5. Onion
The millennia-old cornerstone — fructan + quercetin dual matrix in every kitchen.
Onion in 1 minute
What does it provide? Inulin-type fructans (0.1–2 g/100 g) and quercetin glycosides (especially in the skin) — selective bifidogenic effect and polyphenol-mediated anti-inflammatory action.[803]
How much? ½–1 medium onion (≈ 50–100 g)/day; for IBS-sensitive: onion-infused oil (onion soaked in hot oil, then removed — the flavor is fat-soluble, the FODMAP fructan remains water-soluble, so it doesn't enter the oil).[777]
When to avoid? Large amounts during warfarin or DOAC therapy (anticoagulant interaction), IBS elimination phase, 2 weeks before scheduled surgery in large doses, GERD flare.
Onion is one of the oldest cultivated vegetables: it was already known and used in the ancient Near East and Egypt well before written history — archaeological and literary evidence shows that ancient Egyptians viewed it not only as food but also as a symbolic-cultic object: they saw the concentric layers as an artful sign pointing to eternal life, left onions in pharaohs' tombs, and reportedly placed onions in Ramses IV's eye sockets during mummification. According to Herodotus, the pyramid builders received onion, garlic, and radish as rations — this even appeared on the town's inscriptions.
In Greco-Roman medicine, from the Hippocratic Corpus onward it was mentioned for digestive and general health purposes; Dioscorides and Pliny also noted its anti-inflammatory and diuretic power, though the number of medicinal recipes is smaller compared to kitchen uses.[808] In medieval Europe it became one of the staple vegetables alongside cabbage and bean, due to its excellent preservability and versatility — a cornerstone of peasant household winter supplies, a constant element of monastery herbaria. In modern times the Age of Exploration spread it widely, and it became a friend to American settlers; it remained a constant character in culinary arts and folk medicine — the "food–herb" dual status has persisted for millennia, and this set the stage for modern scientific investigations.
Scientific Background
Onion's two main bioactive groups: (1) inulin-type fructans (ITF), particularly short chain-length fractions (kestose, nystose) — not hydrolyzed in the small intestine, they reach the colon and produce a selective bifidogenic effect; (2) quercetin and quercetin glycosides (especially rutin, isoquercitrin) — the richest dietary quercetin source (purple/red onion skin contains 100–600 mg/kg).[802]
Quercetin bioavailability can be enhanced: the glycoside forms are enzymatically hydrolyzed in the small intestine, then the quercetin aglycone enters into complex bidirectional interaction with the microbiota in the colon — being converted on one hand to microbial metabolites (3,4-dihydroxyphenylacetic acid, hippuric acid), and on the other modifying microbial composition (Firmicutes/Bacteroidetes ratio shift).[804]
Onion fructans' prebiotic effect is dose-dependent — in human interventions, 5–10 g/day ITF intake is bifidogenic, and onion is one of the main non-chicory ITF sources in the daily diet. Quercetin is anti-aggregant: 500 mg/day already causes moderate bleeding-time prolongation, which can be additive with warfarin.[805][807][809]
- + Extra-virgin olive oil + tomato (sofrito): Mediterranean foundation, polyphenol synergy, lycopene release.
- + Legumes (bean, lentil, chickpea): ITF + GOS dual prebiotic matrix, butyrate-positive.
- + Live cultures (yogurt, kefir): synbiotic principle.
- + Lemon/vinegar (in brief pickling): quercetin stabilization, bioavailability enhancement.
- + Leafy greens (spinach, arugula): additional quercetin/flavonol source, antioxidant synergy.
- + Meat (iron absorption support): quercetin assists non-heme iron utilization.
- Warfarin, DOACs (apixaban, rivaroxaban, dabigatran): quercetin's anti-aggregant effect + occasional vitamin K fluctuation — INR monitoring.
- High-dose aspirin/clopidogrel + large amount of raw onion: additive bleeding risk.
- CYP3A4 substrates (certain statins, cyclosporine): quercetin is a weak CYP3A4 inhibitor; theoretical interaction at large amounts.
- GERD/reflux disease flare + raw onion: can cause severe reflux.
- Long boiling with discarded liquid: fructans and water-soluble polyphenols enter the liquid — should be reincorporated.
- Large amount of raw onion on empty stomach: GI irritation.
- IBS elimination phase: avoid in the first 4–6 weeks; small-dose reintroduction.
- Severe GERD/reflux disease (flare): avoid raw onion.
- Active peptic ulcer: avoid raw onion, moderately cooked.
- Warfarin/DOAC + scheduled surgery in 2 weeks: reduce consumption.
- Infant under 6 months: avoid (flavor, spices).
- Allium allergy (rare, IgE-mediated): complete avoidance.
- Not a trigger for histamine intolerance — safe.
- Severe kidney disease: moderate potassium, normal consumption is safe.
Daily/weekly serving
½–1 medium onion (≈ 50–100 g) daily for cooking. For IBS-sensitive: onion-infused oil (fat-soluble flavor, fructan stays out) or green part of leek as substitute.
Preparation pattern
- Peel, halve between the growth poles.
- Slice thinly or dice (flavor focus: thin slices caramelize better; for bite: dice).
