Onion
The invisible foundation of nearly every savory dish on earth, onion is also one of the richest sources of dietary flavonoids -- particularly quercetin, which is better absorbed from onions than from supplements.
Why It Matters for Longevity
A meta-analysis of case-control studies (Turati et al., 2014, PMID 25135917) found high allium vegetable consumption cut gastric cancer risk nearly in half (OR 0.54). An Italian study (Galeone et al., 2006, PMID 17159460) showed 1-7 portions of onion per week significantly reduced colorectal, laryngeal, and ovarian cancer risk with a dose-response relationship.
Red onions pack the most punch. They contain roughly double the flavonols of yellow onions, plus anthocyanins and taxifolin (a potent inhibitor of cancer cell growth identified in Italian Tropea onions). Ontario red onions were the most effective at killing colon and breast cancer cells in vitro (Murayyan et al., 2017, PMID 28578366).
Onions also contain 2-6g of fructooligosaccharides (FOS) per 100g -- prebiotic fibers that selectively feed beneficial Bifidobacteria and Lactobacilli.
How to Use It
Raw for maximum thiosulfinates (the antimicrobial sulfur compounds). Caramelized slowly for deep sweetness -- the Maillard reaction transforms pungent onion into something rich and complex. Red onions in salads and salsas. Yellow onions for the cooking base. Prolonged cooking destroys thiosulfinates but retains quercetin.
What to Pair It With
| Ingredient | Why | Tradition |
|---|---|---|
| Garlic | Fellow alliums; foundational aromatic duo | Global |
| Extra-virgin olive oil | Fat enhances quercetin absorption; base for soffritto | Mediterranean |
| Tomatoes | Complementary flavonoids; base for sauces and curries | Mediterranean / Indian |
| Balsamic vinegar | Acid and sweet balance onion's pungency | Italian (agrodolce) |
| Lentils | Onion is the base of all dal and lentil soups | Indian / Middle Eastern |
| Thyme | Deepens caramelized onion flavor | French (onion soup) |
Flavor Profile
Raw: pungent, sharp, tear-inducing. Caramelized: rich, sweet, silky. Crisp and crunchy raw; melting and soft when slow-cooked. The transformation from raw to cooked onion is one of the greatest tricks in the kitchen.
The Science
Quercetin bioavailability from onions is ~50% higher than from supplements due to the food matrix (Examine.com review). Murayyan et al. (2017, PMID 28578366) identified quercetin and anthocyanins as the key anti-cancer compounds, with red varieties outperforming all others. The Galeone Italian study (2006, PMID 17159460) provides the strongest epidemiological evidence for cancer protection.
Key Nutrients
| Nutrient | Per 100g | Notes |
|---|---|---|
| Quercetin | 20-50 mg (red), 10-20 mg (yellow) | 50% better absorbed from onion matrix than supplements |
| Fructooligosaccharides | 2-6 g | Prebiotic; feeds Bifidobacteria and Lactobacilli |
| Anthocyanins | 25-50 mg (red only) | Low absolute bioavailability but gut metabolites are active |
| Thiosulfinates | Variable (formed on cutting) | Antimicrobial; destroyed by prolonged cooking |