A comprehensive research review about the effects of Quercetin on inflammation and immunity was uploaded to the Nutrients — Open Access Human Nutrition Journal. It covered both effects observed in vitro as well as in vivo (animal and human) and the correlation, or lack of, between those.
The review showed differences in those effects observed in-vitro, in-vivo human and animal. The results suggest that quercetin exhibited anti-inflammation and immune-enhancement in vitro (cells) and also in vivo (animals). However, studies in humans did not fully align with these results from cells and animals. The effect, in which quercetin acts as an immune booster in humans appears to be far less obvious based on todays available results (see the last table in the article) indicating a need for further research before considering future broad application. Apart from the actual findings wrt quercetin it also demonstrates in general that there is need for caution when estimating impacts in humans from effects observed in-vitro and in animal models. It should be noted that quercetin is also touted for other possibly positive effects for example in senolytics or kidney fibrosis.
Quercetin belongs the the flavonoid groups found in fruits and vegetables. There is an increased interest in plant-derived dietary supplements to enhance performance and maintain/improve health. Quercetin is one of the compounds in focus as it has unique biological properties that are likely anti-inflammatory, anti-oxidant, possibly psychostimulant activity and to stimulate mitochondrial biogenesis. However, most information circulating regarding quercetin’s potential is based upon in vitro and animal studies. The aforementioned review examines and summarizes the findings in recent scientific literature regarding effects of quercetin on inflammation and immunity in mental and physical performance and health:
From in vitro experimentation Quercetin is reported to have a long lasting anti-inflammatory substance that possesses strong anti-inflammatory capacities. It possesses anti-inflammatory potential that can be expressed on different cell types, both in animal and human models. It is known to possess both mast cell stabilizing and gastrointestinal cytoprotective activity. It can also play a modulating, biphasic and regulatory action on inflammation and immunity. Additionally, quercetin has an immuno suppressive effect on dendritic cells function.
The researchers developed the below working model on how quercetin blocks tumor necrosis factor-α (TNFα)-mediated inflammation. Quercetin prevents TNF-α from directly activating extracellular signal-related kinase (ERK), c-Jun NH2-terminal kinase (JNK), c-Jun, and nuclear factor-κB (NF-κB), which are potent inducers of inflammatory gene expression and protein secretion. In addition, quercetin may indirectly prevent inflammation by increasing peroxisome proliferator-activated receptor c (PPARγ) activity, thereby antagonizing NF-κB or activator protein-1(AP-1) transcriptional activation of inflammatory genes. Together, these block TNF-α-mediated induction of inflammatory cascades. A schematic of the paths is shown in the figure (click figure for larger view):
The main action of quercetin on inflammation and immune function in vitro is summarized in the below table (click for larger view) which summarizes the various positive effects in both animal and human cell lines:
For the in human effects the researchers analyzed a number of randomized, double-blinded and placebo-controlled trials with quercetin supplementation in humans with dosage of 100, 500 or 1000 mg/day quercetin for upto periods 12 weeks. These were done in groups of people considering themselves fit to endurance athletes thus providing a decent amount of data across a wider spectrum.
The main action of quercetin on inflammation and immune function in vivo was summarized in this table (click to enlarge):
In summary: as a widespread flavonoid, quercetin is a safe and dietary supplement based on its broad range of biological effects in animals. The results of these effects on immune boosting are not consistent, however and appear dependent on the type of subject and their level of health.