What Makes Onions So Healthy? It’s Quercetin

By Molly Knudsen, MS, RDN

Onions are an overlooked vegetable. Whether they’re being sautéed for a soup, sliced for a burger, or pickled for a salad, onions rarely steal the show of a meal, although they often inspire a sob story.

Onions are most known for their pungent flavor, not necessarily their nutritional value. Their white flesh (even in red onions) isn’t always perceived as being as nutritious as vibrantly colored vegetables like kale, broccoli, or beets. But that’s no reason to snub onions. Although some onion varieties are lacking in color, onions in general are one of the highest natural sources of a powerful flavonoid: quercetin.1

What is quercetin?

Quercetin is a flavonoid, which is a chemical compound found in plants. In addition to onions, quercetin is also found in many other fruits and vegetables like capers, asparagus, and apples, as well as tea and red wine.1

A single serving of onions is considered to be one medium onion, which contains around 52 milligrams (mg) of quercetin.2 But most people tend to not eat a whole onion in one sitting (unless they really like onions), so you’re probably not getting quite that much quercetin when you eat onions. 

Even with all these natural sources of quercetin, it’s estimated that most people following a typical Western diet (think higher in simple sugars and animal products and lower in produce) only get 0 to 30 mg a day.1 That’s not a lot, considering research shows that some of quercetin’s health benefits are reached at supplemental intake levels, not from a food source, of 500 to 1,000 mg per day.3

Quercetin and health

Quercetin is one of the most studied dietary flavonoids and is associated with multiple health benefits. Two of this compound’s most notable benefits are supporting heart health and protecting against oxidative stress.

Heart health: Quercetin plays a positive role in supporting healthy blood pressure and endothelial health.4 The endothelium is the thin layer of cells lining the body’s blood vessels and heart. It is considered an active organ in the body as it helps control when blood vessels relax or constrict.5

Protection against oxidative stress: Everyone undergoes oxidative stress, which is caused when there is an imbalance of harmful free radicals, which damage cells and tissues. This process happens inside the body, so although you may not feel immediate effects from oxidative stress, it can affect both health and the immune system over time. Antioxidants can help quell some of that stress. And since quercetin acts as a free radical scavenger and supports antioxidant processes, it’s especially good at this.3

Getting the most quercetin out of onions

Because these health benefits are mainly seen at higher levels of quercetin intake, it’s important to maximize what you can from the diet. Here’s how you can get the most quercetin when eating onions.

  1. Choose the right variety: Quercetin is a major compound in all varieties of red onions and chartreuse onions.6 Yellow onions and shallots also contain quercetin, just not to the same extent. This doesn’t mean that these or other varieties like green onions (scallions), leeks, or pearl onions are less nutritious. All onions are healthful and are rich in antioxidants. But red and chartreuse onions are the way to go if you’re focusing on quercetin content.
  2. Don’t overpeel: Quercetin is unevenly distributed in onions. It’s more highly concentrated in the outer layers of the peel.6 So after you peel back that thin paper covering, the first two to three layers, which tend to be thicker and maybe even a little slimy, have the most quercetin. Don’t peel and discard those layers. Chop or slice them up with the rest of the onion. And it’s always a good idea to wash onions under cold water after peeling to get rid of any dirt or contaminants that may have slipped through the outermost layer.
  3. Cook without water or eat raw: One study looked at the effect that various cooking methods have on flavonoid intake from an onion.7 The results showed that frying onions with oil or butter did not affect quercetin intake status, microwave cooking without water better retained flavonoids than with water, and boiling onions resulted in in about a 30% loss of quercetin into the water. However, eating onions raw is the likely the best way to ensure the quercetin content remains at its peak at the time of ingestion. Try thinly slicing red onions and adding them to sandwiches and salads.

What’s the bottom line?

Onions are a rich, natural source of the flavonoid quercetin. High intakes of quercetin are associated with cardiovascular benefits and oxidative stress protection. However, quercetin is not a “magic bullet” to health. Instead, focus on eating a plant-forward diet with lots of produce and know that onions, albeit smelly, are a healthful choice.

For more information on nutrition and general wellness topics, please visit the Metagenics blog.

References:

  1. D’Andrea G. Fitoterapia. 2015;106:256-271.
  2. Onion health research. Onions-usa.org. https://www.onions-usa.org/all-about-onions/onion-health-research/. Accessed July 30, 2020.
  3. El-Saber Batiha G et al. Foods. 2020;9:374.
  4. Patel RV et al. Eur J Med Chem. 2018;155:889-904.
  5. Deanfield JE et al. Circulation. 2007;115:1285-1295.
  6. Kwak JH et al. Saudi J Biol Sci. 2017;24(6):1387-1392.
  7. Loku K et al. J Nutr Sci Vitaminol (Tokyo). 2001;47(1):78-83.

This entry was posted in Cardiometabolic Health, General Wellness, Science and tagged , on by .

About Molly Knudsen

Molly Knudsen, MS, RDN is a writer at Metagenics. She completed her dietetic training with an emphasis on nutrition education at Texas Christian University and earned a Master of Science in Nutrition Interventions, Communication, and Behavior Change from the Friedman School of Nutrition Science and Policy at Tufts University. Knudsen has experience working with commodity boards and providing student athletes with nutrition coaching. She now practices nutrition education by digesting complex nutrition science through the written word.

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