Quercetin and endurance exercise capacity: a systematic review and meta-analysis

Quercetin is a dietary flavonoid purported to improve human endurance exercise capacity. However, published findings are mixed.

PURPOSE

The study's purpose was to perform a systematic review of the literature and meta-analysis to examine whether quercetin ingestion increases endurance exercise capacity.

METHODS

A search of the literature was conducted using the key words quercetin, performance, exercise, endurance, and aerobic capacity. Eleven studies were identified as meeting the inclusion criteria providing data on 254 human subjects. Across all studies, subject presupplementation VO(2max) ranged from 41 to 64 mL·kg(-1)·min(-1) (median = 46), and median treatment duration was 11 d with a median dosage of 1000 mg·d(-1). Effect sizes (ES) were calculated as the standardized mean difference, and meta-analyses were completed using a random-effects model.

RESULTS

The ES calculated for all studies combining VO(2max) and endurance performance measures indicates a significant effect favoring quercetin over placebo (ES = 0.15, P = 0.021, 95% confidence interval = 0.02-0.27), but the magnitude of effect is considered between trivial and small, equating to a ∼2% [corrected] improvement of quercetin over placebo. Using a subgroup meta-analysis comparing quercetin's effect on endurance exercise performance versus VO(2max), no significant difference was found (P = 0.69). Meta-regression of study ES relative to subjects' fitness level or plasma quercetin concentration achieved by supplementation was also not significant.

CONCLUSIONS

On average, quercetin provides a statistically significant benefit in human endurance exercise capacity (VO(2max) and endurance exercise performance), but the effect is between trivial and small. Experimental factors that explain the between-study variation remain to be elucidated.

An enriched environment reverses the synaptic plasticity deficit induced by chronic cerebral hypoperfusion

Chronic cerebral hypoperfusion (CCH) leads to a long-term, inadequate blood supply in the brain, which eventually causes cognitive impairment. An enriched environment (EE) improves learning and memory by improving synaptic plasticity. The impact of an EE on cognitive impairment induced by CCH is not, however, well known. To investigate this possible effect, we permanently occluded the bilateral common carotid arteries (2-vessel occlusion) in rats to induce CCH and studied EE effects on cognitive impairment and synaptic plasticity following CCH. We found that EE treatment reversed spatial memory deficits induced by CCH. An EE also reversed the deficit in long-term potentiation following CCH, but the input-output curves and paired-pulse facilitation were not affected. CCH led to reduced expression of phosphorylated CREB in the rats, but EE reversed this reduction. In addition, CCH reduced the expression of synaptophysin and microtubule-associated protein 2, whereas EE reversed this reduced expression. Thus, EE reversed CCH-induced spatial cognitive impairment without affecting basal synaptic transmission or the release probability of presynaptic neurotransmitters. The EE effect probably resulted from the regulation of postsynaptic potentiation.

Mechanisms underlying the protective effects of myricetin and quercetin following oxygen-glucose deprivation-induced cell swelling and the reduction in glutamate uptake in glial cells

The protective effects of the flavonoid polyphenols, myricetin and quercetin, were investigated on key features of ischemic injury in cultures including cell swelling and the reduction in glutamate uptake. C6 glial cells were exposed to oxygen-glucose deprivation (OGD) for 5 h and cell swelling was determined 90 min after the end of OGD. OGD-induced swelling was significantly blocked by both quercetin and myricetin although higher concentrations were required for quercetin. OGD-induced free radical production, a contributing factor in cell swelling, was significantly reduced by both myricetin and quercetin. However, depolarization of the inner mitochondrial membrane potential (ΔΨ(m)), the blockade of which generally reduces swelling, was significantly diminished by myricetin, but not quercetin. This indicated that quercetin could reduce swelling despite its inability to prevent depolarization of ΔΨ(m) possibly through other signaling pathways. Increased intracellular calcium ([Ca²+](i)) is an important characteristic of ischemic injury and is implicated in swelling. Both myricetin and quercetin attenuated the increase in [Ca²+](i). Further, a reduction in [Ca²+](i), through the use of nifedipine, nimodipine, verapamil, dantrolene, or BAPTA-AM, significantly reduced OGD-induced cell swelling indicating that one possible mechanism by which such flavonoids attenuate cell swelling may be through regulating [Ca²+](i). OGD-induced decrease in glutamate uptake was attenuated by myricetin, but not quercetin. Cyclosporin A, a blocker of the mitochondrial permeability transition (mPT) pore, but not FK506 (that does not block the mPT), attenuated the decline in glutamate uptake after OGD, indicating the involvement of the mPT in glutamate uptake. Our results indicated that while blockade of ΔΨ(m) may be sufficient to reduce swelling, it may not be a necessary factor, and that flavonoids reduce cell swelling by regulating [Ca²+](i). The differential effects of myricetin and quercetin on OGD-induced reduction on glutamate uptake may be due to their differential effects on mitochondria.

