Onion peels: 7 benefits and 6 uses

red onion on cutting board with some peels broken off

When you cut into onions and start dicing, do you usually toss out the paper-thin onion peels?

If you are like most people, you toss onion peels and possibly an outer layer or two. Onion peels are considered waste, while the inside bulb is considered useful. Don’t feel bad – other people are also contributing to the 500 000+ tons of onion waste in the European Union alone (1).

Onion peels and quercetin

Unknowingly, you are actually tossing out the most valuable part of onions – the part that may give you added protection against COVID-19 and other viruses. Peels and outer layers of onions contain high amounts of quercetin, a flavanol plant compound imparting pigment and defense against pests. This phytochemical (plant compound) is one of the most ubiquitous flavonoids found in herbs, vegetables, fruit, and red wine. It is a well-known antioxidant with anticancer, anti-inflammatory, antihistamine, cardio- and neuroprotective, and antiviral actions.

Quercetin an antiviral agent

Quercetin, as an antiviral agent, is in the spotlight as a possible treatment against COVID-19. Canadian researchers have already established its success against the ZIKA and Ebola viruses in nonhuman primates (2–4). As a broad-spectrum antiviral agent deemed safe by the FDA for supplemental use, these researchers are placing their bets on its usefulness against COVID-19 for humans. Clinical trial in China is already underway to reveal whether quercetin is a viable option. At the very least, it could provide a timely and inexpensive treatment against COVID-19, buying time while other antiviral options are developed.

Research Evidence 

In vitro studies

In vitro studies (those done in petri dishes with human or animal cell lines) show quercetin inhibits viral infections and replication at various stages of several respiratory viruses, including influenza virus, parainfluenza virus, adenovirus, and rhinovirus (5–7). 

Animal studies

An animal study using mice inoculated with human influenza virus A and B tested the antiviral effects of several plant-derived polyphenols (a class of phytochemicals, including flavonoids). Isoquercetin, a common form of quercetin in food, inhibited replication of both strains with the lowest effective concentration compared to other polyphenols (8). In other mice models, forms of quercetin have also demonstrated antiviral effect against ZIKA viral entry into host cells (3) and against Ebola even when given in small amounts 30 minutes prior to infection (4). 

Ideal antiviral agent

While many antiviral agents target one or two virus strains, quercetin has broad-spectrum antiviral activity (6,8). Additionally, many viral strains develop resistance to antiviral medications after interactions. Not so with quercetin! Repeated interactions between quercetin and influenza A and B did not cause resistance. This is extremely beneficial in our current global pandemic where timely treatment and infection prevention is necessary. 

Antioxidants benefits for viral infections

Viral treatment isn’t only about preventing infection and the spread of the virus. It includes managing the inflammatory effects, free radical damage, and tissue damage from the virus as it invades the body. Fortunately, quercetin is not only an antiviral agent. Its anti-inflammatory and antioxidant properties reduce proinflammatory cytokines and lung inflammation caused by the rhinovirus (7).  

A 2011 study found that antioxidant therapy from plant-derived compounds used with antiviral drugs significantly reduced influenza-associated complications from the influenza A (H1N1) virus strain of 2009. Viruses damage tissues, but immune cells that rush to our defence (i.e. macrophages) also release tissue-damaging free radicals (i.e. superoxide anion), which have been implicated in the development of severe influenza-associated complications (9). 

Quercetin safety 

No clinical trials using quercetin on humans to treat or prevent viral infections have been completed; however, many clinical trials testing quercetin’s other health benefits and safety on humans are complete (10–13). 

All we know in respect to safety is with supplemental doses of quercetin. To date, 500-1000 mg of quercetin for 12 weeks is generally considered safe, although people with pre-existing kidney damage are advised against taking supplemental doses. In addition, quercetin interacts with some drugs. Talk with your pharmacist if you take Celebrex, Dilantin, Coumadin, Elavil, Paxil, Prilosec, or Zoloft and are considering taking quercetin supplement. 

Currently the average quercetin intake for men and women in America is 15-17 mg per day (14). That’s a long way off from supplemental doses.  For the time being, we can prudently do our best to increase our daily intake of quercetin, not only for its antiviral effect but also all its other health benefits.

This is where onion peels come in.

