Quercetin, also known as the “king of the flavonoids," is considered among the top plant-based polyphenolic compounds due to its demonstrated powerful antioxidant properties and ability to support a healthy inflammatory response in the body. This potent flavonol is abundant in various foods recognized for their health benefits, including red onions, yellow onions, apples, olive oil, dark berries and grapes, capers, broccoli, salad greens, and culinary herbs such as dill, cilantro, watercress, and radicchio. In fact, this phytochemical contributes to the richly-colored pigments in these foods. 

Seasonal allergic rhinitis, often called seasonal allergies, has an estimated prevalence of 25% to 40% of adults and children globally. Complaints typically include nasal congestion, sneezing, runny nose, and itching. It is an immunoglobulin E (IgE) antibody-mediated, inflammatory condition typically triggered by common environmental triggers, such as pollens. As part of the immune response to allergens, T helper cells (Th2) release interleukin-4 to stimulate B cells into producing IgE. The IgE attaches to basophils and mast cells, sensitizing them to the allergens. Upon exposure to the allergen, these cells degranulate and release mediators, including histamine and prostaglandin. Histamine, prostaglandins, and other mediators trigger an inflammatory response. 

When exposed to allergens, quercetin may help inhibit histamine release from basophils and mast cell degranulation. Histamine is the compound responsible for many unpleasant complaints associated with seasonal allergies. Quercetin can help improve T helper cell (i.e., Th1/Th2) balance and inhibit antigen-specific IgE antibody formation. In an animal model, quercetin significantly reduced epithelial thickness, goblet and mast cell numbers, IgE levels, and immunohistochemical markers compared to untreated mice with allergic airway inflammation. 

Quercetin has an inhibitory effect on histidine decarboxylase (HDC) mRNA transcription, which is the enzyme responsible for catalyzing the synthesis of histamine from histidine and suppresses IL-6 released by human mast cells, which may help resolve in part inflammatory conditions. Furthermore, quercetin interacts with several protein kinase enzymatic reactions that exert anti-inflammatory properties. It has also been shown in vitro to attenuate the effects of oxidative stress by promoting antioxidant enzyme activity, such as superoxide dismutase, catalase, and glutathione peroxidase. 

A review elucidates how the properties of quercetin may be clinically beneficial to individuals with late-phase bronchial asthma responses and allergic rhinitis. Moreover, in a randomized, placebo-controlled, double-blind parallel-group study (n = 66), subjects with allergic symptoms of pollinosis were given 200 mg of quercetin per day for four weeks or a control. The subjects who received quercetin displayed significantly improved JRQLQ (Japanese Rhino-conjunctivitis Quality of Life Questionnaire) scores, including improved eye itching, sneezing, nasal discharge, and sleep disorders.  

Quercetin is a polyphenol flavonoid that may be clinically beneficial in fighting common environmental triggers. Due to its effect on histamine metabolism, it may hold a unique position to support individuals in times of need during allergy season. 

By Danielle Moyer Male, MS, CNS, LDN, and Caitlin Higgins, MS, CNS