Quercetin is a common flavonoid found in many fresh fruits and vegetables.  It has been in widespread use in the dietary supplement industry for the past two decades due to its natural anti-histamine properties.  New research is dramatically expanding our understanding of this nutrient, including its nervous system support, immune support, and weight management properties.

Quercetin is highly concentrated in apples, onions (especially red onions), and green tea.  It is also in red grapes, citrus fruit, tomato, broccoli, leafy greens, cherries, raspberries, cranberries, and many other fruits and vegetables.  One vine-ripened apple contains 50 mg of quercetin. 

Quercetin possesses unique antioxidant activity.  An October 2009 study demonstrates how quercetin activates the key step leading to the production of cellular glutathione (a cell's primary antioxidant).  The researchers showed how this mechanism protected the insulin-producing beta cells of the pancreas, contributing to healthier blood sugar function.  Another October 2009 study demonstrates that quercetin protects against liver injury by antioxidant function.  A January 2009 study showed that quercetin offered significant antioxidant protection for cells lining the sinuses.  And a March 2009 study shows that quercetin offers significant antioxidant protection to the mitochondria (cellular engines).

Quercetin's antioxidant effects also extend to the cardiovascular system.  A study with diabetic rats found that quercetin reduced free radical damage and offered significant cardiovascular protection in terms of the amount of damage done by experimentally induced heart attacks (they were much smaller in quercetin-treated animals).  A study with healthy men found that 200 mg of quercetin enhanced friendly nitric oxide production and endothelial cell function (cells lining arteries), indicating improved circulation and natural support for blood pressure health.  A mouse study also showed that quercetin increased the friendly form of nitric oxide in the circulation, in this case supporting male erection.  A rat study showed that quercetin improved cognitive function in rats with occluded carotid arteries, partly by helping circulation and partly by helping nerves in the brain.

It has also been demonstrated that quercetin modulates the key anti-inflammatory gene signal known as NF-kappaB.  For example, quercetin has been shown to lower NF-kappa B in bone, reducing the number of bone-deteriorating osteoclasts being formed as well as helping to manage existing osteoclasts so that their bone resorption activity does not become excessive.

Another example is an Aug 2009 study involving experimentally induced rheumatoid arthritis which showed that animals taking quercetin had no more inflammation than control animals - demonstrating quercetin's ability to suppress inflammation.  A combination of quercetin, glucosamine, and chondroitin given to patients with osteoarthritis found a significant reduction in joint pain, improved range of motion, as well as documented changes to the synovial fluid of their joints reflecting improvement in lubrication.

Mast Cells, Allergy, and Nerves

The original interest in quercetin as a dietary supplement ingredient was based on its ability to stabilize mast cells, which release histamine and other inflammatory signals.  It became a popular and widely used remedy for sinus congestion, sneezing, the pollen season, and other issues wherein the immune system seemed to behave in the direction of excessive. 

Quercetin's anti-histamine properties are now well established, it has been found to stabilize mast cells in a way that helps lower stress-induced anxiety and allergic reactions.  A chain of recent discoveries helps to place the significance of these discoveries into context, with far ranging implications for human health and improved nerve tolerance for managing stress.

One study shows that stress itself is adequate to begin the migration of immune cells towards your skin, as if preparing to deal with a wound or infection - clearly an evolutionary strategy wherein stress typically implied injury of some type.  Another study shows that stress turns up the volume knob on mast cells, priming them to release inflammatory chemicals that are typically involved with allergies, asthma, skin conditions, and digestive problems.  Furthermore, the communication coming from mast cells feeds back to nerves and modulates behavior through a sense of anxiety.  Mice bred with no mast cells have no fear, and thus boldly venture out and are easy prey.

Quercetin has shown that it can reduce the effects of stress in nerves while preventing depletion of nerve antioxidants due to stress.  The researchers concluded that the "results suggest that neuroprotective properties of quercetin can be used in the treatment and management of stress and related disorders."

One study with mice showed that pre-treating them with quercetin prevented the anxiety response to experimental stress via mast cell stabilization.  The researchers found that quercetin was highly protective to the nervous system.  The dose used translates to 1500 mg - 3000 mg per day for a 150 pound adult.  Quercetin not only prevents mast cells from inappropriately releasing irritant chemicals like histamine it also reduces the inflammatory immune system signals like IL-6 that come from mast cells and are known to talk to nerve cells (glial cells).

An April 2009 cell study showed that quercetin could prevent the formation of beta-amyloid plaque by a variety of mechanisms including its antioxidant function.  The researchers concluded, "These findings provide motivation to test the hypothesis that quercetin may provide a promising approach for the treatment of Alzheimer's Disease and other oxidative-stress-related neurodegenerative diseases."

