioxidant supplements had no significant effect on mortality in a random-effects model meta-analysis (21,484 dead/183,749 (11.7%) versus 11,479 dead/112,958 (10.2%); 78 trials, relative risk (RR) 1.02, 95% confidence interval (CI) 0.98 to 1.05) but significantly increased mortality in a fixed-effect model (RR 1.03, 95% CI 1.01 to 1.05). Heterogeneity was low with an I(2)- of 12%. In meta-regression analysis, the risk of bias and type of antioxidant supplement were the only significant predictors of intertrial heterogeneity. Meta-regression analysis did not find a significant difference in the estimated intervention effect in the primary prevention and the secondary prevention trials. In the 56 trials with a low risk of bias, the antioxidant supplements significantly increased mortality (18,833 dead/146,320 (12.9%) versus 10,320 dead/97,736 (10.6%); RR 1.04, 95% CI 1.01 to 1.07). This effect was confirmed by trial sequential analysis. Excluding factorial trials with potential confounding showed that 38 trials with low risk of bias demonstrated a significant increase in mortality (2822 dead/26,903 (10.5%) versus 2473 dead/26,052 (9.5%); RR 1.10, 95% CI 1.05 to 1.15). In trials with low risk of bias, beta-carotene (13,202 dead/96,003 (13.8%) versus 8556 dead/77,003 (11.1%); 26 trials, RR 1.05, 95% CI 1.01 to 1.09) and vitamin E (11,689 dead/97,523 (12.0%) versus 7561 dead/73,721 (10.3%); 46 trials, RR 1.03, 95% CI 1.00 to 1.05) significantly increased mortality, whereas vitamin A (3444 dead/24,596 (14.0%) versus 2249 dead/16,548 (13.6%); 12 trials, RR 1.07, 95% CI 0.97 to 1.18), vitamin C (3637 dead/36,659 (9.9%) versus 2717 dead/29,283 (9.3%); 29 trials, RR 1.02, 95% CI 0.98 to 1.07), and selenium (2670 dead/39,779 (6.7%) versus 1468 dead/22,961 (6.4%); 17 trials, RR 0.97, 95% CI 0.91 to 1.03) did not significantly affect mortality. In univariate meta-regression analysis, the dose of vitamin A was significantly associated with increased mortality (RR 1.0006, 95% CI 1.0002 to 1.001, P = 0.002). AUTHORS' CONCLUSIONS: We found no evidence to support antioxidant supplements for primary or secondary prevention. Beta-carotene and vitamin E seem to increase mortality, and so may higher doses of vitamin A. Antioxidant supplements need to be considered as medicinal products and should undergo sufficient evaluation before marketing.
↑ Cao G, Alessio HM, Cutler RG; "Oxygen-radical absorbance capacity assay for antioxidants" , https://zenodo.org/record/1258621 , https://doi.org/10.1016/0891-5849(93)90027-R
↑ Ou B, Hampsch-Woodill M, Prior RL; "Development and validation of an improved oxygen radical absorbance capacity assay using fluorescein as the fluorescent probe" , https://doi.org/10.1021/jf010586o
↑ ^8.0 ^8.1 Withdrawn: Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 (2010), http://www.ars.usda.gov/services/docs.htm?docid=15866
↑ Guidance for Industry, Food Labeling; Nutrient Content Claims; Definition for "High Potency" and Definition for "Antioxidant" for Use in Nutrient Content Claims for Dietary Supplements and Conventional Foods U.S. Department of Health and Human Services, Food and Drug Administration, Center for Food Safety and Applied Nutrition, June 2008
↑ "Scientific Opinion on the substantiation of health claims related to various food(s)/food constituent(s) and protection of cells from premature aging, antioxidant activity, antioxidant content and antioxidant properties, and protection of DNA, proteins and lipids from oxidative damage pursuant to Article 13(1) of Regulation (EC) No 1924/20061" , http://www.efsa.europa.eu/en/efsajournal/pub/1489 , https://doi.org/10.2903/j.efsa.2010.1489

[pmid28441057-1] Sies H et al.: Oxidative Stress. Annu Rev Biochem 2017. (PMID 28441057) [PubMed] [DOI] Oxidative stress is two sided: Whereas excessive oxidant challenge causes damage to biomolecules, maintenance of a physiological level of oxidant challenge, termed oxidative eustress, is essential for governing life processes through redox signaling. Recent interest has focused on the intricate ways by which redox signaling integrates these converse properties. Redox balance is maintained by prevention, interception, and repair, and concomitantly the regulatory potential of molecular thiol-driven master switches such as Nrf2/Keap1 or NF-κB/IκB is used for system-wide oxidative stress response. Nonradical species such as hydrogen peroxide (H2O2) or singlet molecular oxygen, rather than free-radical species, perform major second messenger functions. Chemokine-controlled NADPH oxidases and metabolically controlled mitochondrial sources of H2O2 as well as glutathione- and thioredoxin-related pathways, with powerful enzymatic back-up systems, are responsible for fine-tuning physiological redox signaling. This makes for a rich research field spanning from biochemistry and cell biology into nutritional sciences, environmental medicine, and molecular knowledge-based redox medicine.

