Health Benefits of Quercetin

Quercetin is a type of plant pigment known as a flavonoid. It’s naturally found in a variety of foods and beverages. Quercetin is also available in supplement form. 

Quercetin is an antioxidant and has anti-inflammatory effects that may help reduce swelling, regulate blood sugar levels, and prevent heart disease. Research shows that supplemental quercetin may also protect brain health, support immune function, and aid weight management.

Health Benefits

Quercetin has been studied for a variety of conditions. Research suggests that the flavonoid—through its inflammation and antioxidant effects—can have an array of benefits, from aiding the immune system to reducing disease risk. 

Has Antioxidant Effects

As a part of normal metabolism, the human body produces compounds called free radicals. Free radicals damage healthy cells, trigger gene mutations, accelerate aging, and increase the risk of various diseases, including heart disease and diabetes. 

Quercetin has been shown to counter the harmful effect of free radicals through its strong antioxidant activity. This antioxidant effect can help the body maintain a stable state, ward off premature aging, and reduce chronic disease risk. Much of this research has been done in cells or animals, but researchers suggest the effect is likely true for humans too. 

Has Antimicrobial Effects

Several studies have found that quercetin offers broad-spectrum antibacterial properties. Quercetin has been shown to protect against Salmonella enteritidis (salmonella), Staphylococcus aureus (staph), and Escherichia coli (E. coli), in part by destroying bacterial cell walls and changing cell function. 

Quercetin has also been shown to interfere with the growth of drug-resistant microorganisms. This means quercetin may help fight against strains of bacteria that don’t respond to treatment. Quercetin is particularly effective against bacteria that affect the skin, as well as the digestive, respiratory, and urinary systems. It’s also been shown to combat some viruses and fungi.  

Protects Heart Health

Quercetin guards heart health in several ways. It has been shown to reduce both systolic (the top number) and diastolic (the bottom number) blood pressure. Quercetin can also reduce the pressure on arteries. That’s important because, over time, high blood pressure can damage arteries and lead to heart disease.

Quercetin has also been shown to curb total cholesterol and triglycerides. Triglycerides are a type of fat that, in high amounts, can increase your heart disease risk.  

Quercetin can also counter a process called oxidation, which changes “bad” LDL cholesterol in ways that harden arteries. 

Quercetin can also help regenerate blood vessels to improve blood flow, which supports healthy heart function.  

May Help With Weight Management

Only a limited number of human studies have evaluated the effects of quercetin on obesity treatment, but the results are promising. 

In one small study, six men and 30 women who were overweight or had obesity were randomly assigned to take either 100mg of quercetin or a placebo every day for 12 weeks.

The researchers found that the quercetin group had significant decreases in weight; body mass index; and waist, hip, and thigh measurements. The quercetin group also had a reduction in overall body fat percentage, particularly in the arms. 

More research—especially in larger numbers—would be needed to say for certain the effect quercetin can have on weight loss or how quercetin would fit into an overall weight management plan.

May Protect Brain Health

Alzheimer’s disease (AD) is the most common cause of dementia among older adults. Over time, it destroys memory and thinking.  

Flavonoids, the family of antioxidants to which quercetin belongs, have been extensively studied for their antioxidant and anti-inflammatory activities, both of which are important in the prevention of AD. Research has shown that flavonoids as a whole may be able to help prevent the development or progression of AD.

Research on how the flavonoid quercetin affects AD has been primarily animal-based. But one study in humans looked at the impact of quercetin-rich onion powder on cognitive function in 70 healthy Japanese adults aged 60 to 79. The participants were randomly assigned to consume either 11 grams of air-dried, high-quercetin onion powder or a placebo daily for 24 weeks. 

The results showed that quercetin-rich onion powder may delay cognitive decline in elderly people with early stage AD.

The researchers also found that the quercetin group experienced better cognitive function scores, improved depressive symptoms, and increased overall motivation. 

May Support Immune Function 

Studies regarding quercetin and immune function have had mixed results, but there is some promise that the flavonoid can support immune function.  

In one trial, 1,002 participants took 500mg or 1000mg of quercetin or a placebo each day for 12 weeks. Overall, quercetin supplementation did not have a significant effect on rates of upper respiratory tract infections (URTI) compared with the placebo. 

However, in a subgroup of participants aged 40 or older who rated themselves as physically fit, 1,000mg of quercetin per day resulted in fewer total sick days and a reduced severity of URTI symptoms. 

Another small study among athletes found that supplementing with 100mg of quercetin per day did not impact several measures of immune function after three days of intense exercise, but it did significantly reduce URTI risk. In the two-week period after the exercise sessions, just one out of 20 participants in the quercetin group got sick compared to nine of 20 in the placebo group. 

