Onions
Food · Vegetable · Food, Typed Supps
A prebiotic fibre source that feeds gut bacteria much like an inulin supplement.
Stage by stage
- Intake (Releases sulfur compounds): Cutting or chewing releases sulfur compounds responsible for its pungency and eye-watering effect.
- Stomach (Quercetin & fibre released): Quercetin and fructan fibre begin releasing during digestion.
- Small & large intestine (Fructans reach the colon): Much of its fibre (inulin-type fructans) passes undigested into the colon, the same as a standalone inulin supplement.
- Gut bacteria (Fermented as fuel): Gut bacteria ferment these fructans, similar to how they process a prebiotic fibre supplement.
Timing
Raw retains more of the sulfur compounds and quercetin than heavily cooked onion.
Time to effect
- Days: Any reported digestive or gut-microbiome effect in studies.
Allergens
None of the fourteen major allergens. Derived from what this food is, not from a product label.
Eating patterns
Fits: Vegan, Vegetarian, Pescatarian, Halal, Kosher, Keto, Gluten-free. Depends on the product: Low-FODMAP.
In season
August to March in the northern hemisphere, February to September in the southern.
Satiety
The satiety index is measured on 240 kcal servings. At 31 kcal per 100 g that is 784 g of this, which is not a serving anyone eats — so the index has nothing to say about it.
Environmental footprint
0.5 kg CO2e and 14 litres of freshwater per kg, matched to "Onions & leeks" in Poore & Nemecek (2018), Science 360.
Cost band
Cheap — roughly £1.4 per kg, as a band midpoint rather than a price.
Key nutrients
Nutrition per 100g
- Calories: 30.6
- Protein: 1.7g
- Carbohydrate: 4.7g
- of which sugars: 4.7g
- Fat: 0.1g
- of which saturated: 0g
- Fibre: 2.1g
- Sodium: 8mg
Food vs. supplement
Onions are a genuine dietary source of the same inulin-type prebiotic fibre sold as a standalone supplement — alongside quercetin and vitamin C a fibre-only supplement doesn’t include.
Kitchen tip
Letting cut onions sit for 5 to 10 minutes before cooking gives their beneficial sulfur compounds time to fully form.
Common myth
“Cooking onions destroys all their beneficial compounds.” In reality: Cooking reduces some quercetin and sulfur compound content, but a meaningful amount remains — both raw and cooked onions contribute nutritionally.
Recipes that use it
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