[nih-2] 2.0 ^2.1 Antioxidants: In Depth, https://www.nccih.nih.gov/health/antioxidants-in-depth

[pmid24241129-3] 3.0 ^3.1 Bjelakovic G et al.: Antioxidant supplements and mortality. Curr Opin Clin Nutr Metab Care 2014. (PMID 24241129) [PubMed] [DOI] PURPOSE OF REVIEW: Oxidative damage to cells and tissues is considered involved in the aging process and in the development of chronic diseases in humans, including cancer and cardiovascular diseases, the leading causes of death in high-income countries. This has stimulated interest in the preventive potential of antioxidant supplements. Today, more than one half of adults in high-income countries ingest antioxidant supplements hoping to improve their health, oppose unhealthy behaviors, and counteract the ravages of aging. RECENT FINDINGS: Older observational studies and some randomized clinical trials with high risks of systematic errors ('bias') have suggested that antioxidant supplements may improve health and prolong life. A number of randomized clinical trials with adequate methodologies observed neutral or negative results of antioxidant supplements. Recently completed large randomized clinical trials with low risks of bias and systematic reviews of randomized clinical trials taking systematic errors ('bias') and risks of random errors ('play of chance') into account have shown that antioxidant supplements do not seem to prevent cancer, cardiovascular diseases, or death. Even more, beta-carotene, vitamin A, and vitamin E may increase mortality. Some recent large observational studies now support these findings. According to recent dietary guidelines, there is no evidence to support the use of antioxidant supplements in the primary prevention of chronic diseases or mortality. SUMMARY: Antioxidant supplements do not possess preventive effects and may be harmful with unwanted consequences to our health, especially in well-nourished populations. The optimal source of antioxidants seems to come from our diet, not from antioxidant supplements in pills or tablets.

[pmid17327526-4] Bjelakovic G et al.: Mortality in randomized trials of antioxidant supplements for primary and secondary prevention: systematic review and meta-analysis. JAMA 2007. (PMID 17327526) [PubMed] [DOI] CONTEXT: Antioxidant supplements are used for prevention of several diseases. OBJECTIVE: To assess the effect of antioxidant supplements on mortality in randomized primary and secondary prevention trials. DATA SOURCES AND TRIAL SELECTION: We searched electronic databases and bibliographies published by October 2005. All randomized trials involving adults comparing beta carotene, vitamin A, vitamin C (ascorbic acid), vitamin E, and selenium either singly or combined vs placebo or vs no intervention were included in our analysis. Randomization, blinding, and follow-up were considered markers of bias in the included trials. The effect of antioxidant supplements on all-cause mortality was analyzed with random-effects meta-analyses and reported as relative risk (RR) with 95% confidence intervals (CIs). Meta-regression was used to assess the effect of covariates across the trials. DATA EXTRACTION: We included 68 randomized trials with 232 606 participants (385 publications). DATA SYNTHESIS: When all low- and high-bias risk trials of antioxidant supplements were pooled together there was no significant effect on mortality (RR, 1.02; 95% CI, 0.98-1.06). Multivariate meta-regression analyses showed that low-bias risk trials (RR, 1.16; 95% CI, 1.04[corrected]-1.29) and selenium (RR, 0.998; 95% CI, 0.997-0.9995) were significantly associated with mortality. In 47 low-bias trials with 180 938 participants, the antioxidant supplements significantly increased mortality (RR, 1.05; 95% CI, 1.02-1.08). In low-bias risk trials, after exclusion of selenium trials, beta carotene (RR, 1.07; 95% CI, 1.02-1.11), vitamin A (RR, 1.16; 95% CI, 1.10-1.24), and vitamin E (RR, 1.04; 95% CI, 1.01-1.07), singly or combined, significantly increased mortality. Vitamin C and selenium had no significant effect on mortality. CONCLUSIONS: Treatment with beta carotene, vitamin A, and vitamin E may increase mortality. The potential roles of vitamin C and selenium on mortality need further study.