May Help Improve COVID-19 Outcomes

There have been more than a dozen clinical trials to evaluate the effects of quercetin on the prevention and treatment of COVID‐19. 

One of those studies looked at the effect that a product made from quercetin and sunflower lecithin, which increases the rate of quercetin absorption by up to 20 times compared to pure quercetin, had on 152 people with COVID-19. 

Researchers found that when the supplement was used in the early stage of COVID‐19 infection alongside conventional COVID‐19 therapies, patients had improvements in early symptoms and a reduced risk of severe COVID‐19 outcomes, including hospitalization. 

Still, more clinical trials are needed to firmly establish the effectiveness of quercetin supplements against COVID‐19.

The National Institutes of Health (NIH) included quercetin in its review of supplements for COVID-19. In that review, the NIH said that there wasn’t enough information to make a determination for or against the use of any supplement when preventing or treating COVID-19.

May Aid in Autoimmune Conditions

Autoimmune diseases occur when the immune system mistakenly attacks your body’s healthy cells. Some of these conditions include rheumatoid arthritis (RA), inflammatory bowel disease, multiple sclerosis, and lupus. 

Due to its protective anti-inflammatory and antioxidant abilities, quercetin may help prevent or manage autoimmune conditions.

For example, in one small study, 50 women with RA were randomly assigned to receive either 500mg of quercetin or a placebo every day for eight weeks. The quercetin group experienced significant reductions in early morning stiffness, morning pain, and post-activity pain compared with the placebo group.

In addition, the number of patients with active RA disease in the quercetin group significantly decreased. 

May Help Regulate Effects of PCOS

Polycystic ovary syndrome (PCOS) is a common cause of infertility, affecting 6% to 12% of US women of reproductive age. People with PCOS have higher levels of the male hormone androgen than people with female reproductive systems usually have. The levels of androgen can lead to several symptoms, including a stop in ovulation.

People with PCOS are also often insulin resistant, which means their bodies make insulin but can’t use the insulin effectively. This can increase the risk of type 2 diabetes. 

A limited number of human studies have looked at the impact of quercetin supplements vs. a placebo on PCOS for up to 12 weeks. So far, the research has shown positive outcomes, with quercetin users experiencing improvements in both hormone and insulin regulation. 

However, scientists say there are not enough studies to make strong conclusions about the use of quercetin supplements for PCOS. 

Sources of Quercetin

Quercetin is naturally found in a variety of plant foods, including:

Fruits:

• Apples
• Berries
• Cherries
• Citrus fruits
• Grapes
• Pears

Vegetables:

• Asparagus 
• Broccoli
• Kale
• Onion
• Sweet potato
• Tomatoes

Herbs:

• Chives
• Cilantro
• Dill
• Oregano 

Beverages: 

• Green tea
• Black tea
• Red wine

Quercetin can also be taken in supplement form. 

Recommended Intake 

Currently, there is no recommended daily intake specifically for quercetin for general health, wellness, or disease prevention. 

Worth noting is that fruits and vegetables are the major sources of quercetin, and the 2020-2025 Dietary Guidelines for Americans advise adults to consume 1.5 to two cups of fruit and two to three cups of vegetables per day. 

It’s estimated that people take in anywhere from 50-800mg of flavonoid antioxidants per day. It’s believed that quercetin accounts for 75% of that intake, depending on the consumption of fruits, vegetables, and tea.   

When used as an ingredient in food and drinks, quercetin is believed to be safe in amounts up to 500mg per serving, according to the US Food and Drug Administration.

While you can also take quercetin in supplement form, the safety and effectiveness of long-term use hasn’t been well established. Currently, it is only known that up to 1 gram of quercetin supplements a day for 12 weeks has been considered safe. More research on longer use and higher doses is needed.

Side Effects

Quercetin has not been shown to be toxic or to cause any serious side effects in any of the human clinical trials in which it’s been studied.

Generally, quercetin is considered to be safe when gotten naturally through food and drinks.

While less is known about quercetin supplements, no serious side effects have been reported in clinical trials that used up to 1,000mg a day for up to 12 weeks. One study that used 200mg of quercetin twice daily for 30 days did show mild effects. These effects included stomach pain and reflux, constipation, diarrhea, gas, and sleep disorders. However, the rate of these effects was similar to that of the placebo group.

Quercetin might affect certain medications, including pravastatin (used to treat high cholesterol) and fexofenadine (an antihistamine, used to manage allergy symptoms). In addition, quercetin might reduce blood pressure in people with high blood pressure, so it could increase the effects of medications used to treat the condition. 

If you are interested in the potential use of quercetin for a specific condition, talk with a healthcare provider about whether a supplement may benefit you. If the provider thinks it may, discuss proper dose, length of use, and possible interactions with other medications.