The Quercetin Paradox, Evaluation of its Therapeutic Efficacy as a Nutraceutical in Clinical Trials: Evidence and Speculation

Abstract

Introduction

Quercetin, a dietary flavonoid typically consumed as a glycoside, is available in supplements in its aglycone form. This aglycone form is the most studied in preclinical and clinical research because of its antioxidant, anti-inflammatory, and metabolic properties. Several clinical trials have demonstrated the benefits of quercetin for enzymatic, metabolic and cardiovascular health, approximately 60% of these studies fail to verify the actual concentration administered, generating uncertainty on their findings.

Objective

This study aimed to evaluate the concentration, stability, and cost–concentration of commercial quercetin nutraceutical formulations.

Methodology

The concentrations of five commercial brands were quantified by high-performance liquid chromatography. Stability was assessed over 6 months at both room temperature and 40°C.

Results

Only one brand contained the amount labeled; three contained approximately 80%, and one contained merely 14.8%. This study is the first to evaluate the long and thermal stability of quercetin in nutraceutical products, demonstrating that the active ingredient remains stable for at least 6 months. Brand C was identified as the most suitable option.

Conclusion

There was significant variability in the quercetin concentration of commercial supplements. These findings underscore the need for stricter regulatory oversight to ensure the quality, therapeutic efficacy, and safety of nutraceutical products.

Graphical Abstract.

Introduction

Quercetin (QTN) is a flavonoid found throughout the plant kingdom, mainly as QTN glycosides in fruits, vegetables (onion, spinach, parsley, celery, apples, grapes), green tea, and red wine.^1-3 Dietary supplements usually contain it in free form (QTN aglycone),^4,5 which is generally used in vitro and in animal models, supporting its therapeutic potential and encouraging clinical research.^6,7 When supplements exhibit therapeutic or protective properties, they are classified as nutraceuticals, defined as “a food or part of a food with pharmacological properties and health benefits”. ⁸

The biological activity of QTN is attributed to its chemical structure, particularly the catechol group on ring B and hydroxyl group at position 3 of the AC ring,^9,10 which neutralize reactive oxygen species (ROS),^10,11 as shown in Figure 1.

Figure 1.

The chemical structure of quercetin is composed of two aromatic rings (A, B) containing four hydroxyl groups and a heterocyclic ring (C) with a catechol group.^10,11

The properties’ antioxidant, anti-inflammatory, and immunomodulatory effects support its classification as a nutraceutical and justify its investigation in chronic diseases associated with oxidative stress. ¹²

The bioavailability of QTN depends on its chemical form, formulation, and route of administration.^13,14 Its glycosylated form, with attached monosaccharides, allows for greater absorption in the gastrointestinal tract than the aglycone form.^14-18 However, in clinical studies, the latter form has mostly been used (Table 1).

Table 1.

