Onion
A humble flavor base that delivers quercetin, prebiotic fibers, and organosulfur compounds.
Moderate evidence
🥦Vegetables
Evidence tier
Moderate
Research weight
Citations
11 verified / 11
Classification
Vegetables
❝
What the evidence says. Several controlled trials; effects real but modest or context-dependent.
Nutrition per serving 1 cup chopped, raw (160 g)
- Sugars 6.8 g4%
- Fibre 2.7 g2%
- Other carbs 5.4 g3%
- Protein 1.8 g1%
- Other 143.3 g90%
64 kcal1.8 g protein2.7 g fiber6.8 g sugar
| Nutrient | Per serving | % daily value |
|---|---|---|
| Vitamin C | 12 mg | 13% |
| Fiber | 2.7 g | 10% |
| Potassium | 234 mg | 5% |
| Folate | 30 µg | 8% |
| Vitamin A | 0 µg | 0% |
| Vitamin K | 0.6 µg | 1% |
| Vitamin B6 | 0.19 mg | 11% |
| Manganese | 0.21 mg | 9% |
| Copper | 0.06 mg | 7% |
| Vitamin E | 0.03 mg | 0% |
| Magnesium | 16 mg | 4% |
| Calcium | 37 mg | 3% |
Composition data: USDA FoodData Central ↗
What is Onion?
Onion is a vegetable used for lowers systolic blood pressure (quercetin). NutriDex grades the human evidence as Moderate. Onion is one of the richest dietary sources of the flavonol quercetin, plus organosulfur compounds and fructan (prebiotic) fibers. Randomized-trial evidence shows onion and concentrated quercetin supplementation modestly lower systolic blood pressure and improve body fat and lipid markers, with the clearest blood-pressure effect at quercetin doses of at least 500 mg/day. Higher dietary flavonoid intake is also associated with lower cardiovascular and all-cause mortality in large cohorts, though direct whole-onion outcome data remain limited.
Purported Benefits
✓Lowers systolic blood pressure (quercetin)
✓Improves body fat and lipid profile
✓Prebiotic fructans support gut microbiome
✓Rich source of antioxidant flavonols
✓Linked to lower cardiovascular mortality
Dosing & Compounds
Typical Dose
Standard serving: 1 cup chopped, raw (160 g). Eat whole (with skin where edible); favour whole fruit over juice.
Active Compounds
Safety & Cautions
High in FODMAPs (fructans), a common trigger of IBS bloating and gas. Raw onion can cause reflux/heartburn in sensitive people, and handling releases lachrymatory (tear-inducing) compounds. Onion is toxic to dogs and cats. Quercetin supplements (not culinary amounts) may interact with some medications. Educational only — always check with your doctor or pharmacist before combining Onion with any medicine.
Key Studies ★ 11 studies
Systematic review & meta-analysis of RCTs
Hejazi 2023
✓ PubMed
Across 14 RCTs, onion supplementation significantly improved body fat percentage, total/LDL/HDL cholesterol, and systolic blood pressure, with no significant effect on triglycerides in the crude analysis.
Systematic review & meta-analysis of RCTs
Chung 2023
✓ PubMed
In 5 RCTs (147 subjects with obesity), onion intake significantly reduced body weight, BMI, waist circumference, and triglycerides versus placebo, with effects most pronounced for onion peel extract.
Dose-response meta-analysis of RCTs
Hosseini 2024
✓ Full text
Quercetin supplementation significantly reduced fasting blood glucose and systolic blood pressure in adults, with significant fasting-glucose lowering seen at doses >=500 mg/day for >=8 weeks; no effect on triglycerides, HDL, DBP, or waist circumference.
Systematic review & meta-analysis
Wan 2022
✓ Full text
Pooling 22 studies (25 reports), high allium-vegetable consumption showed no significant overall association with cancer risk (RR 0.97, 95% CI 0.92-1.03), with no linear or nonlinear dose-response, tempering earlier cancer-protection claims.
Meta-analysis of RCTs
Serban 2016
✓ PubMed
Pooled across 7 RCTs (9 arms, 587 participants), quercetin supplementation lowered systolic BP by 3.04 mmHg (95% CI -5.75 to -0.33) and diastolic BP by 2.63 mmHg (95% CI -3.26 to -2.01), with effects greater at doses >=500 mg/day.
Meta-analysis (case-control & cohort)
Turati 2015
✓ Source
Across 19 case-control and 2 cohort studies (543,220 subjects), high allium-vegetable intake was associated with reduced gastric cancer risk (OR 0.54, 95% CI 0.43-0.65); each 20 g/day increment gave OR 0.91 (95% CI 0.88-0.94).
Meta-analysis of prospective cohorts
Kim 2018
✓ PubMed
Across 15 prospective cohorts, the highest versus lowest flavonoid intake was associated with lower cardiovascular mortality (pooled RR 0.86, 95% CI 0.75-0.98) and total mortality (RR 0.86, 95% CI 0.73-1.00).
RCT
Onion Peel Extract RCT 2024
✓ PubMed
Randomized double-blind placebo-controlled trial in 80 adults found onion (Allium cepa) peel extract had no overall effect on NK cell activity or cytokines (IL-2/6/12, IFN-gamma, TNF-alpha), but increased NK cell activity (P=0.038) in the subgroup with higher upper-respiratory symptom scores, without significant adverse effects.
Review
Sarkar 2026
✓ PubMed
Review of quercetin — the flavonoid onions are rich in — as a multi-target modulator of inflammation and oxidative stress in COPD.
Review
Boustani 2026
✓ PubMed
Allium cepa And Its-Associated Phytochemicals: Evidence From in Vitro, in Vivo, and Human Studies on Antioxidant Effects
Review
Bayan/MDPI Biology 2025
✓ Full text
2025 review synthesizing >20 years of research on Allium cepa and Allium sativum extracts details antidiabetic and cardioprotective mechanisms, including quercetin-rich onion peel extract reducing triglycerides/glucose and delaying arterial thrombosis via MAPK downregulation in preclinical models.