Supplements like quercetin, vitamin C, and vitamin E get a lot of attention as antioxidants, but the research makes a cautionary point: reactive oxygen species aren’t purely villains, and neutralizing them too well can have tradeoffs worth understanding before recommending a dose.
Quercetin as a Protective Antioxidant
Remigante et al. (2022) studied how quercetin can protect human red blood cells from the oxidative stress associated with aging, particularly its effect on lipid peroxidation. To simulate aging-related damage, they incubated erythrocytes with 100 mM d-galactose — a substance other studies have used to experimentally induce aging-like effects such as lipid peroxidation.
To assess membrane damage, they measured SO₄²⁻ uptake, a marker for the function of the membrane’s bicarbonate/chloride exchanger. Other studies had found that when that exchanger becomes oxidized, it’s recognized by antibodies that flag the cell for macrophages to engulf and remove. After 24 hours of d-galactose exposure, SO₄²⁻ transport into the erythrocytes rose significantly. But when cells were pretreated with just 10 µM of quercetin an hour before the d-galactose, that transport stayed flat for the full 24 hours instead of increasing. Another finding was that d-galactose increased glycated hemoglobin — except when quercetin was given first — leading the authors to suggest that glycation, rather than oxidative stress alone, might be what’s disrupting the bicarbonate/chloride exchanger.
The researchers also checked whether a high concentration of quercetin on its own might backfire and promote oxidation instead of preventing it. They measured levels of Thiobarbituric Acid Reactive Substances (TBARS), which are formed from malondialdehyde during lipid peroxidation. None of the tested concentrations (10 µM, 50 µM, or even 1 mM) raised TBARS levels, even when combined with 10 mM of the reactive oxygen species H₂O₂, which on its own caused a significant increase within an hour (p < 0.001). Quercetin was also able to significantly blunt the increase in TBARS caused by d-galactose, and it was able to protect proteins from oxidation, shown by a smaller drop in sulfhydryl groups after d-galactose exposure when quercetin was added beforehand.
Translating a Cell Study Into a Human Dose
Here’s where it gets more complicated: Burak et al. (2019) ran a randomized controlled trial in 67 healthy adults (ages 19–35) and found that a 190 mg/day quercetin supplement — delivered as a gelatin capsule of onion skin extract — raised plasma quercetin by 464 nmol/L (p < 0.0001) after eight weeks, a dose about 15 times higher than typical dietary intake. Most of that increase happened within the first four weeks, with only a small additional rise over the second four.
The problem is that this human-achievable plasma level looks much lower than the concentration Remigante et al. (2022) used in their cell study. I don’t know whether higher supplemental doses could push plasma levels up further. For context, 190 mg/day is a modest dose compared to the 3,000 mg/day I’ve seen recommended by some functional medicine practitioners as an antihistamine for allergy-related symptoms. My honest takeaway is that the human trial I found doesn’t convince me the antioxidant effect seen in vitro would actually show up at such a comparatively low dose — though I’d frame that the other way too: the burden of proof could just as easily fall on finding evidence that a lower concentration is effective, rather than assuming a higher one is needed.
And dosing up isn’t free of risk. Supplements rarely act through just one mechanism, which is worth remembering before optimizing for a single variable. Sajadi Hezaveh et al. (2019) found that just 500 mg/day of quercetin was enough to meaningfully impair iron absorption, significantly lowering serum iron and ferritin. In their study population — people with thalassemia — lower iron was actually the goal. But for a general client, that’s a real reason to monitor iron status during quercetin supplementation. It’s also a good idea to check a database for potential interactions between any supplement and the medications a client is taking, since many supplements interact with CYP enzymes.
Might Antioxidants Blunt a Desired Role of ROS
Physical activity increases superoxide production, which can damage cell membranes. Morrison et al. (2015) ran a randomized controlled trial giving young men vitamin C (500 mg twice daily) and vitamin E (400 IU/day) for eight weeks — four weeks before and four weeks during an endurance cycling program. They thought the antioxidant supplementation might blunt the expected gains in VO2 peak and maximal power output, since the reactive oxygen species generated during exercise also help regulate the muscle adaptations that make training effective in the first place. It didn’t: athletic performance improved as expected. But the supplemented group didn’t show the usual post-training rise in muscle tissue superoxide dismutase. The authors suggest that vitamin C was directly neutralizing excess superoxide, reducing the physiological need to upregulate superoxide dismutase in response.
The Takeaway
Both threads point to the same underlying caution: antioxidant supplementation isn’t just “reduce oxidative stress, get better outcomes.” Quercetin shows real protective effects in cell studies, but the doses shown effective in vitro may not translate cleanly to what’s achievable — or advisable — in humans. We also need to look out for the possibility that antioxidants might interfere with signaling roles that reactive oxygen species often play in our physiology. Reactive oxygen species have a normal place in our bodies; they’re not just a problem to be neutralized, and any recommendation needs to account for that nuance.
The content of this blog is for educational purposes and is not intended as medical advice. Please work with a qualified healthcare provider for personalized guidance.
References
Burak, C., Wolffram, S., Zur, B., Langguth, P., Fimmers, R., Alteheld, B., Stehle, P., & Egert, S. (2019). Effect of alpha-linolenic acid in combination with the flavonol quercetin on markers of cardiovascular disease risk in healthy, non-obese adults: A randomized, double-blinded placebo-controlled crossover trial. Nutrition, 58, 47–56.
Morrison, D., Hughes, J., Della Gatta, P. A., Mason, S., Lamon, S., Russell, A. P., & Wadley, G. D. (2015). Vitamin C and E supplementation prevents some of the cellular adaptations to endurance-training in humans. Free Radical Biology & Medicine, 89, 852–862.
Remigante, A., Spinelli, S., Basile, N., Caruso, D., Falliti, G., Dossena, S., Marino, A., & Morabito, R. (2022). Oxidation stress as a mechanism of aging in human erythrocytes: Protective effect of quercetin. International Journal of Molecular Sciences, 23(14).
Sajadi Hezaveh, Z., Azarkeivan, A., Janani, L., Hosseini, S., & Shidfar, F. (2019). The effect of quercetin on iron overload and inflammation in β-thalassemia major patients: A double-blind randomized clinical trial. Complementary Therapies in Medicine, 46, 24–28.
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