Benefits of Quercetin in Clinical Trials

Population characteristics	Quercetin dosage (mg/day)	Formulation/Brand	Treatment interval (days)	Results or benefits
Heart disease (VP)	120	ND/ND	60	↓IL-1β (p = 0.008), ↓TNF-α (p = 0.060), ↓IL-10 (p = 0.064). 25
Metabolic syndrome (HV)	150	Capsules/ND	56	↓Waist (p = 0.004), ↓Systolic pressure (p = 0.044), ↑HDL (p = 0.025), ↓triglycerides (p = 0.138), ↑TNF-α (p = 0.024). 19
	150	Capsules/ND	42	Unchanged lipids and biomarkers of inflammation. 23
Diabetes mellitus 2 (VP)	250	Capsules/Life extension USA	56	↑TAC (p = 0.043), ↓Oxidized LDL (p < 0.001), but glucose, insulin, glycosylated hemoglobin, and lipid profile unchanged (p > 0.05). 31
	500	Capsules/Lamberts	84	↓Glycosylated hemoglobin (p = 0.011), ↑Sleep (p < 0.001), ↓Anxiety (p < 0.001), ↓Waist circumference (p = 0.455), ↓Blood pressure (p = 0.029) and ↑improves quality of life (p < 0.001).32,45
Post-heart attack (VP)	500	Tablets/Sigma a	60	↑TAC (p < 0.001), ↓TNF-α (p = 0.009). 26
Rheumatoid arthritis (VP)	500	Solaray® capsules	60	↓Symptoms (p < 0.05), ↓hs-TNF-α (p < 0.05). 34
Hyperuricemia (HV)	500	Tablets/Nature’s best	30	↓Plasma uric acid (p = 0·008), no change in glucose and blood pressure. 24
Physical performance (HV)	500	Capsules/Solaray®	60	↑Endurance performance (p < 0.05). 20
	1000	Capsules/GNC®	7	↓IL-6, ↓Creatin kinase and ↓MDA, post-exercise, ↑TAC and ↑Superoxide dismutase activity (p < 0.05). 10
	1000	Capsules/ND	14	↓Creatin kinase (p = 0.026), ↓Lactate dehydrogenase (p = 0.001), ↑Endurance performance. 22
	1000	Capsules/ND	14	↓Erythrocyte lipid peroxidative (p < 0.05). 21
COVID-19 (VP)	1000	Capsules/(Jarrow)®	7	↓Alkaline phosphatase (p = 0.002), ↓quantitative CRP (p = 0.004), ↓Lactate dehydrogenase (p = 0.032). 30
PCOS (VP)	1000	Capsules/Jarrow®	90	↓Testosterone (p = 0.001), ↓insulin resistance (p = 0.009). 27
	1000	Capsules/Jarrow®	90	↑AMP kinase (p < 0.001),↑Adiponectin (p = 0.001), ↓Luteinizing hormone (p = 0.009), ↓Insulin resistance (p < 0.001).28,33
Sarcoidosis (HV)	2000	Capsules/ND	1	↓MDA (p < 0.05), ↓TNFα/IL-10 (p < 0.05), ↓IL-8/IL-10 (p < 0.05), ↑TAC (p < 0.05). 12
Fanconi anemia children ≥2 years	4000	Powder a /CCHMC lab	730	↓Reactive oxygen species (p = 0.04), ↓Neutrophils (p = 0.01) ↓Hemoglobin(p < 0.001).29

Abbreviations: ND, not described; IL, interleukin; TNFα, tumor necrosis factor-alpha; hs-TNFα, high-sensitivity tumor necrosis factor-alpha; HDL, high-density lipoprotein; LDL, low-density lipoprotein; TAC, total antioxidant capacity; MDA, malondialdehyde; CRP, C-reactive protein; AMP Kinase, adenosine monophosphate-activated protein kinase. PCOS, polycystic ovary syndrome; VP, volunteer patients; HV, healthy volunteers; The upward arrow (↑) indicates an increase; the downward arrow (↓) indicates a decrease.

^aQuercetin powder from Sigma® was used.

In these studies, QTN supplementation has been examined across various conditions, including heart disease, metabolic syndrome, type 2 diabetes (DM2), polycystic ovary syndrome (PCOS), rheumatoid arthritis, viral infections (COVID-19), Fanconi anemia, and post-exercise stress.^10,19-30

In patients with metabolic syndrome or DM2, the QTN administration has been associated with reductions in blood pressure, triglyceride levels and glycated hemoglobin.^31,32 It has also demonstrated improvements in metabolic regulation among women with PCOS^27,28,33 and alleviation of symptoms in rheumatoid arthritis patients. ³⁴ The quercetin exerted hepatoprotective effects and facilitated accelerated recovery in COVID-19 patients, whereas in those with Fanconi anemia, attenuation of ROS contributed to platelet stability. ²⁹ Furthermore, in healthy volunteers, QTN reduced post-exercise oxidative stress and enhanced physical performance.^10,20,22

A broad range of QTN doses and dosage forms was observed across the clinical studies. However, nearly 60% of the trials did not corroborate the administered concentrations, a significant methodological limitation that hinders the interpretation and compromises the reproducibility of the findings (Table 1).

Thus, the aim of this study was to quantify the quercetin concentration, assess its stability, and analyze the cost of five commercial nutraceutical formulations.

Materials and Methods

Database Search Algorithm

A search in the PubMed database using the keywords quercetin, dietary, supplements and human was performed; the filters applied were clinical trials published in English in the past 25 years. Abstract articles were reviewed by at least two authors, and studies where quercetin was combined with other components, such as vitamin C, were excluded. Data concerning condition, dose, interval of administration, number of patients and/or volunteers, and reported pharmacological effects were extracted from the selected articles.

Reagents

Acetonitrile (ACN) and methanol (MeOH) were High Performance Liquid Chromatography (HPLC) grade, and the quercetin standard (USP grade) was obtained from Sigma‒Aldrich® (Missouri, USA). In addition, undenatured 96% ethanol was purchased from a local drugstore, and ortho-phosphoric acid [H₃PO₄] (≥85%) and 99.9% dimethyl sulfoxide (DMSO) were obtained from J.T. Baker® (Phillipsburg, NJ). Milli-Q® ultrapure water was used.

