Quercetin is a natural flavonoid acting as both an antioxidant and anti-inflammatory. This potent flavonoid is found in low levels in many fruits and vegetables, including elderberry, apples, and onions.
As a supplement, quercetin has many positive health benefits. This article focuses on the results of clinical trials involving quercetin as well as linking to specific genetic topics. By using your genetic data, you can make a more informed decision on whether quercetin is worth trying.
Quercetin has shown in cell studies to be a fabulous, wonder-supplement for lots of different conditions. But…the studies in humans don’t always match up with the cell studies and animal studies. I’m going to focus mainly on the human trials of quercetin and go into the cell studies just a bit in reference to genetics.
Food sources of quercetin:
- Capers: 234mg/100g
- Black elderberry: 42 mg/100g
- Dark chocolate: 25 mg/100g
- Shallots and onions: 10 – 31mg/100g
- Apples, with skin: 2 -4 mg/100g
- Bilberry: 1.27/100g
- Red Wine: 0.83 mg/100 ml
- Apple juice: 0.48 mg/ 100ml
Blood pressure reduction studies that use quercetin:
In a double-blind randomized placebo-controlled study, quercetin reduced blood pressure in men with hypertension. The study used 730 mg/day of quercetin and found that it reduced systolic blood pressure by 7 mm Hg and diastolic blood pressure by 5 mg Hg.[ref ]
Another study using a smaller dosage of quercetin had a smaller decrease in blood pressure. The study results showed a decrease in systolic blood pressure of 3.6 mm HG in overweight patients with high blood pressure using only 162 mg/day of quercetin.[ref ]
A meta-analysis that combined the data from 7 clinical trials found there was a significant reduction in blood pressure in randomized controlled trials that used doses of more than 500mg/day.[ref ]
Quercetin for oxidative stress and oxidized LDL:
When a cell has an imbalance of reactive oxygen species (ROS) to antioxidants, it is referred to as oxidative stress. These reactive oxygen species contain an unstable balance of electrons and can cause reactions that are damaging to the cell. Too much oxidative stress can cause DNA damage, the production of inflammatory signals, and eventually cell death.
Quercetin is a free radical scavenger shown in studies to help prevent oxidized cholesterol.[ref ] This is important because oxidized cholesterol may accelerate atherosclerosis or plaque build-up in the arteries.[ref ]
A double-blinded, placebo-controlled cross-over trial in overweight adults with metabolic syndrome found that 150mg/day of quercetin decreased the concentration of oxidized LDL cholesterol. There wasn’t much of an effect on any other health markers else at this dosage, but just decreasing the oxidized LDL should decrease the risk of cardiovascular disease.[ref ]
Researchers theorize that oxidative stress contributes to Alzheimer’s disease pathology. Animal and cell studies show that quercetin can protect against oxidative stress in the brain and partially prevent the associated neuronal toxicity.[ref ]
Most of the time, preventing oxidative stress is something that you want to do – but not always. One of the benefits of exercise is to create stress, which causes cells to respond by adapting and producing more mitochondria. Quercetin’s effect on exercise performance has been researched. The results of the studies have varied, but most show that quercetin doesn’t increase exercise performance.[ref ] If you are supplementing with quercetin, consider whether you should take it at a time that it won’t interfere with the benefits of exercise-induced stress.
Related article: Athletic Performance Genes
Quercetin as a senolytic (longevity benefits):
Cellular senescence occurs when a cell can no longer divide or do its normal function. Basically, the cell just sits there giving off proinflammatory signals. Those inflammatory factors can then impact the surrounding cells. Kind of like a meth dealer moving into the neighborhood…not good for the neighbors.
The body can get rid of senescent cells pretty well – up to a point. But when too many senescent cells accumulate, things start going downhill. Recent research points to senescent cells actually causing a lot of the diseases of aging, rather than just being a symptom of aging.
Clearing out senescent cells could either delay or possibly reverse aging. That would be pretty cool…
But what about human trials? When quercetin is combined with Dasatinib (a leukemia drug), it clearly reduces senescent cells.[ref ][ref ] This is an exciting field of study that shows a lot of promise for the future.
One more way that quercetin may improve atherosclerosis and oxidized LDL is by reducing senescent cells in the endothelium (lining of the arteries). A new study looked at quercetin’s effect on a cell model of atherosclerosis. The study found that quercetin inhibited the foam cells created by oxidized LDL in atherosclerosis and it also decreased senescent cells. While just a cell study, this points to quercetin possibly having multiple beneficial effects in cardiovascular disease.[ref ]
Related article: NAD+
Advanced glycation end products:
The production of advanced glycation end products (AGEs) in the body (and through foods we eat) increases the diseases of aging. For AGEs that are produced in the body, methylglyoxal levels are important.