Onion peels to the rescue

Red onions, particularly the papery peel of red onions, are among the top 10 richest sources of quercetin (15). Other foods high in quercetin are berries, broccoli, red wine, green tea, apples, and red lettuce. Lactic acid fermentation modifies quercetin into a form that is better absorbed and delivers more antioxidant effects (18) and, presumably, more antiviral effects.

Onions: organic vs. conventionally grown?

Onions are not on the Dirty Dozen List, meaning pesticide residue on onions is low, but this is not the only factor to consider when choosing onions. While organically grown onions do not have higher nutrient values (minerals and vitamins) than conventionally grown onions, a long-term study over 6 years showed that organic Red Baron onions (common red onions) and Hyskin onions (common yellow onions) contain higher antioxidant activity and 10-50% higher total flavonoids than conventionally grown onions (16). 

A major 2012 systematic review reported similar findings in organic produce versus conventionally grown produce (17). Annual rainfall and temperature contribute to overall polyphenol content, but soil management and pest control are likely larger contributing factors. Absence of synthetic pesticides to fend off microbial attack cause onions to generate more quercetin (and other plant flavonoids) as a defense mechanism.

If quercetin is one benefit you want from onions, purchase organic red onions when possible and utilize the onion peel.

Tips for using more onion peels

Since organic onion peels are a rich source of quercetin, using them in food preparation will lead us to consuming more quercetin. Here are some ways to incorporate more onion peels, thus more quercetin, into your diet.

1. Fermented onions with the onion peels.

Fermented onions have quercetin in a highly bioavailability form. Fermentation also makes raw onions more palatable and gut-healthy due to lactic acid bacteria. These good bacteria alone boost immunity, yet fermented onions have the added benefit of quercetin.

When fermentation is complete, use the fermented onions as you would raw onions (e.g. in soups, stews, hamburgers, tacos). As you use the onions, take a one-tablespoon shot of onion brine before meals. Always ensure brine submerges remaining onions; that is, keep the brine levels above the onion slices. Onion brine is a very rich source of quercetin. Discard the onion peels once you reach the bottom of the jar.  

2. Broth, stock, and bone broth

Wash the onion before quartering, then add the whole onion – peel, fleshy bulb/leaves, and root – into your bone broth to impart extra nutritional and quercetin value. Strain the papery peel along with the bones, vegetables, and spices afterward.

3. Soups and stews

Add onion peels to soups and stews. Removing them before eating is challenging, so consider putting peels in a tea bag, cheesecloth, or muslin packet before adding them to your soup or stew. This is what the french call sachet d’épices or spice sachet. This makes for easier removal. Who wants to be eating a piece of papery onion peel when they are expecting vegetables, broth, legumes, grains, or meat? – really! 

4. Cooked grains and rice

Whenever you cook rice or intact whole grains – millet, amaranth, quinoa, teff, sorghum – add some leftover onion peels to the bottom of the pot or put into a spice sachet as explained above. The skins should be submerged under water to ensure nutrients and quercetin filter into the water and absorb into the grain or rice as it cooks. 

5. Tea

Bring onion peels and any other tea herbs or roots to a boil, then reduce heat to a gentle simmer for 10-20 minutes. Strain the tea and peels (or use a tea ball), and enjoy the benefits.

6. Ground onion peel

Using a mortar and pestle or a coffee grinder, grind onion peels into a powder and add to casseroles or meatloaves for extra onion flavour, nutrients, and quercetin. Add a teaspoon or more to savory bread dough for a mild onion flavour. 

Final thoughts

It’s unknown whether significantly increasing the amount and bioavailability of quercetin from foods can translate into therapeutic effects. Dosage certainly matters for achieving therapeutic effect; however, optimal dosage of quercetin as an effective treatment against COVID-19 in humans is still unknown. 

While we wait for results from clinical trials with quercetin against COVID-19, there is little harm in increasing your dietary intake. At the very least, its high antioxidants will improve other aspects of health.

onions cut crosswise with onions in the background

REFERENCES

1.         Benítez V, Mollá E, Martin-Cabrejas M, Aguilera Y, López-Andréu F, Cools K, Terry L, Esteban R. Characterization of Industrial Onion Wastes (Allium cepa L.): Dietary Fibre and Bioactive Compounds. Plant Foods Hum Nutr Dordr Neth. 2011;66:48–57. 