Nutraceuticals

Five brands of nutraceuticals containing QTN, named A, B, C, D and E, were purchased. The information included in the label, such as the place of manufacture, indications for use, other ingredients present in the formulation, expiration date and warnings, were recorded.

Quantification of Quercetin by High-Performance Liquid Chromatography

The conditions reported by Zhang were modified, and then, ⁴ the HPLC method was validated, complying with the parameters of linearity, repeatability, and reproducibility, among others, according to NOM-177-SSA1-2013, which is harmonized with international standards of the Food and Drug Administration and the European Medicines Agency.^35-37

Quercetin separation was carried out in a Waters® chromatographic system (Milford, MA, USA) composed of an autosampler model 717, a pump model 515, a UV/Vis detector model 2487, and a column heater. A Zorbax® Eclipse Plus C18 column (4.6 × 150 mm, 5 μm, Agilent®) maintained at a temperature of 30°C was used. The mobile phase consisted of ACN and H₃PO₄ at 0.1% in a 40:60 (v/v) ratio, which was prepared daily; the flow rate was 0.9 mL/min. Detection was performed at 365 nm, and the data were analyzed with Empower® 2.0 software (Waters, Inc).

Preparation of the Quercetin Stock Solution and Calibrators

Ten milligrams of the QTN standard were weighed and transferred to a 10 mL volumetric flask, 2 mL of DMSO was added, and the mixture was homogenized by shaking and ultrasonication for 2 min at 25°C. The mixture was diluted to 10 mL with MeOH, and homogenization (shaking and ultrasonication) was repeated. The final concentration of the stock solution was 1 mg/mL.

From the stock solution of QTN (1 mg/mL), serial dilutions were made with the mobile phase, and the following calibrator solutions were obtained: 1, 5, 10, 20, 30, 40, and 50 μg/mL. For each calibrator, 10 μL was injected into the chromatographic system to obtain the calibration curve.

Weight Variation per Unit of Nutraceutical

The tablets or capsules were selected as units (U) and weighed directly. In the case of the capsules, the content (total weight) was weighed, the content was subsequently drained, and the empty capsule was weighed. The net content was calculated from the difference in total weight and the weight of the empty capsule. Individual data and the average weights of the 10 U samples were obtained. ³⁸

Evaluation of Quercetin Concentration in Nutraceuticals

The QTN quantification of the nutraceuticals was performed with 10 units. Each sample was dissolved in 500 mL of 70% ethanol and shaken at 260 rpm for 5 min at room temperature (RT). The mixture was homogenized in an ultrasonic bath for 5 min at 25°C, followed by a second homogenization at 260 rpm and RT for 10 min. A 1 mL aliquot was taken from this mixture and centrifuged at 12 000 rpm for 10 min, and the resulting supernatant was diluted 1:40 (v/v) with the mobile phase.

Then, 10 μL was injected into the chromatographic system, and the QTN concentration was quantified by the previously validated HPLC method. ³⁵ The variation in content was expressed as a percentage and C.V. compared to the response of the quercetin standard (100%).

Cost Analysis

Two cost analyses were performed. First, the QTN content specified on the label was used. For this purpose, the price of the nutraceutical bottle was divided by the number of units contained, adjusting the amount of U consumed daily to reach a dose of 500 mg. The value obtained was multiplied by 30 to obtain the monthly cost of the treatment.

The second analysis was performed based on the actual QTN content (quantified by HPLC). Thus, the price of the nutraceutical bottle was also divided by the number of units but adjusted according to the quantity of units needed to reach the daily dose of 500 mg. The daily cost was subsequently multiplied by 30 to obtain the monthly payment of treatment. ³⁹

Stability of the Quercetin Content in the Nutraceuticals

Stability studies of drugs, medicines, and herbal remedies provide scientific evidence that demonstrates the shelf-life of a drug. Since there is no specific regulation for nutraceuticals, we relied on the Mexican Standard NOM-073-SSA1-2015, regarding the “Stability of drugs and medicines, as well as herbal remedies”, ⁴⁰ which establishes that the active ingredient of any pharmaceutical product must remain 90% stable for 6 months at 40°C. This served as a basis for determining the percentage at which we would expect quercetin to remain stable.

