You’ve heard countless times that an apple a day keeps the doctor away. Well, there’s good reason one of the oldest clichés in the book still gets play: Apples pack a potent nutrient punch that helps keep the body healthy and functioning at full tilt. In addition to fiber and vitamins A and C, one of apples’ star constituents is the plant pigment quercetin, a high-powered flavonoid that acts as an antioxidant, anti-inflammatory and antihistamine to help ward off disease. 

While apples are one of the best sources of quercetin, they’re not the only food that supplies it. Onions, tomatoes, broccoli, kale and tea also deliver quercetin, as do berries and grapes—a key reason reason red wine has health benefits. 

Here’s a rundown of quercetin’s science-backed health boons.

Cancer prevention

Quercetin’s antioxidant properties make it a fierce free-radical fighter, meaning it protects cells against oxidative stress and possibly disease, including cancer. A study published in the American Journal of Clinical Nutrition in September 2002 examined the flavonoid intakes of 10,000 adults and found that men who got more quercetin had lower lung cancer incidence. 

In addition, research published in the journal Carcinogenesis in April 2010 showed a correlation between quercetin, lung cancer and cigarette smoking (a known cause of lung cancer) in approximately 4,000 people. Researchers noted an inverse association between diets heavy in quercetin-rich foods and lung cancer risk and pinpointed a possible mechanism by which quercetin changes the expression of genes involved in metabolizing tobacco carcinogens.

Quercetin also shows promise in staving off pancreatic cancer. In October 2007, the American Journal of Epidemiology published a human trial that showed intake of quercetin, along with the flavonoids kaempferol and myricetin (abundant in fruit skins), led to significantly lower incidence of this deadly disease. As in the lung cancer research mentioned above, the flavonoids were especially protective among current cigarette smokers. 

In other research, published in the journal Translational Oncology in March 2008, disease-quelling quercetin, resveratrol and catechins inhibited breast tumor growth in mice, suggesting these compounds may prevent breast cancer from spreading. 

Cardioprotection

Both in vitro and animal studies, along with epidemiological research, have suggested quercetin offers cardioprotective benefits. Research published in the Journal of Cardiovascular Pharmacology in March 2008 showed that quercetin lowered the blood pressure of hypertensive rats. Another study in the May 2002 issue of the American Journal of Clinical Nutrition, showed that quercetin intake was associated with fewer heart attack deaths in a group of 4,800 women ages 55 and older. In October 2009, the British Journal of Nutrition published research that demonstrated quercetin reduced systolic blood pressure and high-density lipoprotein (HDL, or “bad cholesterol”) in overweight participants, bolstering the flavonoid’s potential role in staving off coronary heart disease. 

Asthma, allergy and respiratory relief

A handful of laboratory studies conducted in the 1980s found quercetin effective for reducing allergic reactions such as watery eyes, facial swelling and runny noses, because it stops immune cells from firing off histamines aimed at defending against allergens. Via the same mechanism—blocking histamine release, which leads to congestion—and by taming inflammation of the lungs, quercetin may also help relieve asthma and hay fever. 

More recently, the aforementioned 2002 study of 10,000 subjects also found asthma risk to be lower in people with greater quercetin intakes. Finally, cell research presented at the 2010 American Thoracic Society International Conference suggests quercetin may also combat rhinovirus, a common asthma trigger and instigator of the common cold, by unleashing its arsenal of antihistamine, anti-inflammatory and antioxidant properties.