Fresh apples -- the peel in particular -- have some of the highest levels of quercetin (also found in onions, broccoli, kale, blueberries, cranberries and red grapes). Some of the most lab exciting studies on this flavonol suggest it may help fight Alzheimer's disease by protecting brain cells against oxidative stress. In an animal study at Cornell University, quercetin proved more powerful than the antioxidant vitamin C in neutralizing the kind of neural damage done by free radicals. "Fresh apples have some of the highest levels of quercetin ... and may be among the best food choices for fighting Alzheimer's," said study author C.Y. Lee.
What is Quercetin?
Quercetin belongs to a group of plant pigments called flavonoids that give many fruits, flowers, and vegetables their color.
Flavonoids such as quercetin are antioxidants -- they scavenge damaging particles in the body known as free radicals, which damage cell membranes, tamper with DNA, and even cause cell death. Antioxidants can neutralize free radicals and may reduce or even help prevent some of the damage they cause. They also help keep LDL ("bad") cholesterol from being damaged, which scientists think may contribute to heart disease. In test tubes, quercetin has strong antioxidant properties, but researchers aren't sure whether taking quercetin (and many other antioxidants) has the same effects inside the body.
Quercetin acts like an antihistamine and an anti-inflammatory, and may help protect against Alzhemier’s disease, heart disease, and cancer.
Cornell University Study
In this latest research, Lee and his colleagues exposed rat brain cells to varying amounts of quercetin or vitamin C, a well-established antioxidant and disease fighter. The brain cells were then exposed to hydrogen peroxide, a substance that can mimic some of the kinds of cell damage that occurs in the brains of those with Alzheimer’s disease. The cells that had been treated with quercetin showed significantly less damage than the vitamin-C treated cells or brain cells that did not get any antioxidant protection. Quercetin also protected nerve cell membranes more than did vitamin C. This is significant because the researchers believe that loss of cell membrane integrity contributes to neurotoxicity. Larger doses of quercetin or vitamin C conferred greater protection from oxidative stress.
Scientists are not sure exactly how the quercetin in apples protects brain cells in the lab, although its antioxidant effects are thought to neutralize cell-damaging compounds called free radicals. Other foods that are rich in quercetin, such as onions, blueberries, cranberries, and tea, may also afford protection against Alzheimer’s. Quercetin is also sold in nutritional supplements. The apple, however, remains at the top of the list when it comes to natural sources for disease-fighting quercetin.
“People should eat more apples, especially fresh ones,” says study leader C.Y. Lee, Ph.D., professor and chairman of the Department of Food Science & Technology at Cornell University in Geneva, N.Y. He cautions that protection against Alzheimer’s using any food product is currently theoretical and adds that genetics and environment are also believed to play a role in the disease. Despite these caveats, the researcher predicts that “eating at least one fresh apple a day might help.” But Lee also points out that results so far are limited to cell studies and that more advanced research, particularly in animals, is still needed to confirm the findings.
Querecetin Natural Sources
Even though quercetin is relatively stable during cooking, fresh apples are better sources of quercetin than cooked or processed apple products because the compound is mainly concentrated in the skin of apples rather than the flesh, Lee says. Products such as apple juice and apple sauce do not contain significant amounts of skin. In general, red apples tend to have more of the antioxidant than green or yellow ones, although any apple variety is a good source of quercetin.
For those who don’t like apples or may have difficulty eating the whole fruit, there are some promising alternatives, Lee suggests. Other foods containing high levels of quercetin include onions, which have some of the highest levels of quercetin among vegetables, as well as berries, particularly blueberries and cranberries. Like other antioxidants, quercetin has been associated with an increasing number of potential health benefits, including protection against cancer.
How to Take Quercetin?
Recommended adult dosages of quercetin vary depending on the condition being treated. The recommendations for the general supplementation are 100 - 250 mg 3 times per day.
Precautions
Quercetin is generally considered safe. Side effects may include headache and upset stomach. Preliminary evidence suggests that a byproduct of quercetin can lead to a loss of protein function.
Pregnant and breastfeeding women and people with kidney disease should avoid quercetin.
At high doses (greater than 1 g per day), there are some reports of damage to the kidneys.
Possible Drug Interactions
If you are being treated with any of the following medications, you should not use quercetin supplements without first talking to your health care provider.
Anticoagulants (blood thinners) -- Quercetin may enhance the effect of these drugs, increasing your risk for bleeding:
- Warfarin (Coumadin)
- Clopidogrel (Plavix)
- Aspirin
Chemotherapy -- Test tube and animal studies suggest that quercetin may enhance the effects of doxorubicin and cisplatin, two chemotherapy medications used to treat cancer. In addition, some doctors believe taking antioxidants at the same time as chemotherapy can be harmful, while others believe it can be helpful. Talk to your oncologist before taking any supplements if you are undergoing chemotherapy.
Corticosteroids -- Quercetin may cause these drugs to stay in the body longer.
Cyclosporine -- Quercetin may interfere with the body's absorption of this drug, which is used to suppress the immune system.
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