To evaluate the stability of the QTN content in the nutraceuticals of each of the brands, the units were stored in amber glass bottles for 1, 3, and 6 months at RT (long-term stability) and 40°C (accelerated stability). The QTN content was quantified in triplicate once the storage time had elapsed.

Statistical Analysis

The descriptive analysis used measures of central tendency: mean (X̄), standard deviation (S.D.), and coefficient of variation (C.V.). Differences in concentration and cost across the five brands were analyzed using one-way ANOVA, followed by Tukey’s post hoc test in SPSS (version 24). Each brand was compared with the others; p ≤ 0.05 was considered statistically significant.

Results

Database Search Results

From the PubMed search, 86 articles were identified, and 69 were excluded for several reasons; for example, studies where quercetin was combined with other compounds, in vitro or animal studies, studies published in languages other than English, and analyses performed with quercetin glycosides instead of the aglycone form or not available in the full text. Seventeen documents with clinical trials were included.

Information Contained in the Nutraceutical Labels

The information provided on the labels of the five brands (named A, B, C, D and E) tested is shown in Table 2.^41,42

Table 2.

Information Provided in the Brand Labels

Information	Brands
	A	B	C	D	E
Place of manufacture and distribution.	Yes	Yes	Yes	Yes	Yes
Batch and expiration date.	Yes	Yes	Yes	Yes	Yes
Content (mg/U)	500	100	500	500	250
Instructions for use and recommended doses.	One capsule daily with food or by medical prescription.	Two capsules per day.	Do not exceed one tablet per day.	Take two capsules daily, preferably with food.	Only for adults. Two capsules per day.
Other ingredients.	Vegetable cellulose, magnesium stearate, silicon dioxide and microcrystalline cellulose.	Microcrystalline cellulose.	Cellulose, stearic acid, magnesium stearate, oleic acid, camauba wax, ethyl-cellulose, polyethylene glycol.	Gelatin	Partially pregelatinized starch, polyvinylpyrrolidone.
Warnings	Do not exceed the recommended dose. Pregnant, breastfeeding or under 18 years of age, taking any medication or have a medical condition, consult your doctor before use. Discontinue use if any adverse reactions occur.	It is not a medicine. The use of this product is the responsibility of the person recommending it and the person using it.	It is not a medicine. The use of this product is the responsibility of the person recommending it and the person using it. Keep out of reach of children. Store in a cool, dry place.	Do not use during pregnancy, breastfeeding, or if there is an intolerance to the ingredients. Keep in a cool, dry place and out of reach of children. Its use is the responsibility of the user and the person recommending it.	Not recommended for pregnancy or breastfeeding. Discontinue use if you have an allergic reaction. Keep out of reach of children. It is not a medicine; its consumption is the responsibility of the person who recommends it and the person who uses it.

The amount of QTN present in the different nutraceuticals ranged from 100 to 500 mg/U. Similarly, the recommended daily intake on the labels of the nutraceuticals is between 200 and 1000 mg/day (Table 2).

Weight Variation

The net weight range of U was as follows: for brand A, 486.6-533.4 mg; for brand B, 438.5-521.1 mg; for brand C, 1000.5-1038.5 mg; for brand D, 420.6-526.7 mg; and for brand E, 411.1-477.5 mg. The brand C tablets had the lowest coefficient of variation of 1.2%, whereas the brand D capsules had the highest C.V. of 7.9%. This same behaviour was observed for the standard deviation of brand C, which was 12.3%, and for brand D, it was 37.9%. The weight variation coefficient was below 10% for brands A, B, D, and E (capsules), and below 5% for brand C (tablets). Nevertheless, all brands met the established acceptance criterion for weight uniformity ³⁸ (Table 3).

Table 3.

Weight Variation in Units of the Nutraceutical Brands

Unit	Net weight (mg/U)
	A	B	C	D	E
1	533.4	521.1	1025.6	426.6	411.1
2	513.6	480.1	1017.7	526.7	422.1
3	505.0	462.0	1031.8	502.5	468.3
4	520.6	507.8	1034.6	503.5	463.0
5	502.7	438.5	1038.5	443.5	433.8
6	503.3	491.1	1000.5	514.7	477.5
7	489.8	474.2	1017.1	420.6	431.3
8	517.4	491.5	1011.1	501.1	475.9
9	486.6	501.0	1017.0	463.9	456.0
10	519.1	472.8	1007.4	490.5	437.6
X̄	509.2	484.0	1020.1	479.4	447.7
S. D.	14.5	23.9	12.3	37.9	23.5
% C. V.	2.8	4.9	1.2	7.9	5.2

Individual weight and average of ten units (U) of each brand. Mean (X̄), Standard deviation (S. D), Coefficient of variation (C. V).