A September 2009 study of animals with experimentally induced spinal cord injury showed that 50% of animals receiving quercetin twice daily for 3 or 10 days recovered the ability to walk, whereas no control animals recovered.  Quercetin was found to support the health of tissue bridges at the site of nerve injury, facilitating healing.

Collectively, this data shows a wide-range of nervous system support by quercetin.  Since many problems in today's world involve overheated nerves from stress that lead to other health problems, quercetin is an ideal tool to help calm down over-excited nerves as well as protecting nerves from free radical decline due to wear and tear.

Quercetin and Immunity

Quercetin's ability to modulate mast cells and allergy is significant, as covered in the preceding section.  It has also been shown to decrease the proliferation of prostate cancer cells, as well as knocking out such cells.  An animal study also showed that quercetin prevented early lesions in the digestive tract that lead to colon cancer, while also reducing the proliferation of colon cancer cells and once again knocking them out.  While such cell and animal studies are preliminary, they still point to quercetin's multiple roles in health protection.

Of significant interest in current times are quercetin's potential anti-flu properties.  Last year an animal study showed that quercetin significantly reduces susceptibility to the flu.  Mice were exposed to the influenza virus A/Puerto Rico/8/34 (H1N1) flu virus under various conditions, including increased stress from exercise.  The mice given quercetin were able to offset the adverse effects of stress and showed much less susceptibility to the flu.

There have been a number of studies showing that quercetin directly interacts with different types of viral infections, reducing their ability to infect.  An August 2009 cell study showed that quercetin interfered with the gene signals that enable hepatitis C virus production.  The researchers concluded that "Quercetin may allow for dissection of the viral life cycle and has potential therapeutic use to reduce virus production with low associated toxicity."

A June 2009 study showed that quercetin could reduce viral replication of influenza A by directly interacting with viral particles.  A 2005 study showed quercetin significantly boosted the respiratory antioxidant defense system in mice exposed to influenza A, offering significant protection for the lungs during a time of high stress. 

At this time there are no published studies on quercetin and the H1N1 swine flu.

Immunometabolism:  Linking Immunity and Obesity

This past summer the journal Nature Medicine published three groundbreaking articles linking the function of immune cells to obesity and diabetes - data which opens the door to solving all kinds of health problems including the obesity issue itself.

It has been known for a number of years that the extra pounds of fat in an overweight person are generating significant amounts of immune-related inflammatory signals such as TNFa and IL6.  Such inflammation not only damages the stored fat so that it is less metabolically responsive, it has been shown to induce inflammatory damage around the body.

What hasn't been understood are the changes within stored fat that result in this inflammatory state.  The new research goes a long way towards explaining exactly how this happens and the mechanism is startling.  It involves the function of various T regulator cells of the immune system, cells that until this point were never thought to have anything to do with metabolism and body weight. 

Here are links to abstracts of these studies:
Linking T cells and Glucose Uptake by Fat Cells - A study explaining changes in T cells that directly influence metabolism.
How Fat Inflammation Gets Going - A study explaining how these changes in T cells cause inflammation.
Antihistamines Prevent Obesity and Diabetes - A study showing one way to help address this issue is with the use of antihistamines (one of the drugs tested is a synthetic version of quercetin, nature's most potent natural anti-histamine).

These new studies show that in animals that are not overweight there is a high level of T Helper cells (CD4*) and regulatory T cells in their white adipose tissue.  However, in the fat of overweight animals and obese diabetic humans this population of immune cells is virtually gone and has been replaced with a population of CD8+ T cells (also called cytotoxic T cells or T killer Cells),  Cytotoxic T cells kill cancerous cells and virally infected cells.  Here they are in excess amounts within stored fat - apparently responding to initial stress within white adipose tissue from too much extra fat.

It was shown that these cytotoxic T cells were behind the recruitment of excessive macrophages into the extra pounds of fat.  The macrophages, in turn, generate the massive inflammation associated with being overweight or obese.  This problem, in turn, results in even more cyctoxic T cells and we end up with one large inflammatory party that is self-perpetuating as well as damaging the metabolism of calories in white adipose tissue (locking in excess pounds of stagnant fat that won't budge).

This means that the proper T helper cells and regulatory T cells are needed to keep white adipose tissue in a non-inflammatory condition.  The researchers also showed that when this slides out of balance then glucose uptake by fat cells is dysregulated leading to perpetuation of obesity, a nasty catch 22 that most certainly applies to any person who is having trouble losing weight by eating and exercising.

The researchers showed that as part of this immune cell problem, there were excessive numbers of T Helper 1 cells and a lack of T Helper 2 cells within the fat.  Excess T Helper 1 cells are associated with autoimmune problems, allergy, skin problems, etc.  It is quite likely that just as excess inflammation coming from fat cells can wreak havoc around the body, so it is that fat may be tilting overall immunity into T Helper 1 excess, leading to multiple health problems.  Or, a T Helper 1 health problem may in reverse help set the stage for obesity.  Either way it is not a good situation.

