The best natural sources of quercetin: apples, onions, capers and green tea

From apple to onion: why quercetin matters for healthy ageing

Quercetin is a plant compound found in many everyday foods – especially apples, onions, capers and green tea. It belongs to a group of polyphenols called flavonols and has antioxidant and anti-inflammatory properties. Researchers are particularly interested in how quercetin-rich foods may support cardiovascular and metabolic health over the long term.

Human studies so far suggest quercetin can modestly influence markers such as blood pressure, blood lipids and CRP (a marker of inflammation). There are early data on “senolytic” effects (removing damaged cells linked with ageing), but these usually involve high, drug-like doses of quercetin combined with medicines such as dasatinib. This is very different from what you obtain through normal food intake and should not be confused with a dietary approach.

For more curated ideas, see our longevity collection.

Further reading: meta‑analysis on blood pressure effects of quercetin; lipid/CRP data; pilot study on dasatinib + quercetin (senolytic).

Quercetin 101 – forms, absorption and what really shows up in the blood

• What it is: Quercetin is a flavonol. In foods it usually occurs as glycosides (for example quercetin glucosides, rutin) rather than as free quercetin.
• Why bioavailability matters: Not all quercetin foods are absorbed equally well. The chemical form and the food matrix influence how much actually reaches your bloodstream.

• Bioavailability – practical points
  • Onions: Quercetin in onions (mainly glucosides) is relatively well absorbed in humans. Human ileostomy study, food comparison.
  • Apples: Most quercetin is in the peel. The peel is much richer than the flesh, with moderate absorption overall. Peel ≫ flesh.
  • Capers: Very high quercetin density, often as rutin (quercetin‑rutinoside). Rutin is absorbed less efficiently than quercetin glucosides, but capers remain an extremely concentrated food source. HPLC analysis of commercial capers.
  • Green tea: Contains small amounts of quercetin but is mainly valued for catechins such as EGCG. These have their own evidence base for cardiovascular and metabolic health. EFSA opinion on catechins.
• Simple ways to improve quercetin absorption
  • Add some fat: A source of fat (for example walnuts, olive oil, yoghurt) alongside quercetin foods can increase absorption. Human crossover study.
  • Heat and cooking: Quercetin itself is relatively heat‑stable. Long boiling mainly transfers it into the cooking water, so using the stock or braising liquid helps you keep more of it. Onion cooking study, Bioaccessibility after cooking.
  • Lemon in tea: Adding lemon or other vitamin‑C‑rich citrus to tea can help stabilise catechins. Milk proteins may interact with tea polyphenols, so if you are keen to maximise them, have some cups without milk. Purdue summary, brewing study.

The four best natural quercetin sources – and how to use them

1) Apples (with peel): a convenient daily quercetin food

• Shopping: Choose fresh, firm, strongly coloured apples. Organic may be worth considering if you routinely eat the peel, but thoroughly washing non‑organic apples is also reasonable.
• To maximise quercetin: Avoid peeling; rinse briefly under running water; slice or bite just before eating to limit oxidation.
• Cooking: Raw apples retain the most polyphenols, but baking or a quick sauté are good compromises. Long stewing reduces some content, especially if you discard the liquid.
• Serving ideas:
  • Apple‑walnut salad (apple 150 g, walnuts 20 g) with a lemon‑mustard and olive oil dressing (about 10 ml oil, 10 ml lemon juice).
  • Overnight oats (50 g oats, 120 ml milk or plant drink) with 100 g diced apple and cinnamon.
  • A whole apple as an afternoon snack with 100 g low‑fat quark or 1 tbsp nut butter.
• Realistic intake: 1 medium apple per day is a practical habit for most adults.
• Content overview: Whole apples typically contain around 2–4 mg quercetin per 100 g, but the peel contains much higher concentrations than the flesh. For detailed data by variety and processing, see resources such as the USDA Flavonoid Database 3.3.

More on peel content: analysis of old and new apple varieties (peel ≫ flesh).

2) Onions (especially red; use the outer layers)

• Shopping: Choose red or yellow onions with firm bulbs and dry, intact skins. Red onions often contain more flavonols than white.
• Preparation: Enjoy onions raw, sautéed or roasted. If you boil them for a long time, try to use the broth so you do not lose too much quercetin into the water.
• Practical tip: After slicing, let onions rest for 5–10 minutes while you prepare other ingredients. This is mainly relevant for other sulphur compounds rather than quercetin, but it is a convenient kitchen habit.
• Serving ideas:
  • Red onion salad with apple cider vinegar, fresh parsley and olive oil.
  • Sautéed onions on wholegrain bread with goat’s cheese.
  • Lentil soup with onions – and make sure you consume the broth.
• Realistic intake: Including onions in 1–2 meals per day (around 30–60 g per meal) can noticeably increase quercetin intake over time.
• Content overview: Onions provide roughly 35–120 mg quercetin per 100 g. Red onions tend to be higher than yellow, which are higher than white. The outer layers contain more than the inner ones. Key references: J. Agric. Food Chem. review, polyphenol table, onion review (2021).