Chromatographic Analysis of the Nutraceutical Quercetin

The HPLC method was validated because it met the requirements of linearity, repeatability, and reproducibility according to international guidelines. The retention time of the QTN compound was 3.68 min, and a representative chromatogram is shown in Figure 2.

Figure 2.

Representative chromatograms of 10 μL of quercetin standard at 10 μg/mL (blue) and nutraceutical samples (10 μL of 1:40 v/v dilutions for unity) for brands A (red), B (gray), C (black), D (green) and E (yellow)

Quercetin Concentration in Nutraceuticals

The concentration of QTN was quantified using HPLC. For comparative analysis, the content of each brand was normalized to a dose of 500 mg, five units of brand B, two of brand E and one of A, C and D. Considerable variability was observed among the brands. Only brand C met the amount declared on its label, with a content of 533.7 mg (106.7%). In contrast, brands A, B, and E shown approximately 80% of the declared amount. Brand D showed the greatest discrepancy, with only 73.8 mg, representing a deficit of 85.2%. Statistical analysis shown that brand D was an outlier, with a QTN concentration significantly lower than the others. Brand C also showed a significant difference because it was the only one whose actual value matched the amount declared on the label (Table 4).

Table 4.

Evaluation of Quercetin Concentrations in Nutraceuticals

Unit	A			B		C		D			E
	500 mg/U	%	100 mg/U	500 mg*	%	500 mg/U	%	500 mg/U	%	250 mg/U	500 mg*	%
1	324.0	64.8	75.4	377.0	75.4	512.0	102.4	79.2	15.9	186.7	373.4	74.7
2	392.7	78.5	78.4	392.0	78.4	489.0	97.8	68.3	13.7	211.1	422.2	84.4
3	355.5	71.1	74.3	371.5	74.3	533.2	106.6	77.8	15.6	192.5	385.0	77.0
4	437.6	87.5	81.1	405.5	81.1	534.1	106.8	78.0	15.6	206.2	412.4	82.5
5	384.7	76.9	73.0	365.0	73.0	553.1	110.6	63.8	12.8	188.0	376.0	75.2
6	364.9	73.0	88.9	444.5	88.9	544.7	108.9	76.6	15.3	219.3	438.6	87.7
7	396.9	79.4	77.8	389.0	77.8	547.1	109.4	66.2	13.2	185.2	370.4	74.1
8	403.9	80.8	66.6	333.0	66.6	562.5	112.5	74.7	14.9	220.6	441.2	88.2
9	388.2	77.7	86.3	431.5	86.3	589.4	117.9	70.6	14.1	190.0	380.0	76.0
10	413.8	82.8	81.1	405.5	81.1	521.7	104.3	83.2	16.7	206.6	413.2	82.6
X̄	386.2	77.3	78.3	391.5	78.3	533.7	106.7	73.8	14.8	200.6	401.2	80.2
S. D.	31.9	6.4	6.5	32.6	6.5	21.5	4.3	6.3	1.3	13.7	27.4	5.5
%C. V.	8.3	8.3	8.3	8.3	8.3	4.0	4.0	8.6	8.6	6.8	6.8	6.8

Quercetin (QTN) content in nutraceuticals. The contents are labelled as follows: brands A, C, and D, 500 mg/unit; brand B, 100 mg/unit; and brand E, 250 mg/unit, adjusted to 500 mg*. Mean (X̄), Standard Deviation (S.D.), Coefficient of Variation (C.V.). The percentage was calculated based on the actual amount in relation to 500 mg of QTN Differences between groups were analyzed using a one-way ANOVA, followed by Tukey´s post-hoc test for multiple comparisons. Statistical significance (^a) was set at p ≤ 0.05.

Cost Analysis

Table 5 shows the cost analysis, establishing the relationship between the QTN content mentioned on the label and the true QTN concentration, both for a daily dose of 500 mg of QTN and for monthly treatment. The costs of 1 month of treatment, considering the amount of QTN specified on the label, for brands A, B, C, D and E were USD $9.90, $76.40, $9.90, $7.50 and $18.60, respectively.

Table 5.

