The Benefits of Quercetin in Onions and Apples

In 2015, flavonoid quercetin was found to be a natural senolytic, killing off pro-inflammatory zombie cells. It’s found concentrated in onions, kale, and apples. Those consuming more quercetin (and flavonoids in general) appear to have less than half the risk of dying from heart disease, the leading killer throughout much of the Western world. Quercetin intake may just be a proxy for eating a healthier diet, but even controlling for proxies of healthy plant-based food intake, such as fiber and vitamin C, a 68 percent lower risk remained. The leading source of flavonoid intake was tea, though, which may extend lifespan through a variety of mechanisms.

Even if some of the benefits can be attributed to the quercetin, that doesn’t necessarily mean it was through a senolytic pathway. For example, quercetin is a potent antioxidant, which is thought to be why it extends the lifespan of C. elegans by 15 percent and the lifespan of yeast by 60 percent. It also has anticlotting properties. Onions are the leading food source of quercetin in the United States. Eating onion soup can thin your blood within hours of consumption, which could potentially explain the reduced risk of blood clots and cardiovascular disease in those who eat more quercetin-rich foods and beverages.

Onion consumption can also lower cholesterol. Researchers fed people nearly an entire stick of butter, and their cholesterol shot up about 30 points within hours of consumption. But, combine that with a third cup of raw or boiled onion and cholesterol only goes up a fraction of the amount. (Of course, the real moral of the story is…don’t eat a stick of butter!)

Apples are also a leading source of quercetin. The wisdom of the public health aphorism dating back to 1866, “An apple a day keeps the doctor away” seems to have borne fruit. Those who eating more apples tend to live longer than those who eat fewer apples. Regular apple eaters may even live longer than berry or greens aficionados. One study of elderly women found that “moderate” apple consumption, by which they meant approximately an apple or two a week, was associated with a 20 percent lower risk of dying prematurely, whereas those who ate an apple a day had a 35 percent lower risk. Why is that the case? It seems to be less the apple of one’s eye than the apple of one’s arteries.

Even a fraction of an apple a day is associated with 24 percent lower odds of having severe major artery calcifications, a marker of vascular disease. It appears to be more than just a generic fruit effect, since there was no such link with the consumption of pears, oranges, or bananas. (Wait, did I just compare apples to oranges? Yes!)

Interventional studies show that it may not even be an apple effect, but rather an apple peel effect, as you get significantly better improvement within hours eating unpeeled apples than you do after peeled. This would be consistent with a quercetin effect (that’s what gives apple peels their bitter taste), but you don’t know until you put it to the test. And indeed, randomized controlled trials have shown that isolated quercetin supplements may lower blood pressure, lower cholesterol, and lower inflammation.

Although most of the quercetin supplement studies used doses not achievable naturally, even just a teaspoon of fresh onion can acutely improve blood pressure and fluidity compared to placebo, and a spoonful of dried apple peels a day over 12 weeks seemed to improve the range of motion and soreness of those with chronic joint pain. This study was uncontrolled; so, the placebo effect couldn’t be discounted, but an apple a day can’t hurt. In fact, modelling suggests prescribing an apple a day could prevent about as many deaths from vascular disease on a population scale than prescribing everyone a cholesterol-lowering statin drug—with fewer side-effects. (Though ironically, now that drugs like Lipitor are available in generic form, the drugs would likely be cheaper than the fruit.)