Cooking losses: During boiling, around 30–75% of quercetin glycosides can move into the cooking water. Frying, sautéing and baking tend to preserve more quercetin within the food. J Food Sci, Foods (MDPI).

3) Capers: small portions, highly concentrated quercetin source

• Shopping: Jarred capers in brine are the most common in UK supermarkets. Look for “nonpareilles” (smaller buds) for a more delicate texture.
• Preparation: Rinse briefly under water to reduce sodium and add towards the end of cooking to preserve their flavour and polyphenols.
• Serving ideas:
  • Lemon‑caper piccata with chicken or tofu.
  • Sprinkled over roasted vegetables or stirred into potato salad.
  • Tuna‑caper‑bean salad (white beans 150 g, tuna 100 g, capers 5–10 g, lemon, olive oil).
• Realistic intake: 1–2 teaspoons (5–10 g) per serving are usually enough for flavour and provide a meaningful quercetin contribution.
• Content overview: Capers are among the richest quercetin foods. Commercial products average around 5.2 mg flavonoid glycosides per gram (about 3.9 mg quercetin equivalents/g), meaning 10 g capers may deliver roughly 40 mg quercetin equivalents, depending on variety and processing. European Food Research & Technology.
• Form and bioavailability: The dominant form in capers is often rutin. Its absorption is lower than that of quercetin glucosides, but the very high flavonoid content still makes capers a valuable, quercetin-rich food. Newer data suggest good bioaccessibility even after processing. Bioaccessibility study 2023.
• Sodium note: Capers in brine are salty. If you have high blood pressure or kidney issues, rinse well and use sparingly as a flavour accent rather than a main ingredient.

4) Green tea: modest quercetin, valuable catechins

• Shopping: Choose good‑quality loose-leaf green tea (for example sencha or gyokuro) or reliable tea bags. Store in a dry, airtight container away from light.
• Brewing for polyphenols: For sencha, 70–80 °C water for 2–3 minutes works well. Studies often find a high catechin yield around 85 °C for 3 minutes. A 12‑hour cold brew in the fridge is a gentler option for summer. Brewing conditions & catechins, temperature‑time effects.
• With lemon: Adding lemon or other citrus helps stabilise catechins and may improve their availability. Purdue summary.
• Iron absorption: Green tea polyphenols can reduce non‑haem iron absorption from plant-based meals. If you are prone to low iron or anaemia, it may be sensible to leave a 1–2 hour gap between iron‑rich meals and green tea. Human study, 1‑hour interval.

How much quercetin from food is “enough” for everyday health?

Because quercetin content varies by variety, season, storage and cooking method, it is more realistic to think in terms of a food pattern rather than counting precise milligrams. A practical “food‑first” combination for many adults could be:

• 1 medium apple (with peel)
• Onions totalling around 60–100 g per day (for example spread across two meals)
• Capers 1–2 teaspoons (5–10 g)
• 1–2 cups (200–400 ml) green tea

Indicative outcome: This mix typically provides somewhere in the region of 40–90 mg quercetin equivalents per day, sometimes more, depending on the exact foods and their preparation. Main data sources: USDA Flavonoid Database 3.3, polyphenol tables, caper analysis.

How to maximise quercetin in everyday cooking

Shopping tips for quercetin-rich foods

• Apples: Go for firm fruits with good colour. Include different varieties across the year.
• Onions: Choose heavy onions with dry skins. Use red onions where possible as they tend to contain more polyphenols.
• Capers: Check the ingredients in the brine and compare sizes. Smaller buds are often more delicate.
• Green tea: Buy from reputable suppliers. If possible, choose teas that list harvest season or origin.

Storing to preserve polyphenols

• Apples: Keep chilled in the fridge crisper, away from very ethylene-sensitive produce.
• Onions: Store in a cool, dark, well‑ventilated place, not next to potatoes.
• Capers: Refrigerate once opened and use within the recommended time.
• Green tea: Store airtight and away from light, moisture and strong odours.

Cooking methods that protect quercetin

• Keep apple peels on where you can. Favour baking or brief sautéing rather than very long stewing if you want to preserve more polyphenols.
• Use onion outer layers and include pan juices or deglazing liquids in your meals.
• Rinse capers to reduce salt, then add near the end of cooking.
• Brew green tea at the right temperature and consider adding lemon for polyphenol stability.