Analysis of the Cost of Monthly Quercetin Treatment

Analysis according to label information	A	B	C	D	E
Cost of vial ($)	19.70	20.40	19.70	15.25	18.70
Units per vial	60	40	60	60	60
QTN (mg/U)	500	100	500	500	250
Units for 500 mg	1	5	1	1	2
Cost per unit ($)	0.33	0.51	0.33	0.25	0.31
Cost per 500 mg ($)	0.33	2.55	0.33	0.25	0.62
Monthly treatment ($)	9.90	76.50	9.90	7.50	18.60
Analysis according to QTN content	A	B	C	D	E
Real QTN (mg/U)	386.2	78.3	533.70	73.8	200.6
Units for 500 mg	1.3	6.4	1	6.8	2.5
Cost per 500 mg/day ($)	0.43	3.26	0.33	1.70	0.77
Monthly treatment ($)	12.90	97.80*	9.90	51.00*	23.10*
Comparative statistical analysis of costs between brands
A	----	0.00a	0.182	0.00a	0.00a
B	0.00a	----	0.00a	0.00a	0.00a
C	0.182	0.00a	-----	0.00a	0.00a
D	0.00a	0.00a	0.00a	----	0.00a
E	0.00a	0.00a	0.00a	0.00a	----

The costs per vial, unit, daily intake (500 mg) and monthly treatment (30 days) are shown in United Stated Dollar ($). First, cost analysis was performed according to the label information; second, cost analysis was performed on the basis of the true quercetin (QTN) content quantified by chromatographic analysis. Differences between groups were analyzed using a one-way ANOVA, followed by Tukey´s post-hoc test for multiple comparisons. Statistical significance (^a) was set at p ≤ 0.05.

Additionally, we investigated the relationship between the real QTN content and the daily 500 mg treatment over 1 month. Brand C has a content of 533.7 mg, equivalent to 106.7% of the active compound specified on the label, with a monthly cost of USD $9.90.

Brands A, B, and E contained an actual QTN concentration of approximately 80% of that stated on the label, so adjusting the dose to 500 mg of QTN increased the monthly treatment cost to USD $12.90, $97.80, and $23.10, respectively. Brand D had the lowest QTN content, 73.8 mg/U (14.8%), and to reach the daily dose of 500 mg, almost seven units are needed, increasing the monthly treatment cost to USD $51.00 (Table 5).

Therefore, brands A and C are suitable options for the consumer; brand A has an actual concentration of 386.2 mg/U and a monthly cost of USD $12.80, whereas brand C contains 533.7 mg/U and a monthly cost of USD $9.87, making it the best option in terms of the cost/concentration ratio.

Quercetin Stability

The stability of quercetin was assessed over 6 months. The initial concentration (time zero) for each brand was set as 100%. Under RT storage conditions, the remaining QTN was 107.3% for brand A, 106.3% for B, 94.1% for C, 104.5% for D, and 98.1% for E (Table 6).

Table 6.

Stability of Quercetin at Room Temperature and 40°C

Brand time	A (X̄ ± S. D.)		B (X̄ ± S. D.)		C (X̄ ± S. D.)		D (X̄ ± S. D.)		E (X̄ ± S. D.)
	mg/U	% QTN	mg/U	% QTN	mg/U	% QTN	mg/U	% QTN	mg/U	% QTN
Room temperature
T (0)	386.2 ± 31.9	100	78.2 ± 6.5	100	533.7 ± 27.8	100	73.8 ± 6.3	100	200.6 ± 13.7	100
Month 1	361.7 ± 29.7	93.7 ± 7.7	79.2 ± 1.0	101.1 ± 1.3	493.1 ± 28.4	92.4 ± 5.3	78.6 ± 2.9	106.5 ± 4.0	199.2 ± 9.8	96.3 ± 4.8
Month 3	409.7 ± 6.5	106.1 ± 1.7	80.3 ± 8.4	102.7 ± 11.0	565.5 ± 11.5	106.0 ± 2.1	74.5 ± 2.7	100.9 ± 3.6	199.1 ± 5.0	99.2 ± 2.5
Month 6	414.4 ± 25.2	107.3 ± 6.5	83.2 ± 10.2	106.3 ± 13.0	501.4 ± 43.0	94.1 ± 7.9	77.4 ± 0.6	104.5 ± 0.9	196.7 ± 4.1	98.1 ± 2.0
Temperature at 40°C
Month 1	390.8 ± 17.1	101.2 ± 4.4	77.1 ± 1.1	98.5 ± 1.4	511.1 ± 37.6	95.8 ± 7	76.6 ± 2.5	103.7 ± 3.5	192.6 ± 2.0	96.0 ± 1.4
Month 3	447.3 ± 19.4	115.8 ± 5.0	80.5 ± 9.1	102.9 ± 11.6	586.1 ± 8.7	109.8 ± 1.6	74.5 ± 1.6	100.9 ± 2.2	198.6 ± 4.6	99.0 ± 2.0
Month 6	379.1 ± 7.8	98.3 ± 2.3	77.8 ± 6.3	99.3 ± 8.0	491.9 ± 7.1	92.1 ± 1.3	73.6 ± 2.0	99.8 ± 2.7	199.1 ± 4.7	99.3 ± 2.3

