Quercetin: The Most Promising Natural Antioxidant We Have?

Getting your Trinity Audio player ready...

Quercetin, a compound found in many fruits and vegetables, is consumed by humans daily and is also a popular dietary supplement. Quercetin is taken mainly for its antioxidant and anti-inflammatory properties, but this compound has quite a wide range of biological effects that might be beneficial for slowing the ageing process and preventing age-related diseases. For example, quercetin can influence the control of cell division to stop cancer cells from dividing, and appears to reverse age-related senescence (when healthy cells become unable to divide) when combined with a drug called dasatinib. In this review article, researchers discuss what we know about quercetin, how to get it, and how it might prevent age related diseases.

Sources of quercetin:

The researchers present a table showing the amount of quercetin contained per 100 grams of various quercetin-rich foods. While some foods have much higher concentrations of quercetin, the researchers highlighted onions as being a particularly good source that are easy to incorporate into a diet. These numbers aren’t definitive though, as growing and storage conditions can affect the quantity of quercetin contained within a food.

Bioavailability:

Bioavailability is the proportion of a substance that reaches the blood, which is of course necessary for it to reach one’s tissues and cells to exert its effects. Unfortunately, the bioavailability of quercetin is low, with only 10% or less of ingested quercetin reaching the blood. This is because much of the quercetin we consume is broken down before it has a chance to be absorbed in the gut, and because quercetin doesn’t dissolve well in water, making it harder to absorb. The absorption of quercetin can be increased by consuming it with fats. Quercetin supplements usually enclose quercetin within capsules designed to protect it from breakdown and enhance its absorption.

Health benefits of quercetin:

Next comes a summary of the evidence that quercetin can treat or prevent various age-related diseases. 

Cardiovascular disease:

Quercetin has the ability to dilate blood vessels and as such has been shown to lower blood pressure in both animals and humans. One study found that taking 730mg of quercetin per day for 12 weeks significantly lowered blood pressure in overweight adults with hypertension.

120mg of quercetin per day has also been shown to decrease inflammatory signalling in patients with coronary artery disease. These benefits are probably due to quercetin’s antioxidant abilities.

Alzheimer’s disease:

Quercetin inhibits inflammation (a key player in neurodegenerative diseases), limits the accumulation of amyloid and tau proteins thought to be involved in Alzheimer’s disease, and also enhances the activity of enzymes that break these proteins down. Cell culture studies also show that quercetin boosts the survival of brain cells.

All these effects suggest that quercetin might be valuable for treating or preventing Alzheimer’s and other neurodegenerative diseases, but as of yet there are no human studies demonstrating this.

Cancer:

One application of quercetin that has garnered particular interest among scientists and the general public alike is in the treatment or prevention of cancer. Quercetin has the ability to interfere with the division of multiple types of cancer cells and promote their destruction, which could be a very valuable property for enhancing existing cancer therapies. Indeed in the lab, quercetin has been found to have synergistic effects when used in combination with drugs targeting breast cancer, prostate cancer and leukaemia to name a few.

There are currently around half a dozen ongoing clinical trials investigating whether quercetin supplementation can benefit cancer patients, but there are no available results just yet.

Diabetes:

Quercetin increases sensitivity to the blood sugar-lowering hormone insulin, boosts the activity of the proteins that transport glucose into cells from the blood, and triggers the proliferation of insulin-producing cells in the pancreas. These properties suggest that quercetin might be a good antidiabetic supplement. There are some human clinical trials that back this up, but further larger studies are needed.

Immunity:

Quercetin has antimicrobial properties: it is able to inhibit the replication of bacteria and viruses, though exactly how it does this is still being investigated. It may also enhance the body’s own immune defences. A few clinical trials have investigated whether quercetin can prevent or aid recovery from infections in humans, but the results so far are not conclusive.

Arthritis:

In mice, quercetin has shown potential for treating both rheumatoid and osteoarthritis by reducing inflammation and inhibiting the main enzyme that degrades cartilage within the joints. Evidence from human clinical trials is promising but somewhat inconsistent.

Liver health:

Cell culture studies suggest that quercetin might help prevent liver scarring by suppressing inflammation and halting the division of cells that lay down scar tissue. Liver scarring mostly occurs as the result of long-term damage caused by infection, alcohol abuse or other toxic exposure, but the ability of the liver to regenerate after being damaged also decreases with age, so quercetin could be beneficial for maintaining liver health in old age by reducing scarring over time. However, there’s currently no human evidence for this.

The take-home message:

Quercetin is a natural compound that shows great promise in the prevention and treatment of a wide range of age-related diseases, but there’s still not much human evidence to support its benefits. Since quercetin isn’t very bioavailable, utilising its full potential might rely on new technologies like lipid nanoparticles that can deliver quercetin to target cells more efficiently.

In the meantime, is striving to obtain as much quercetin as possible from dietary sources or taking quercetin supplements justifiable? This review doesn’t comment on the merits of quercetin supplementation for general health and wellbeing. We can say that quercetin appears to be very safe, with no reported side effects in doses up to a few grams. Given that quercetin is a plant compound and that plant-based diets appear increasingly beneficial for healthy ageing, there is little to lose from consuming more quercetin-containing foods. However, some of the clinical trials showing therapeutic effects for quercetin used doses that are difficult to achieve through diet alone, so supplementation might provide additional benefits, though a doctor should be consulted before taking any dietary supplement.