Quercetin A compound from fruits and vegetables with a wide range of health benefits

Quercetin is a member of the flavonoid family of compounds. Flavonoids, sometimes referred to as bioflavonoids, are part of what give fruits and vegetables colour. Quercetin is more specifically a flavonol, which is a subclass of flavonoids. There are many subclasses of flavonoids, and within each subclass, there are many compounds. Quercetin is just one of the compounds in the flavonol subclass. The compounds in each subclass have a similar chemical structure. It is no great secret that a diet rich in fruits and vegetables is associated with better health and reduced risk of chronic disease.^[1] The compounds responsible for those health benefits have been an area of interest and have led researchers to specifically isolate certain compounds and study them in depth. Epidemiological studies have shown that higher dietary flavonoid intake is associated with health benefits.^[2]

Many foods contain quercetin, including apples, berries, Brassica vegetables (broccoli, cabbage, cauliflower, kale, etc.), capers, grapes, onions, shallots, tea, and tomatoes. Nuts, seeds, flowers, and barks also contain quercetin. The skin of the fruit often contains most of the quercetin. The United States Department of Agriculture reports that a Red Delicious apple without the peel has 0.41 mg of quercetin per 100 g, but with the peel has 3.86 mg of quercetin per 100 g. There are a number of naturally occurring forms of quercetin, and some are better absorbed by the body than others.^[3] In general, quercetin is absorbable by the body from natural food sources. Studies have shown that diets high in vegetable, berry, and other fruits had increased levels of quercetin in the blood, while individuals with a low intake of vegetables, berries, and other fruits had a 30% decrease in levels of quercetin in the blood.^[4] Onions, in particular, have been found to significantly increase blood levels of quercetin, and the elevated blood levels remained for 24 hours.^[5] Other components in the diet may affect the absorption of quercetin by the body. Animal studies have shown that when consumed with a meal that contains a higher fat content, quercetin is better absorbed.^{[6, 7]}

Quercetin and Cardiovascular Health

Quercetin has many proposed actions in the body, including being a potent antioxidant. Antioxidants are well-known for their role in protecting the body from cancer, but antioxidants also play a role cardiovascular health. Since the 1990s, epidemiological evidence has shown that intake of quercetin is inversely related to cardiovascular disease.

Both animal and human studies have shown that quercetin reduces blood pressure.^[8] Subjects in the most recent human trial had stage‑1 hypertension, which means they had a systolic pressure between 140 and 159 mmHg, and/or a diastolic blood pressure between 90 and 99 mmHg. Subjects took a supplement containing 738 mg of quercetin per day for 28 days. At the end of the trial, diastolic blood pressure was reduced by 7–9 mmHg, and systolic blood pressure was reduced by 5–7 mmHg.^[9] Other studies have also shown a blood pressure–lowering effect, some only showing a reduction in systolic pressure.^[10] The actions by which quercetin lowers blood pressure have yet to be fully discovered; however, the antioxidant properties are thought to play a role. It is also thought that quercetin has a vasorelaxant effect,^[11] and possibly an effect on angiotensin-converting enzyme,^[12] which if inhibited, has a blood pressure–reducing effect.

Low-density lipoprotein, or LDL is well-known as the ‘unhealthy’ or bad cholesterol in the body. Oxidized LDL has an even greater negative impact on the body since it is what is thought to be the leading cause of atherosclerosis, or the buildup of plaque in arteries. Oxidized LDL forms when LDL interacts with free radicals in the body. A number of studies have investigated the use of quercetin to prevent the oxidization of LDL cholesterol. A recent in vitro or cell study investigated the use of an apple-peel extract for the prevention of oxidation of LDL cholesterol. The quercetin-rich applepeel extract used in the study did in fact inhibit the oxidation of LDL cholesterol.^[13] In humans, as calculated from a three-day diet record, as quercetin levels increased in the diet, total cholesterol and oxidized LDL cholesterol levels decreased.^[14] In a doubleblind, placebo-controlled, crossover study, subjects were given a 150 mg supplement of quercetin per day for 6 weeks. Subjects in the study were either overweight or obese, and had metabolic syndrome traits. Results showed that quercetin reduced oxidized LDL cholesterol in this population of individuals who were at high risk for cardiovascular disease.^[10] The results of such studies at reducing oxidized cholesterol and blood pressure provides promising evidence that quercetin may be a useful treatment for people with cardiovascular disease or those at increased risk.

Quercetin and Allergies

One of more well-known applications of quercetin in the alternative health world is as a natural solution for allergy relief. Allergies such as sneezing, runny nose, and itchy, watery eyes are due to the release of histamine from immune system cells known as mast cells or basophils. Cell studies have shown that quercetin inhibited the release of histamine from cells that were stimulated with an allergen.^[15] Two randomized controlled trials have shown beneficial effects of supplemental quercetin with regards to allergy symptoms due to pollen release, especially with eye symptoms like itching.^{[16, 17]} Study participants started the supplemental quercetin 4 weeks prior to the release of pollens from the trees in their local area that typically cause them to have allergy symptoms.

A recent study also showed that quercetin significantly decreased contact dermatitis and photosensitivity, which are two skin conditions that do not typically respond to conventional treatments.^[18] Another interesting finding of this study was that quercetin could be used prophylactically, while cromolyn, the only pharmaceutical formula that is marketed as a “mast cell stabilizer”, must be added at the same time as a trigger or it rapidly loses its effect.^[18] Quercetin has also been studied for its use as an asthma treatment. Cell studies have shown that quercetin is able to relax airway tissue that was previously stimulated to contract.^[19] Animal studies have shown that even just a single dose of quercetin can cause bronchodilation.^[20]

Quercetin and Anti-inflammation

Many of quercetin’s beneficial effects can be attributed to its anti-inflammatory actions. In cell studies, it has been shown to inhibit inflammatory cytokine production such as tumor necrosis factor-alpha (TNF‑α),^[21] and interleukin‑8 (IL‑8),^[22] to name a few. Along with inhibiting proinflammatory cytokine secretion, quercetin has antioxidant effects, which may also contribute to its anti-inflammatory effects. The anti-inflammatory benefits of quercetin have been seen in studies of Graves’ orbitopathy (GO),^[23] an autoimmune disease of the thyroid that can have significant effects on the eyes, causing redness, swelling, and bulging. Another complication of GO is fibrosis of tissues, which quercetin was also found to positively benefit.^[24]

Sarcoidosis, another disease with significant inflammation, has been found to be positively affected by quercetin supplementation.^[25] Subjects in the treatment group consumed 2000 mg of quercetin and showed reduced markers of inflammation in blood samples that were taken within 24 h. Cell studies using quercetin as the treatment have shown decreased inflammatory cytokines, as well as other inflammatory compounds involved in the joint destruction seen in rheumatoid arthritis.^[26] Animal studies using oral quercetin have also shown clear decreases in clinical signs of arthritis, compared to untreated controls.^[27] These results suggest it may be a potential adjunctive treatment for rheumatoid arthritis.

Inflammation plays a pivotal role in cancer, and many therapies aim at reducing inflammation as part of prevention and treatment. Quercetin is no exception, and coupled with its antioxidant effects, as well as other potential anticancer mechanisms,^[28] it has shown promising results in a wide variety of cancers. Interestingly, quercetin has also been found to be beneficial in improving the effectiveness of some chemotherapeutic drugs by increasing the cancer cells’ sensitivity to the drug,^[29] as well as protecting tissues against radiation-induced damage in animal models.^[30] As always, we recommend seeking advice from your health-care practitioner prior to initiating any new therapies.