Units (U) were stored in an amber bottle at room temperature (20.04 ± 0.66°C and 56% humidity) or at 40.96 ± 0.53°C and 29% humidity for accelerated stability. The quercetin (QTN) concentration at time zero (T) was taken as 100%. Three units were analyzed by high-performance liquid chromatography, and the mean (X̄) and standard deviation (S. D.) are shown.

Under accelerated stability conditions (40°C), the values were 98.3% (Brand A), 99.3% (Brand B), 92.1% (Brand C), 99.8% (Brand D), and 99.3% (Brand E).

Therefore, all five brands maintained at least 90% of their initial QTN concentration under both storage conditions, demonstrating high stability. ⁴⁰

Discussion

QTN Concentration in Nutraceuticals

A review of the literature revealed that several clinical studies failed to verify the actual concentration of quercetin in the nutraceuticals analyzed (Table 1). This methodological omission represents a significant limitation, as it compromises the reproducibility and validity of the pharmacological effects attributed to this compound.

Our study found that the intra-brand weight variation was less than 10%, indicating consistency in dosage and product quality. However, a significant discrepancy was observed between the quercetin concentrations reported on commercial product labels and the actual concentrations determined by HPLC. A remarkably low concentration (14.8%) was quantified in brand D, whereas brand C contained the amount indicated on the label (106.7%), equivalent to 73.8 and 533.7 mg/U, respectively.

In Hungary, Vida et al. evaluated nine brands of quercetin and reported concentrations ranged from 96.8% to 128.1%. ⁴³ This is in contrast to our findings (14.8 to 106.7%).

Likewise, Pardo Barrela, in Spain, reported variability between 100.58% and 123.84% in the concentration of QTN in six brands. ⁴⁴ Taken together, these data suggest that these brands, which are available in the European market, usually comply with the concentration of QTN indicated on their label, in contrast to what was observed in the present study.

Our high variability is attributable to the low concentration of quercetin in brand D. We speculate that the low QTN content in Brand D likely due to its formulation rather than degradation, as it demonstrated stability over 6 months.

With the respect of QTN concentration, from the seventeen clinical studies, only one quantified the actual quercetin concentration, which was 544 mg. ²⁴ The brand used was Nature’s Best, and its results are consistent with those reported by Vida et al, ⁴³ who reported a concentration of 558.5 mg.

In six studies, the actual concentration of quercetin was not quantified at the time of the clinical trial, as was the case in the works of Shohan, Khorshidi and Rezvan^27,28,30,33 who used Jarrow brand supplements; the two clinical trials of Mantadaki^32,45 with Lambert brand nutraceuticals; and the study of where the GNC brand was used. ¹⁰ However, these clinical trials could be considered more solid and robust, given that the Jarrow, Lambert, and GNC brands were shown to contain the concentration indicated on their label, as reported by Vida et al. ⁴³ and the results of this study about label C.

In the remaining ten studies, the actual concentrations of the tested products were not determined, representing a major methodological limitation, as the pharmacological effect is directly dependent on the administered dose. Consequently, any discrepancy in QTN content could compromise the validity of the reported therapeutic effects.^19,23,25,31

It is noteworthy that, in studies where QTN concentration was not assessed, this omission generally reflected the fact that such determination was not the primary objective of the clinical trial, with investigators relying on the accuracy of the label-declared concentrations. The lack of analytical verification of the actual concentration of QTN in nutraceuticals represents a critical limitation in clinical research. This omission generates uncertainty about the effective dose administered, hinders the understanding of the patient’s true exposure to the active ingredient and compromises the interpretation of the therapeutic results, also imply risks of adverse effects, especially in vulnerable populations such as pediatric patients ²⁹ or those with chronic illnesses.^{25,29,32,34,45}

As has been documented in studies such as those of Pfeuffer and Egert,^19,23 the absence of significant effects could be related to subtherapeutic concentrations or to the instability of the active compound.