- In oil over medium heat 8–15 min stirring until caramelization (sweeter flavor), or high heat 2-3 min (translucent state).
Classic patterns
Sofrito (Mediterranean): olive oil + finely chopped onion + garlic + tomato — slowly, 15-20 min.
Caramelized onion paste (French): in butter + oil slowly 30-40 min — for pizza, soup, cheese board.
Onion pickling (quick): sliced red onion + vinegar + sugar + salt, 30 min — for tacos, salads.
French onion soup (soupe à l'oignon gratinée): caramelized onion + beef broth + wine + baguette slice + Gruyère + golden brown in oven.
Storage
Whole raw: dry, dark, ventilated place 2-3 months. Cut: refrigerated in airtight container 3-5 days. Cooked onion: refrigerated after quick cooling max 3 days.
What not to do
Don't store with potatoes (ethylene production and moisture → rapid spoilage). Don't cook 30+ min in strong acid (fructan hydrolysis). Don't leave cooked onion at room temperature longer than 2 hours.
References
[777] . Monash UniversityMonash FODMAP database. High and Low FODMAP foods. Link
The Monash University FODMAP database, classifying foods as high or low in FODMAP content.
[802] Slimestad R et al. Onions: a source of unique dietary flavonoids2007;55(25):10067–10080. J Agric Food Chem. Link
Onion bulbs (Allium cepa L.) are among the richest sources of dietary flavonoids and contribute to a large extent to the overall intake of flavonoids. This review includes a compilation of the existing qualitative and quantitative information about flavonoids reported to occur in onion bulbs, including NMR spectroscopic evidence used for structural characterization. In addition, a summary is given to index onion cultivars according to their content of flavonoids measured as quercetin. Only compounds belonging to the flavonols, the anthocyanins, and the dihydroflavonols have been reported to occur in onion bulbs. Yellow onions contain 270-1187 mg of flavonols per kilogram of fresh weight (FW), whereas red onions contain 415-1917 mg of flavonols per kilogram of FW. Flavonols are the predominant pigments of onions.
[803] Galeone C et al. Onion and garlic use and human cancer2006;84(5):1027–1032. Am J Clin Nutr. Link
BACKGROUND: Interest in the potential benefits of allium vegetables, in particular, onion (Allium cepa) and garlic (Allium sativum), has its origin in antiquity, but the details of these benefits are still open to discussion. OBJECTIVE: We investigated the role of allium vegetables in the etiology of various neoplasms. Previous data are scanty and are based mainly on Chinese studies. DESIGN: Using data from an integrated network of Italian and Swiss case-control studies, we analyzed the relation between frequency of onion and garlic use and cancer at several sites. We calculated odds ratios (ORs) by using multivariate logistic regression models that were adjusted for energy intake and other major covariates. RESULTS: Consumption of onions varied between 0-14 and 0-22 portions/wk among cases and controls, respectively.
[804] Suleria HAR et al. Onion: nature protection against physiological threats2015;55(1):50–66. Crit Rev Food Sci Nutr. Link
Onion (Allium cepa L.) is found in various regions of Europe, North America, Asia, and Africa. It is one of the classic examples of Allium species used not only for culinary preparations but also for medicinal purposes. Onion with a variety of purposes is often used as a raw material in many dishes and accepts almost all of the traditions and culture. Owing to its storage characteristics and durability of shipping, onions have been traded more widely than most vegetables. The pungent fractions of garlic are mostly sulfur-containing moieties while its two chemical groups have marked effect on human health. These are flavonoids and ALK (EN)-based cysteine sulfoxides (ACSOs).
[805] Egert S et al. Quercetin reduces systolic blood pressure and plasma oxidised low-density lipoprotein concentrations in overweight subjects: a double-blind, placebo-controlled cross-over study2009;102(7):1065–1074. Br J Nutr. Link
A double-blind, placebo-controlled cross-over study examining the effect of quercetin on systolic blood pressure and oxidised LDL concentrations in overweight subjects (Br J Nutr).
[807] Kleijnen J, Knipschild P, ter Riet G. Garlic, onions and cardiovascular risk factors: a review of the evidence from human experiments with emphasis on commercially available preparations1989;28(5):535–544. Br J Clin Pharmacol. Link
1. Claims for beneficial effects on cholesterol levels, fibrinolytic activity, and platelet aggregation are attributed both to fresh garlic and onions (or their extracts) and to commercially available preparations. 2. Regarding fresh garlic, the claims have been confirmed, but so far only at very high dosages. 3. For onions and commercially available supplements contradictory results have been reported. 4. All published trials showed severe methodological shortcomings. Some trials were not randomized and/or not blinded whilst this was possible, and in only one of every three studies more than 25 patients participated in each treatment group.
[808] . EMA/HMPCAssessment report. Community herbal monograph on Allium cepa L., bulbus. Link
An EMA/HMPC assessment report and community herbal monograph on onion (Allium cepa L., bulbus).
[809] Reinhart KM et al. The impact of garlic on lipid parameters: a systematic review 2009 (kontextus Allium-családra). Nutr Res Rev. 2009.
A systematic review of the impact of garlic on lipid parameters, in the context of the Allium family (Nutr Res Rev).