7‑day “quercetin habit” – simple ideas to rotate

Breakfast options

• Overnight oats (50 g oats, 120 ml milk or plant drink) with 100–150 g apple, 20 g walnuts and cinnamon.
• Quark bowl (200 g low‑fat quark or strained yoghurt) with 100 g apple slices and 15 g pumpkin seeds.

Lunch ideas

• Beetroot‑lentil salad (150 g cooked lentils) with 40 g red onions and 5–10 g capers.
• Two slices (~100 g) wholegrain bread with 40 g herb goat’s cheese, 40 g sautéed onions and tomato slices.

Dinner suggestions

• Tofu or chicken piccata (120–150 g protein) with 5–10 g capers and lemon, plus roasted vegetables including about 80 g onions.
• Potato‑bean salad (200 g potatoes, 120 g green beans) with 40 g red onions and 5 g capers.

Snacks and drinks

• A whole apple (150–180 g) as a mid‑afternoon routine.
• Green tea in the morning and/or afternoon, with a slice of lemon if you enjoy it.

Special cases and alternatives

• Low‑FODMAP or onion‑sensitive: Many people find they tolerate small amounts of green spring onion tops or chives better than whole onions. Well‑cooked onions may also be easier to manage than raw. Depending on tolerance, you may rely more on apples and capers for quercetin.
• If you dislike green tea: Try a mild cold‑brew version, or simply focus on the other three quercetin foods and your overall diet quality.
• Seasonality: Outside the main UK apple season, look for stored local varieties or reliable imports. Frozen onion mixes can be convenient and still contribute quercetin.
• Other quercetin‑containing foods: Buckwheat (rich in rutin), kale and various berries also contribute flavonols and other polyphenols, but this article focuses on the four easiest, high‑impact options.

Safety, interactions and who should be cautious

• Normal food amounts: Quercetin-rich foods such as apples, onions, capers and green tea are generally regarded as acceptable parts of a balanced diet for most people.
• Sodium from capers: Capers can contain substantial salt from the brine. If you have high blood pressure, cardiovascular disease or kidney problems, rinse thoroughly and use modest amounts. Speak to your doctor or dietitian about overall sodium intake.
• Green tea safety: Brewed green tea, consumed in typical amounts, is widely used around the world. Very high intakes of isolated EGCG from supplements, however, have been associated with raised liver enzymes in some reports. EFSA opinion.
• Quercetin supplements: Capsule or tablet forms provide doses far above usual food intakes. Quercetin can influence drug‑metabolising enzymes and transporters (such as CYP3A4 and P‑gp) and may interact with certain prescribed medicines, including some statins, calcium channel blockers, anticoagulants/antiplatelets, immunosuppressants and macrolide antibiotics. Always speak with a doctor or pharmacist before starting quercetin supplements, especially if you take regular medication. More on long‑term use and safety of quercetin. Evidence on inhibition/modulation: J. Agric. Food Chem., quercetin & CYP3A4/P‑gp.
• Pregnancy and breastfeeding: During pregnancy and breastfeeding it is generally advisable to focus on quercetin from foods and avoid high‑dose supplements unless specifically recommended by a healthcare professional who knows your history.

What is well supported – and what is still uncertain?

• Better established: Diets rich in flavonols (including quercetin foods) are consistently associated with more favourable cardiovascular and inflammatory profiles in observational and intervention studies. JAHA meta‑analysis, RCT overview.
• Promising but early: Senolytic effects (helping clear so‑called “senescent” cells) are mainly shown in cell and animal models or in small pilot human studies using combinations such as dasatinib plus high‑dose quercetin. EBioMedicine pilot.
• Key takeaway: It is sensible to focus on long‑term dietary patterns that include quercetin-rich foods, within a generally healthy lifestyle. This should not be confused with the use of pharmacological doses in experimental protocols.

“Try this today” – simple checklist to boost quercetin intake

• Buy: Around 6 apples, 4 red onions, 1 jar of capers and 100 g of sencha or another good‑quality green tea.
• Prep: Pre‑chop onions for the next day or two (store in the fridge), rinse a portion of capers and keep them ready to use, and store tea in a dry, airtight container.
• Build habits:
  • Apple at around 3 pm most days.
  • Onions included at lunch and/or dinner.
  • Green tea mid‑morning (for example 10:30) and/or mid‑afternoon.
  • Capers as a flavour boost instead of adding extra salt.

Conclusion: For health‑conscious adults, regularly including quercetin-rich foods such as apples with peel, onions, capers and green tea is a simple, food‑based way to support cardiovascular and inflammatory health over time. These habits sit best within an overall pattern of balanced eating, movement, sleep and clinical check‑ups, rather than as a stand‑alone “longevity hack”.