2.         Wong G, He S, Siragam V, Bi Y, Mbikay M, Chretien M, Qiu X. Antiviral activity of quercetin-3-β-O-D-glucoside against Zika virus infection. Virol Sin. 2017;32:545–7. 

3.         Gaudry A, Bos S, Viranaicken W, Roche M, Krejbich-Trotot P, Gadea G, Desprès P, El-Kalamouni C. The Flavonoid Isoquercitrin Precludes Initiation of Zika Virus Infection in Human Cells. Int J Mol Sci. 2018;19. 

4.         Qiu X, Kroeker A, He S, Kozak R, Audet J, Mbikay M, Chrétien M. Prophylactic Efficacy of Quercetin 3-β-O-d-Glucoside against Ebola Virus Infection. Antimicrob Agents Chemother. 2016;60:5182–8. 

5.         Kaul TN, Middleton E, Ogra PL. Antiviral effect of flavonoids on human viruses. J Med Virol. 1985;15:71–9. 

6.         Chiang LC, Chiang W, Liu MC, Lin CC. In vitro antiviral activities of Caesalpinia pulcherrima and its related flavonoids. J Antimicrob Chemother. 2003;52:194–8. 

7.         Ganesan S, Faris AN, Comstock AT, Wang Q, Nanua S, Hershenson MB, Sajjan US. Quercetin inhibits rhinovirus replication in vitro and in vivo. Antiviral Res. 2012;94:258–71. 

8.         Kim Y, Narayanan S, Chang K-O. Inhibition of influenza virus replication by plant-derived isoquercetin. Antiviral Res. 2010;88:227–35. 

9.         Uchide N, Toyoda H. Antioxidant therapy as a potential approach to severe influenza-associated complications. Mol Basel Switz. 2011;16:2032–52. 

10.       Heinz SA, Henson DA, Austin MD, Jin F, Nieman DC. Quercetin supplementation and upper respiratory tract infection: A randomized community clinical trial. Pharmacol Res. 2010;62:237–42. 

11.       Mlcek J, Jurikova T, Skrovankova S, Sochor J. Quercetin and Its Anti-Allergic Immune Response. Mol Basel Switz. 2016;21. 

12.       Heinz SA, Henson DA, Nieman DC, Austin MD, Jin F. A 12-week supplementation with quercetin does not affect natural killer cell activity, granulocyte oxidative burst activity or granulocyte phagocytosis in female human subjects. Br J Nutr. 2010;104:849–57. 

13.       Jin F, Nieman DC, Shanely RA, Knab AM, Austin MD, Sha W. The variable plasma quercetin response to 12-week quercetin supplementation in humans. Eur J Clin Nutr. Nature Publishing Group; 2010;64:692–7. 

14.       Sampson L, Rimm E, Hollman PCH, de Vries JHM, Katan MB. Flavonol and flavone intakes in US health professionals. J Am Diet Assoc. 2002;102:1414–20. 

15.       Showing all foods in which the polyphenol Quercetin 3-O-glucoside is found [Internet]. Phenol-Explorer. [cited 2020 Mar 19]. Available from: http://phenol-explorer.eu/contents/polyphenol/293

16.       Ren F, Reilly K, Kerry JP, Gaffney M, Hossain M, Rai DK. Higher Antioxidant Activity, Total Flavonols, and Specific Quercetin Glucosides in Two Different Onion (Allium cepa L.) Varieties Grown under Organic Production: Results from a 6-Year Field Study. J Agric Food Chem. 2017;65:5122–32. 

17.       Smith-Spangler C, Brandeau ML, Hunter GE, Bavinger JC, Pearson M, Eschbach PJ, Sundaram V, Liu H, Schirmer P, Stave C, et al. Are organic foods safer or healthier than conventional alternatives?: a systematic review. Ann Intern Med. 2012;157:348–66. 

18.       Kimoto-Nira H, Ohashi Y, Amamiya M, Moriya N, Ohmori H, Sekiyama Y. Fermentation of onion (Allium cepa L.) peel by lactic acid bacteria for production of functional food. J Food Meas Charact. 2020;14:142–9. 

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