⏱ 3 min read
Antioxidation is one of the most frequently mentioned biological mechanisms in nutritional outreach and, paradoxically, one of the least understood: the popular imagination presents it as a simple battle between "good antioxidants" and "bad free radicals," when the real physiology is much more nuanced and shows that both a deficit and an excess of antioxidant activity produce relevant clinical problems, a U-shaped curve that explains why intensive interventions with high-dose antioxidant vitamins have been disappointing in large clinical trials despite the apparently solid mechanistic basis.
Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are highly reactive molecules that the body generates continuously as inevitable byproducts of mitochondrial respiration, of the activity of immune cells (neutrophils and macrophages use them deliberately to kill pathogens), and as physiological signaling mediators in multiple cellular pathways; the body has a sophisticated endogenous antioxidant system (superoxide dismutase, catalase, glutathione peroxidase, thioredoxin, peroxiredoxins) that keeps ROS levels within an optimal functional range, neither too low (because useful signals are then lost) nor too high (because they then damage DNA, proteins, and lipids cumulatively).
One of the most counterintuitive ideas in the modern biology of aging is that small doses of oxidative stress are not only harmless but clearly beneficial: aerobic exercise produces an acute rise in muscle ROS that stimulates mitochondrial biogenesis, improves insulin sensitivity, and strengthens the endogenous antioxidant system; modest caloric restriction, intermittent fasting, and cold exposure produce similar effects; this phenomenon is called mitohormesis or oxidative hormesis, and it explains why trying to neutralize all oxidative stress with pharmacological antioxidants can cancel out precisely the adaptive benefits we are looking for.
Several studies have documented that high-dose vitamin C and E supplementation during aerobic training significantly blunts the beneficial adaptations to exercise (improvements in insulin sensitivity, mitochondrial biogenesis, endogenous antioxidant capacity), a clinically important observation for athletes and for people who combine exercise with intensive antioxidant supplementation.
Glutathione is the most abundant and most important intracellular antioxidant, present at millimolar concentrations in practically every cell in the body and especially concentrated in the liver; its function is multifaceted: it directly neutralizes reactive species, regenerates vitamin C and vitamin E after their oxidative use, takes part in the conjugation and elimination of xenobiotics (phase II of hepatic detoxification), and keeps protein sulfhydryl groups in their functional reduced state.
N-acetylcysteine (NAC) is probably the most effective intervention for raising intracellular glutathione concentrations because it supplies cysteine, the limiting amino acid in endogenous synthesis; alpha-lipoic acid, broccoli sulforaphane, and selenium (a cofactor of glutathione peroxidase) are complementary interventions with a solid mechanistic basis and clinical support for specific indications such as diabetic neuropathy, liver disease, or exposure to toxins.
The population evidence points consistently in one direction: diets rich in fruits, vegetables, nuts, seeds, spices, cocoa, and tea (all naturally rich in polyphenols, carotenoids, vitamin C, and vitamin E within their complete food matrix) are associated with lower cardiovascular, cancer, and all-cause mortality; however, large clinical trials of intensive supplementation with isolated antioxidant vitamins (CARET, ATBC, SELECT, HOPE) have been neutral or slightly unfavorable.
The vitamin E paradox is a paradigmatic example: epidemiologically it is associated with a better cardiovascular profile, but trials using high supplemental doses (more than 400 IU/day) show neutral or slightly adverse results, possibly because the food matrix of vitamin E-rich foods supplies other valuable components that the isolated supplement does not, or because high doses of isolated alpha-tocopherol paradoxically displace gamma-tocopherol and other tocopherols with partly different profiles.
Despite the disappointing results in population-level cardiovascular and cancer prevention, antioxidant supplementation has specific, supported indications: coenzyme Q10 in statin myopathy and heart failure (Q-SYMBIO study); vitamin E within AREDS2 in age-related macular degeneration; NAC in acetaminophen hepatotoxicity; alpha-lipoic acid in painful diabetic neuropathy; selenium in autoimmune thyroiditis.
The reasonable conclusion: in healthy people eating Mediterranean or varied diets, antioxidants are already well supplied by whole foods; intensive supplementation with isolated antioxidant vitamins offers no population-level benefits and may interfere with beneficial adaptations to exercise; targeted supplementation with clinical support for particular indications does have a place, with realistic expectations and supervision when appropriate.
Information, not medical advice.