Stability

The stability of a nutraceutical is crucial for ensuring its efficacy and safety, particularly under conditions of prolonged storage; however, we did not find studies on the stability of QTN in nutraceuticals; only a technical report of QTN in raw material was carried out by a pharmaceutical company, which indicated adequate stability for 5 years under controlled conditions of 30 ± 2°C and a relative humidity of 70 ± 5%. ⁴⁶

The present study is the first to perform accelerated stability analysis of QTN at 40°C for 6 months in five different commercial brands. The results showed that the remaining concentration was greater than 90%, which suggests adequate stability under high-temperature conditions. This information is valuable and can serve as an indicator of the shelf life of nutraceuticals under real conditions, especially in countries with warm climates.

Cost

The cost of a product is a problem faced by a consumer, and the acquisition of a nutraceutical that does not contain the declared amount of active ingredient represents an unjustified economic investment and a risk to their health, given that a lower concentration of active compound will have a poor or no therapeutic effect. ⁴⁷ Our analysis of the cost/content could not be compared because of an absence of reports in the literature.

It is evident that consumers are exposed to the risk of purchasing a product that does not comply with the nutraceutical content because, worldwide, there is no regulation or monitoring of the quality control of these products; consequently, there is no guarantee of their pharmacological and therapeutic effects.

Dose-Response QTN

Despite considerable methodological heterogeneity in clinical trials, varying in population, dose, and outcomes, a consistent trend supporting a dose-response relationship for quercetin emerges. A dose of 500 mg/day, derived from studies of various commercial formulations, has been established as safe and clinically relevant. Efficacy is demonstrated by a significant reduction in hs-TNFα in rheumatoid arthritis patients, ³⁴ while safety is corroborated by a significant decrease in uric acid without impacts on glycemia or blood pressure. ²⁴

In sports science, higher quercetin doses (≥1000 mg/day) demonstrate significant ergogenic and antioxidant benefits. Daneshvar et al, ²⁰ reported that such doses markedly improved time to exhaustion, while Tsao et al. ¹⁰ observed enhanced total antioxidant capacity, SOD activity, and reduced levels of malondialdehyde, IL-6, and creatine kinase after only 7 days of supplementation. Furthermore, the safety and efficacy of high-dose QTN are supported in chronic conditions; doses ranging from 750 to 4000 mg/day for up to 2 years were well-tolerated in pediatric patients with Fanconi anemia and led to a sustained reduction in oxidative stress. ²⁹

Clinical evidence further supports the efficacy of a 500 mg/day QTN dose. In DM2 patients, this dose significantly improved glycemic control, blood pressure, respiratory function, and mental well-being.^32,45 Similarly, in post-infarction patients, it reduced inflammation and enhanced antioxidant capacity, leading to improved quality of life. ²⁶ These findings indicate a positive dose-response relationship, with consistent and sustainable clinical benefits emerging at doses of 500 mg/day.

Conclusions

Significant variability in the concentration of quercetin nutraceuticals was found across brands; quercetin has been shown to be stable under different storage conditions but the price is not a reliable indicator of quality. Therefore, it is crucial to implement strict quality controls and independent verification to validate quercetin content and ensure efficacy and safety in patients.

Limitations

The main limitations included the small number of brands analyzed due to the limited availability of products at the time of purchase of quercetin aglycone; different batches per brand were not evaluated, which restricts the reproducibility of the results. In addition, no microbiological analyses were performed, so the presence of microorganisms cannot be ruled out. The concentrations of additives were also not determined, which makes it impossible to verify their correlation with the limits established by food safety regulations.

Appendix.

Abbreviations

QTN

Quercetin

ACN

Acetonitrile

MeOH

Methanol

HPLC

High performance liquid chromatography

DMSO

Dimethyl sulfoxide

U

Units

C.V

Coefficient of variation

RT

Room temperature

S.D.

Standard deviation

ANOVA

Analysis of variance

X̄

Mean

TAC

Total antioxidant capacity.

ORCID iDs

Raquel García Alvarez https://orcid.org/0000-0003-3563-134X

Juan Luis Chávez-Pacheco https://orcid.org/0000-0001-6919-4497

Edelmira Solorio-López https://orcid.org/0000-0003-2981-2347

Josefina Gómez-Garduño https://orcid.org/0000-0002-0226-3651

Liliana Rivera-Espinosa https://orcid.org/0000-0002-1334-0026