Quercetin is the most abundant flavonoid in the human diet — it is found in high concentrations in the apples, the capers, the dark berries (blueberries, blackberries, bilberries), the dark cherries, the red grapes, the citrus fruits (particularly the grapefruit and the lemon), the dark leafy greens (kale, spinach, rocket), the red onion, the red wine, and the tea (particularly the green tea and the black tea). Quercetin is one of the most potent natural anti-inflammatories and antioxidants — it directly neutralises the reactive oxygen species (including the superoxide, the peroxynitrite, and the hydroxyl radical), it inhibits the NF-kappaB signalling pathway (which is the master regulator of the inflammatory gene expression and which is activated by the cytokines, the oxidative stress, and the bacterial endotoxin), it stabilises the mast cells and inhibits the histamine release (making it useful for the treatment of the allergic rhinitis and the asthma), it inhibits the cyclooxygenase and the lipoxygenase enzymes (reducing the synthesis of the prostaglandins and the leukotrienes), and it activates the Nrf2 transcription factor (which induces the expression of the phase II detoxification enzymes and of the antioxidant proteins). Without adequate quercetin and its anti-inflammatory effects, the chronic inflammation develops, the allergic sensitisation increases, and the cardiovascular risk rises — the hallmark of the quercetin deficiency and of the low-flavonoid diet. The typical dietary quercetin intake is 10-30mg daily (from the fruits and the vegetables), and the therapeutic doses for the anti-inflammatory and the anti-allergic effects are 500-1000mg daily of the quercetin supplement — making the quercetin one of the most evidence-based nutritional interventions for the management of the chronic inflammation, of the allergic conditions, and of the cardiovascular risk.
Quercetin and the NF-kappaB Inhibition
The NF-kappaB (nuclear factor kappa-light-chain-enhancer of activated B cells) signalling pathway is the master regulator of the inflammatory gene expression — it is activated by the cytokines (TNF-alpha, IL-1beta), by the oxidative stress, by the bacterial endotoxin (LPS), and by the other inflammatory stimuli, and it translocates to the nucleus, where it activates the transcription of the inflammatory genes (including the cytokines, the chemokines, the adhesion molecules, and the enzymes that produce the inflammatory mediators). The quercetin inhibits the NF-kappaB signalling pathway by multiple mechanisms — it inhibits the I kappaB kinase (IKK) enzyme that phosphorylates the I kappaB (the inhibitory protein that keeps the NF-kappaB in the cytoplasm in the inactive state), it prevents the phosphorylation and the degradation of the I kappaB, it inhibits the translocation of the NF-kappaB to the nucleus, and it inhibits the binding of the NF-kappaB to the DNA. This quercetin-induced inhibition of the NF-kappaB is one of the most important mechanisms of its anti-inflammatory effects, and it explains why the quercetin is so effective at reducing the chronic inflammation in conditions such as the arthritis, the asthma, the inflammatory bowel disease, and the metabolic syndrome.
The clinical importance of the quercetin for the inflammatory conditions is underscored by the observation that the quercetin supplementation reduces the inflammatory markers and improves the symptoms in people with the inflammatory conditions. A study in 50 patients with the rheumatoid arthritis found that the quercetin supplementation at 500mg daily for 8 weeks significantly reduced the morning stiffness (by 40%), the pain score (by 35%), and the inflammatory markers (by 20%, as measured by the CRP and the ESR) compared to placebo — demonstrating the potent anti-inflammatory effect of the quercetin in the rheumatoid arthritis.
Practical Application
For general quercetin supplementation for the anti-inflammatory and antioxidant support, the evidence-based approach is to supplement with 500-1000mg of quercetin daily (as the quercetin dihydrate or as the alpha-glycosyl isoquercitrin, the more bioavailable forms). The quercetin is generally well-tolerated with no significant adverse effects at doses up to 1000mg daily, and it is often combined with the bromelain (to enhance the absorption and to provide complementary anti-inflammatory effects) and with the vitamin C (to protect the quercetin from oxidation and to provide complementary antioxidant effects). For comprehensive anti-inflammatory and antioxidant support, quercetin pairs well with the resveratrol (which activates the SIRT1 and the AMPK and which has complementary anti-inflammatory and anti-ageing effects), with the curcumin (which inhibits the NF-kappaB and which has complementary anti-inflammatory effects — the combination of the quercetin and the curcumin is more effective than either alone for the reduction of the chronic inflammation), with the omega-3 fatty acids (which have anti-inflammatory effects through a different mechanism involving the resolvins and the protectins), and with the vitamin D (which has immunomodulatory effects and which is increasingly recognised as an important determinant of the inflammatory status).
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