Quercetin

Quercetin is a flavonol, one of the most abundant flavonoids in the human diet. Red onions, capers, apples with skin, kale and tea are the richest common sources.

Supplements typically use quercetin extracted from Sophora japonica flower buds. Plain quercetin dihydrate is cheap and very poorly absorbed — bioavailability is low enough that large doses of the plain material achieve modest plasma levels.

That is why enhanced forms exist and why they are worth considering rather than dismissing as marketing. Phytosome preparations, isoquercetin, and combinations with bromelain or vitamin C all aim at the same problem. In food, quercetin occurs as glycosides which are better absorbed than the free form used in cheap supplements.

Quercetin has antioxidant activity and effects on inflammatory signalling pathways, which is common to many flavonoids and accounts for much of the general research interest.

The endurance rationale rests on reported effects on mitochondrial biogenesis, the process by which cells build more mitochondria. If real at achievable doses, that would improve aerobic capacity over time rather than acutely.

Quercetin also acts as a mild adenosine receptor antagonist, the same broad mechanism as caffeine though far weaker, which may contribute to any perceived effect on fatigue. It additionally stabilises the immune cells that release histamine, which is the basis for its use in seasonal support products.

Endurance research has produced mixed results. Meta-analyses have reported small improvements in endurance capacity, with effects more evident in untrained participants than in trained athletes — a familiar pattern across this index.

There is a reasonable body of work reporting fewer upper respiratory symptoms in people undertaking heavy training blocks, which is one of the more consistent findings.

Research on blood pressure has been modestly favourable. The recurring problem across all of it is bioavailability: many trials used plain quercetin at doses that achieve limited plasma concentrations, which makes negative findings hard to interpret and positive ones hard to attribute confidently.

Quercetin has extensive dietary exposure and is well tolerated at typical supplemental doses. Reported effects are mild and infrequent, mainly headache and digestive upset.

At high doses over long periods there have been isolated reports of kidney-related concerns, so sustained very high intake is not advisable.

Quercetin affects some of the enzyme systems the body uses to process other compounds, so it can alter how much of other things reaches the bloodstream. Speak to your doctor before use if you take any regular medication.