[pmid22419320-5] Bjelakovic G et al.: Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database Syst Rev 2012. (PMID 22419320) [PubMed] [DOI] [Full text] BACKGROUND: Our systematic review has demonstrated that antioxidant supplements may increase mortality. We have now updated this review. OBJECTIVES: To assess the beneficial and harmful effects of antioxidant supplements for prevention of mortality in adults. SEARCH METHODS: We searched The Cochrane Library, MEDLINE, EMBASE, LILACS, the Science Citation Index Expanded, and Conference Proceedings Citation Index-Science to February 2011. We scanned bibliographies of relevant publications and asked pharmaceutical companies for additional trials. SELECTION CRITERIA: We included all primary and secondary prevention randomised clinical trials on antioxidant supplements (beta-carotene, vitamin A, vitamin C, vitamin E, and selenium) versus placebo or no intervention. DATA COLLECTION AND ANALYSIS: Three authors extracted data. Random-effects and fixed-effect model meta-analyses were conducted. Risk of bias was considered in order to minimise the risk of systematic errors. Trial sequential analyses were conducted to minimise the risk of random errors. Random-effects model meta-regression analyses were performed to assess sources of intertrial heterogeneity. MAIN RESULTS: Seventy-eight randomised trials with 296,707 participants were included. Fifty-six trials including 244,056 participants had low risk of bias. Twenty-six trials included 215,900 healthy participants. Fifty-two trials included 80,807 participants with various diseases in a stable phase. The mean age was 63 years (range 18 to 103 years). The mean proportion of women was 46%. Of the 78 trials, 46 used the parallel-group design, 30 the factorial design, and 2 the cross-over design. All antioxidants were administered orally, either alone or in combination with vitamins, minerals, or other interventions. The duration of supplementation varied from 28 days to 12 years (mean duration 3 years; median duration 2 years). Overall, the antioxidant supplements had no significant effect on mortality in a random-effects model meta-analysis (21,484 dead/183,749 (11.7%) versus 11,479 dead/112,958 (10.2%); 78 trials, relative risk (RR) 1.02, 95% confidence interval (CI) 0.98 to 1.05) but significantly increased mortality in a fixed-effect model (RR 1.03, 95% CI 1.01 to 1.05). Heterogeneity was low with an I(2)- of 12%. In meta-regression analysis, the risk of bias and type of antioxidant supplement were the only significant predictors of intertrial heterogeneity. Meta-regression analysis did not find a significant difference in the estimated intervention effect in the primary prevention and the secondary prevention trials. In the 56 trials with a low risk of bias, the antioxidant supplements significantly increased mortality (18,833 dead/146,320 (12.9%) versus 10,320 dead/97,736 (10.6%); RR 1.04, 95% CI 1.01 to 1.07). This effect was confirmed by trial sequential analysis. Excluding factorial trials with potential confounding showed that 38 trials with low risk of bias demonstrated a significant increase in mortality (2822 dead/26,903 (10.5%) versus 2473 dead/26,052 (9.5%); RR 1.10, 95% CI 1.05 to 1.15). In trials with low risk of bias, beta-carotene (13,202 dead/96,003 (13.8%) versus 8556 dead/77,003 (11.1%); 26 trials, RR 1.05, 95% CI 1.01 to 1.09) and vitamin E (11,689 dead/97,523 (12.0%) versus 7561 dead/73,721 (10.3%); 46 trials, RR 1.03, 95% CI 1.00 to 1.05) significantly increased mortality, whereas vitamin A (3444 dead/24,596 (14.0%) versus 2249 dead/16,548 (13.6%); 12 trials, RR 1.07, 95% CI 0.97 to 1.18), vitamin C (3637 dead/36,659 (9.9%) versus 2717 dead/29,283 (9.3%); 29 trials, RR 1.02, 95% CI 0.98 to 1.07), and selenium (2670 dead/39,779 (6.7%) versus 1468 dead/22,961 (6.4%); 17 trials, RR 0.97, 95% CI 0.91 to 1.03) did not significantly affect mortality. In univariate meta-regression analysis, the dose of vitamin A was significantly associated with increased mortality (RR 1.0006, 95% CI 1.0002 to 1.001, P = 0.002). AUTHORS' CONCLUSIONS: We found no evidence to support antioxidant supplements for primary or secondary prevention. Beta-carotene and vitamin E seem to increase mortality, and so may higher doses of vitamin A. Antioxidant supplements need to be considered as medicinal products and should undergo sufficient evaluation before marketing.

[6] Cao G, Alessio HM, Cutler RG; "Oxygen-radical absorbance capacity assay for antioxidants" , https://zenodo.org/record/1258621 , https://doi.org/10.1016/0891-5849(93)90027-R

[7] Ou B, Hampsch-Woodill M, Prior RL; "Development and validation of an improved oxygen radical absorbance capacity assay using fluorescein as the fluorescent probe" , https://doi.org/10.1021/jf010586o

[USDAx-8] 8.0 ^8.1 Withdrawn: Oxygen Radical Absorbance Capacity (ORAC) of Selected Foods, Release 2 (2010), http://www.ars.usda.gov/services/docs.htm?docid=15866

[9] Guidance for Industry, Food Labeling; Nutrient Content Claims; Definition for "High Potency" and Definition for "Antioxidant" for Use in Nutrient Content Claims for Dietary Supplements and Conventional Foods U.S. Department of Health and Human Services, Food and Drug Administration, Center for Food Safety and Applied Nutrition, June 2008

[efsa-10] "Scientific Opinion on the substantiation of health claims related to various food(s)/food constituent(s) and protection of cells from premature aging, antioxidant activity, antioxidant content and antioxidant properties, and protection of DNA, proteins and lipids from oxidative damage pursuant to Article 13(1) of Regulation (EC) No 1924/20061" , http://www.efsa.europa.eu/en/efsajournal/pub/1489 , https://doi.org/10.2903/j.efsa.2010.1489

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