Evaluation of antigenotoxic effects of plant flavonoids quercetin and rutin on HepG2 cells

The flavonoid quercetin and its derivative rutin were investigated for genotoxicity/antigenotoxicity activity in human hepatoma HepG2 cells using the comet assay. The extract cytotoxicity was evaluated using the trypan blue exclusion dye method with quercetin and rutin concentrations ranging from 0.1 to 200.0 μg/mL of culture medium. Three minor non-cytotoxic concentrations were chosen to evaluate the genotoxicity and antigenotoxicity of the flavonoids (0.1, 1.0 and 5.0 μg/mL) through comet assay. The cultures were treated with three different concentrations of rutin or quercetin (genotoxicity) or their association with Aflatoxin B1 (AFB1), methyl methanesulfonate (MMS) or doxorubicin (DXR) (antigenotoxicity test) in three protocols: pre-treatment, simultaneous treatment and post-treatment. The cell cultures were also treated with 1% DMSO (control group), AFB1, MMS and DXR (positive-control). Statistical analyses were performed using ANOVA and Dunnett's test (p ≤ 0.05). Quercetin at concentrations higher than 10.0 μg/mL or rutin higher than 50.0 μg/mL exhibited a cytotoxic effect on the cells, showing that quercetin is more cytotoxic than rutin. Furthermore, neither compound was able to induce genotoxicity in the concentrations evaluated. On the other hand, both flavonoids reduced DNA damage induced by AFB1, MMS and DXR in all treatment protocols.

Quercetin enhances human α7 nicotinic acetylcholine receptor-mediated ion current through interactions with Ca(2+) binding sites

The flavonoid quercetin is a low molecular weight substance found in fruits and vegetables. Aside from its anti-oxidative effect, quercetin, like other flavonoids, has a wide range of neuropharmacological actions. The α7 nicotinic acetylcholine receptor (α7 nAChR) has a Ca(2+)-binding site, is highly permeable to the Ca(2+) ion, and plays important roles in Ca(2+)-related normal brain functions. Dysfunctions of α7 nAChR are associated with a variety of neurological disorders. In the present study, we investigated the effects of quercetin on the ACh-induced inward peak current (I(ACh)) in Xenopus oocytes that heterologously express human α7 nAChR. I(ACh) was measured with the two-electrode voltage clamp technique. In oocytes injected with α7 nAChR cRNA, the effects of the co-application of quercetin on I(ACh) were concentration-dependent and reversible. The ED(50) was 36.1 + 6.1 μM. Quercetin-mediated enhancement of I(ACh) caused more potentiation when quercetin was pre-applied. The degree of I(ACh) potentiation by quercetin pre-application was time-dependent and saturated after 1 min. Quercetin-mediated I(ACh) enhancement was not affected by ACh concentration and was voltage-independent. However, quercetin-mediated I(ACh) enhancement was dependent on extracellular Ca(2+) concentrations and was specific to the Ca(2+) ion, since the removal of extracellular Ca(2+) or the addition of Ba(2+) instead of Ca(2+) greatly diminished quercetin enhancement of I(ACh). The mutation of Glu195 to Gln195, in the Ca(2+)-binding site, almost completely diminished quercetin-mediated I(ACh) enhancement. These results indicate that quercetin-mediated I(ACh) enhancement human α7 nAChR heterologously expressed in Xenopus oocytes could be achieved through interactions with the Ca(2+)-binding site of the receptor.

Ameliorative effect of quercetin on memory dysfunction in streptozotocin-induced diabetic rats

Diabetes-related cognitive dysfunction is a consequence of changes within the central nervous system that are secondary to chronic hyperglycemia, oxidative stress, and cholinergic dysfunction, and probably therefore anti-diabetics, anti-oxidants, and acetylcholine esterase (AChE) inhibitors were found to have beneficial effects in animal models. Quercetin, a bioflavonoid widely distributed in the plants is reported to be a potent anti-diabetic, anti-oxidant, AChE inhibitor, and memory enhancer. Therefore, we screened its influence against diabetes-induced cognitive dysfunction in streptozotocin-induced diabetic rats using Morris water and elevated plus maze (EPM) paradigms. Thirty days after diabetes induction rats exhibited marked and persistent hyperglycemia, weight loss, higher escape latency during training trials and reduced time spent in target quadrant in probe trial in Morris water maze test, and increased escape latency in EPM task. Treatment with quercetin (5-20 mg/kg, p.o., twice daily, 30 days) in streptozotocin-induced diabetic rats prevented the changes in blood glucose, body weight, and performance in Morris water and elevated plus maze tasks. In another set of experiment, quercetin (40 mg/kg, p.o., twice daily) treatment during training trials (31-35 days) markedly decreased escape latency and increased time spent in target quadrant during Morris water maze task. This treatment also decreased blood glucose levels, but had no influence on body weights. These effects were comparable to vitamin C (100 mg/kg, twice daily, 30 days) and donepezil (3 mg/kg day 31-day 35, during training trials), and devoid of any motor deficit and anxiety-like effect when tested in open field test. In conclusion, quercetin may provide a new potential option for prevention of the cognitive dysfunction in diabetes.