Quercetin is a bioflavonoid widely used in naturopathic medicine. It has been prescribed as a natural antihistamine for allergic rhinitis and related acute conditions. It has some anti-viral and anti-inflammatory properties. It has been applied in several metabolic disorders such as hyperlipidemia, non-alcoholic fatty liver disease, and diabetes. In diabetes it bypasses GLUT transporters via AMPK signalling. It reduces oxidative stress and is believed to be protective of the cardiovascular system and the liver. Its anti-fibrotic properties have been applied in ocular diseases. Pre-clinical data suggests use for the glycolytic flux in cancer, the epigenetic reprogramming of malignant cells, cancer stem cell management, and immunoregulation in cancer. It modulates the p-glycoprotein porter system, making it a valuable adjunct to many chemotherapy drugs. Decades ago, the Ames test showed mutagenicity in bacteria from quercetin, but animal studies verify there is zero genotoxicity.
In this review paper on quercetin in metabolic diseases by Yi et al., we find many biochemical actions determined by pre-clinical studies, and finally some modest human trials that confirm clinical efficacy. The outcomes in diabetes were more modest than for NAFLD and hyperlipidemia, but it was confirmed that it is completely compatible with metformin. While the level of evidence remains weak, further trials are clearly warranted. Inconsistent studies are mentioned, but with the important caveat that under-dosing may be a key factor. A marked discrepancy between the doses being used in clinical practice and those used in research trials is commonly seen in the database of natural medicines. Compounding this is the issue of low absorption and rapid excretion. Improved delivery by phytosomes/liposomes is noted. It also notes the induction of improved absorption by long-term intake. It is entirely possible the medicinal effects are due to metabolites such as dihydroquercetin, or from an impact on gut microbiota. New formulations of quercetin may open up a much broader use of this natural substance.
Yi, Peng, Wu, et al., The Therapeutic Effects and Mechanisms of Quercetin on Metabolic Diseases: Pharmacological Data and Clinical Evidence, Oxid. Med. Cell. Longev. 2021; 6678662. doi.org/10.1155/2021/6678662