Quercetin and Vitamin C Benefits: A 2021 Review
Ascorbic acid (vitamin C) is an essential vitamin with known antiviral properties (PubMed) which is under investigation for its other beneficial effects during the stress response in sepsis and critically ill patients [PubMed].
Quercetin is a widely distributed plant flavonoid, found in several vegetables, leaves, seeds, and grains, where it is conjugated with residual sugars to form quercetin glycosides [PubMed]. Studies suggest that quercetin supplementation may promote antioxidant, anti-inflammatory, antiviral and immuno-protective effects.
In this review, we collate the evidence of the benefits of quercetin, benefits of vitamin C, discuss their synergistic actions and their use for the prevention and treatment of COVID-19 pandemic.
Benefits of Quercetin
Quercetin has been shown to reduce inflammation in multiple studies. In this 8 week study involving 50 women with rheumatoid arthritis, participants took 500mg of quercetin per day or a placebo. The quercetin group reported less early morning stiffness, morning pain, and after-activity pain.
Early studies on quercetin and inflammation are promising, although more large scale human studies need to be performed to verify these benefits.
There’s evidence that quercetin reduces allergy symptoms. Researchers believe quercetin’s anti-inflammatory effects may relieve allergy symptoms.
This study published in Molecules in 2016, for example, found that quercetin could be effective for treating the anaphylactic (allergic) reaction in someone with peanut allergies. A similar study from 2006 concluded that quercetin was a “safe, natural therapy that may be used as primary therapy or in conjunction with conventional methods” for blocking allergies.
It’s possible that quercetin has the same anti-allergy effect in humans, although more research needs to be done.
There’s also some evidence that quercetin has cancer-fighting properties. Researchers believe quercetin can fight cancer cells with its powerful antioxidant properties.
In 2015, researchers reviewed available test tube and animal studies on quercetin and prostate cancer. After reviewing available evidence, researchers found that quercetin suppressed cell growth and induced cell death in prostate cancer cells.
This study published in 2017 in Oncology Reports took things a step further, finding that quercetin induced cancer cell death in nine types of cancer, including prostate cancer, colon cancer, and breast caner.
In another 2017 study, researchers gave quercetin to mice with tumors. Researchers found that mice in the quercetin-treated group showed delayed tumor growth, no significant changes in daily behavior, significantly better survival ratings, and increased rates of cell death.
Quercetin may also target bladder cancer. In 2016, researchers published a landmark study in the American Journal of Cancer Research. Researchers analyzed quercetin’s effect on cancer cells in a test tube. They concluded, “We are the first to show that quercetin displays potent inhibition on bladder cancer cells via activation of AMPK pathway.”
Early research on the cancer-fighting benefits of quercetin is promising, although more research needs to be performed to verify these effects in humans.
Other evidence suggests quercetin lowers your risk of chronic brain disorders, including Alzheimer’s and dementia.
In this study published in Neuropharmacology in 2015, researchers gave quercetin to mice with Alzheimer’s, injecting them with quercetin every two days for three months. By the end of the study, the injections had reversed several markers of Alzheimer’s, and the mice performed much better on learning tests.
In a separate study published in 2018, researchers gave mice with Alzheimer’s a quercetin-rich diet. Researchers found the diet improved brain function in mice with early-middle stage Alzheimer’s, although it had no significant effect on middle-late stage Alzheimer’s.
You may have heard that coffee is linked to a reduced risk of Alzheimer’s. There’s certainly some research to back that claim up. However, recent research has suggested that quercetin (not caffeine) is the primary compound in coffee responsible for protective effects against Alzheimer’s.
Some people use quercetin to reduce blood pressure and improve other measurements of cardiovascular health. High blood pressure raises your risk of heart disease, which is the leading cause of death in the United States. Approximately 1 in 3 American adults have high blood pressure.
Research suggests that quercetin reduces blood pressure levels. In this study published in 2002, researchers found that quercetin exhibited vasodilator effects, widening blood vessels and reducing blood pressure.
Multiple studies on humans have suggested similar benefits. Researchers reviewed nine human studies involving 580 people. After reviewing available evidence, researchers found that taking more than 500 mg of quercetin supplement per day reduced systolic and diastolic blood pressure by 5.8mm Hg and 2.6 mm Hg, respectively. In other words, quercetin was shown to significantly improve blood pressure readings.
Some people take quercetin for its anti-aging effects. Antioxidants and anti-inflammatories – including quercetin – seem to have powerful anti-aging benefits. One study on young mice found that quercetin reduced mortality and extended lifespan. Another study from 2017 on human cells in test tubes found that quercetin reduced aging markers, with researchers concluding there was “anti-aging potential” for quercetin.
Others take quercetin supplements to improve endurance and exercise performance. This 2011 study reviewed 11 quercetin studies involving 254 human subjects and found that quercetin provided a statistically significant benefit in human endurance exercise capacity (VO2 max) and endurance exercise performance, although the effect “is between trivial and small”.
Finally, some diabetics take quercetin to help manage blood sugar. This study from 2019 found that taking 500mg of quercetin per day significantly reduced fasting plasma glucose (FPG) levels, suggesting that quercetin helped control fasting blood sugar. This study in 2019 found similar results, finding that quercetin lowered serum glucose levels at doses of 10, 25, and 50mg per kg.
Some suggest that quercetin could treat the COVID-19 coronavirus. Quercetin was shown to be effective against SARS, the Ebola virus, and the Zika virus. This 2019 study by researchers in Korea found that quercetin and other flavonoids may inhibit the proteolytic activity of SARS-CoV 3C-like protease, creating an antiviral effect. The combined role of quercetin and vitamin C in the prevention and treatment of COVID-19 will be described further below.
Benefits of Vitamin C
First, vitamin C helps encourage the production of white blood cells known as lymphocytes and phagocytes, which help protect the body against infection (PubMed).
Second, vitamin C helps these white blood cells function more effectively while protecting them from damage by potentially harmful molecules, such as free radicals.
Third, vitamin C is an essential part of the skin’s defense system. It’s actively transported to the skin, where it can act as an antioxidant and help strengthen the skin’s barriers (ScienceDirect).
Studies have also shown that taking vitamin C may shorten wound healing time (PubMed, PubMed).
What’s more, low vitamin C levels have been linked to poor health outcomes.
Other than it's immune-boosting property, there are also other benefits of vitamin C.
People who have pneumonia tend to have lower vitamin C levels, and vitamin C supplements have been shown to shorten the recovery time (PubMed, PubMed).
A large review of 29 studies in 11,306 people demonstrated that regularly supplementing with vitamin C at an average dose of 1–2 grams per day reduced the duration of colds by 8% in adults and 14% in children (PubMed).
Interestingly, the review also demonstrated that regularly taking vitamin C supplements reduced common cold occurrence in individuals under high physical stress, including marathon runners and soldiers, by up to 50% (PubMed, PubMed).
Quercetin and Vitamin C Synergy
June 19, 2020, Dr Marik published the paper “Quercetin and Vitamin C: An Experimental, Synergistic Therapy for the Prevention and Treatment of SARS-CoV-2 Related Disease (COVID-19)” in the journal Frontiers in Immunology, which notes:
"There is evidence that vitamin C and quercetin co-administration exerts a synergistic antiviral action due to overlapping antiviral and immunomodulatory properties and the capacity of ascorbate to recycle quercetin, increasing its efficacy."
Quercetin and Vitamin C for the prevention and treatment of COVID-19
The FLCCC team has developed the I-MASK+ protocol for prophylaxis and at home treatment of early stage disease.