A randomized, double-blind, placebo-controlled crossover trial found that 160 mg/d of quercetin reduced methylglyoxal, a precursor for AGEs by an average of 11% after 4 weeks.[ref ]
Related article: Advanced Glycation End Products
High uric acid levels are a risk factor for gout. A double-blind, placebo-controlled cross-over trial in healthy men with higher uric acid found that quercetin lowered uric acid levels. The trial used 500mg/day of quercetin for four weeks and decreased uric acid by 26·5 µmol/l on average.[ref ]
Since quercetin can reduce both pain and inflammation, it makes sense that it could help with rheumatoid arthritis. Indeed, a two-month placebo-controlled trial found that quercetin reduced morning pain, stiffness, and post-activity pain. Quercetin also reduced TNF-alpha (inflammatory cytokine) levels. The trial included 50 women with RA who took either 500mg/day of quercetin or a placebo.[ref ]
Related article: TNF-alpha and rheumatoid arthritis
A randomized placebo-controlled trial found that 12 weeks of quercetin at 1000mg/day reduced upper respiratory tract infections.[ref ]
Excessive exercise can make you more susceptible to getting sick. In a mouse trial where the mice exercised to fatigue (treadmill) for days, researchers found that quercetin offset the increased propensity to get sick after exercising to fatigue.[ref ] This may be something to try if you are training for an upcoming exercise-intensive event.
A human study showed that quercetin was safe (>5g/day) and effective for some people in reducing the viral load in hepatitis C patients.[ref ]
Mast Cell Blocker:
Mast cells are an important part of the immune system that can degranulate and signal for an inflammatory response. Overactive mast cells can be a problem, leading to allergic responses or to mast cell activation syndrome. One compound that mast cells can release is histamine.
Quercetin has shown to stabilize mast cells and inhibit the release of histamine.[ref ]
Related article: Histamine and Mast Cells
Caffeine and quercetin (CYP1A2 gene):
Quercetin inhibits CYP1A2, which is the enzyme the body uses to metabolize caffeine.[ref] If you are a slow metabolizer of caffeine, quercetin along with caffeine may mean that you feel the effects of caffeine for a longer period of time.
Relevant articles: CYP1A2 Variants
Absorption, transport, and metabolism
For any substance to have an effect in the body, it needs to be absorbed (usually in the intestines), transported throughout the body, and broken down and eliminated (usually through the liver). One big difference between cell studies and in vitro human studies is the absorption and metabolism of substances.
Absorption of quercetin:
A study found that the bioavailability of quercetin in humans is about 45%. The half-life of quercetin ranges between 11-28 hours, so taking it daily will build up more than just what was consumed that day.[ref ]
Researchers have looked at the interaction between different types of foods and the absorption of quercetin. They found that eating a meal that contains fat along with the quercetin supplement can increase bioavailability.[ref ] Alternatively, if you normally take a fish oil supplement, you may want to take it at the same time as your quercetin.
Transdermal absorption of quercetin is an interesting idea, especially with the antioxidant properties and the possibility of enhancing the skin. Since it is hydrophobic, quercetin won’t dissolve well in water. One study found olive oil and soybean oils were better for transdermal absorption, compared to avocado, raspberry seed, and coconut oils.[ref ] Another study used solid lipid nanoparticles with quercetin to enhance skin absorption.[ref ] (Personal note…quercetin is a yellow-ish powder that stains everything it touches. When mixed with olive oil, it lends a jaundiced-hue to skin.)
Metabolism and interactions:
Quercetin is an inhibitor of the CYP2C8 enzyme. If you have genetic variants that slow down the CYP2C8 variant and stack quercetin with a medication that uses that enzyme, you could majorly impair the metabolism of the medication.
Related article: CYP2C8 Genetic Variants
Quercetin has a catechol structure and is partly metabolized through the COMT enzyme. If you carry the slower version of COMT, you may want to be careful and not go overboard with quercetin.
Related article: COMT variants
Transport of quercetin:
The MATE1 (multidrug and toxic compound extrusion transporter-1) protein is the quercetin transporter. It is coded for by the SLC47A1 gene.[ref ]
What quercetin doesn’t do…
I don’t want to leave you with the impression that quercetin is a miracle cure for everything. Far from it! Instead, it may be another tool to use for a specific purpose at the right dosage.
For example, a trial of 162 mg/d of quercetin for six weeks showed little effect on C-reactive protein, leptin levels, or blood glucose levels in overweight adults with metabolic syndrome. The study did show that the 162 mg dose was safe.[ref ]
Another study in healthy adult women found no significant effect from either 500 mg/day or 1000 mg/day on inflammatory markers and immune function.[ref ]
The studies that show more of an effect seem to be using higher dosages (500-1,000 mg/day). Plus, it may be more effective in people who have a problem (e.g. high blood pressure or cholesterol) rather than having a large effect on people who are healthy.
Where to get quercetin:
You can get quercetin as a supplement in capsules or as a powder. The advantage of getting it as a powder and either putting it in a smoothie or into capsules yourself is that you can eliminate the excipients in most capsule formulas. The drawback to the powdered form is that it is yellow and will stain everything that it touches a bright yellow. The stains do eventually come out of kitchen towels…lessons learned.
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