A 2025 Heart Trial Found This Compound in the Onion Layer You Peel Off

Quercetin is a flavonoid found throughout onions, but concentration varies enormously by variety and by which part of the onion you're eating — and it's currently one of the more actively researched compounds in aging and cardiovascular science.

The July 2025 heart-surgery trial

A Canadian clinical trial found quercetin supplementation reduced the risk of post-operative atrial fibrillation — a common and serious complication after heart surgery — to just 4% of patients, compared to 18% in the placebo group.¹ That's a substantial difference for a single supplemented compound in a surgical population.

What quercetin is actually doing

Quercetin acts as both a "senolytic" — helping clear out aging, dysfunctional cells that accumulate in tissue over time — and a "senomorphic," suppressing the inflammatory signals those cells release. That dual mechanism is part of why it's drawn broader research interest beyond this one trial; a separate 2025 pilot trial combining quercetin with the drug dasatinib found improved cognitive scores in older adults at risk for Alzheimer's, though that combination involved a pharmaceutical partner, not food-level quercetin alone.²

Where it's concentrated in the onion

Red onions, and specifically their outer, more deeply pigmented layers, contain the highest quercetin concentrations of common onion varieties. The pale, tightly-wound inner rings most people are left with after peeling away the outer layers carry meaningfully less.

The practical takeaway

• This trial used a supplement-level dose, not a food-level claim — it doesn't mean eating onions replicates the heart-surgery result
• Still, if quercetin from food is the goal, don't peel red onions down to the palest inner rings — leave more of the outer layer
• Light cooking preserves more quercetin than boiling