Furthermore, a lack of T Helper 2 cells, especially if the body is tilted to T Helper 1 excess by obesity, would impair the formation of antibodies needed to fight an infection such as the Swine Flu.

The researchers also identified that mast cells were far more abundant in fat tissue from obese and diabetic humans and mice compared to those of normal weight.  Giving antihistamine drugs (Zaditor and cromolyn) to mice significantly improved their metabolism.  Cromolyn is a drug patterned after the quercetin molecule, which is the best natural antihistamine available.

In their experiments the mice were divided into four groups. The first was the control group; the second group was simply switched to a healthy diet; the third was given cromolyn or ketotifen fumarate; and the fourth was both given the drug and switched to a healthy diet.

While symptoms of the second group improved moderately, the third group demonstrated dramatic improvements in both body weight and diabetes. The fourth group exhibited nearly 100 percent recovery in all areas.

To bolster these findings, researchers then took a group of mice whose ability to produce mast cells was genetically impaired. Despite three months of a diet rich in sugar and fat, these mice neither became obese nor developed diabetes.

These studies are indeed groundbreaking and open the door for new strategies to help individuals manage their weight, especially those who cannot get weight under control simply by eating better and exercising more.

Quercetin's Role In Metabolism and Weight Loss

In addition to the new discovery of excessive mast cell activity and poor metabolism, quercetin is demonstrating a variety of other ways in which it helps metabolism and weight management.

One of the great problems in becoming overweight is that your fat cells expand in size and multiply in number.  There appears to be no shortage of baby fat cells willing to mature into fat-storing goliaths.  Quercetin has been found to block baby fat cells from maturing as well as inducing cell death (appropriately) in the baby fat cell population.  Test showed that quercetin had a 71% inhibitory rate on new fat cell formation, far higher than any other flavonoid.  Another study using quercetin and resveratrol showed similar findings, suppression of fat cell formation and enhanced fat cell death.

Furthermore, quercetin has been shown to be absorbed into fat cells where it induces significant antioxidant activity.  This will lower inflammation coming from fat cells, such as the problematic excess of TNFa typically experienced by overweight individuals.  A metabolic study with quercetin showed it lowered all inflammatory markers tested, offsetting the stress of a high fat diet.

One new study calculates obesity risk by the percentage of healthy plant compounds from fruits and vegetables as a percentage of the diet.  There is no question that quercetin is the most abundant flavonoid in such a fresh food diet - and is definitely one of the most potent. 

Obese Zucker rats have leptin receptor problems resulting in excessive appetite, high cholesterol, high triglycerides, high blood pressure, insulin resistance, excessive numbers of fat cells, and obesity (the typical problems of obese humans).  Two doses of quercetin were tested over a ten week period (human equivalent is 136 mg or 681 mg for a 150 pound adult).  Both doses lower the blood pressure, triglycerides, cholesterol, and insulin resistance in these obese animal model.  The higher dose of quercetin lowered the inflammatory TNFa within adipose tissue, whereas adiponectin (which prevents insulin resistance) was increased.  It lowered unfriendly nitric oxide (iNOS) while boosting circulatory-friendly nitric oxide (eNOS).  These are amazing findings for any study.

Another problem in overweight individuals is that glucose is too easily taken up by fat cells after a meal, in turn stimulating excessive leptin production by fat cells and locking in leptin and insulin resistance.  It has been demonstrated that quercetin directly blunts this inappropriate uptake of glucose by fat cells.  Indeed, flavonoids in grapes such as quercetin are associated with less risk for developing type II diabetes.

The proper function of leptin and adiponectin within fat is vital for healthy metabolism.  When these hormones are working properly they turn on a pivotal enzyme called AMP-activated protein kinase (AMPK).  In turn, AMPK activates numerous metabolic signals that facilitate healthy metabolism and longevity.  One way to activate AMPK is to exercise.  Recently, researchers demonstrated that much of quercetin's anti-obesity effects are due to activation of AMPK.

Quercetin helps reduce inflammation and free radical problems occurring within fat.  It helps reduce the number of fat cells and prevent the development of new fat cells, key issues in the battle of the bulge.  It also boosts adiponectin levels that support healthy blood sugar metabolism.  It activates the AMPK enzyme system that facilitates healthy fat burning.  And its supreme ability to stabilize mast cells indicates that it is likely to change the function of immune cells operating within fat in a way that is conducive to having an easier time with weight management efforts. 

The collective body of quercetin research shows that it is a useful tool for immune stability, immune function, cardiovascular health, bone health, joint issues, nerve health, and metabolism.  It helps reduce inflammation while enhancing antioxidant function - facilitating numerous metabolic signals associated with health.