No evidence for a thermic effect of the dietary flavonol quercetin: a pilot study in healthy normal-weight women

Our objective was to investigate the effect of quercetin supplementation on fasting resting energy expenditure (REE) and respiratory quotient (RQ) in humans. Six healthy, normal-weight women (mean age 25.5 ± 1.6 years, body mass index 21.4 ± 1.5 kg/m²) participated in a randomized, placebo-controlled, double-blinded crossover study. Treatments were administered as capsules of 150 mg quercetin (aglycone) or placebo. The acute response was measured by indirect calorimetry for 3 h following ingestion. Blood pressure and pulse rate were assessed in 30-min intervals. On the following day, 24 h after capsule intake, a follow-up measurement was performed. Baseline (t ₀) REE adjusted for fat-free mass was 4.7 ± 0.26 kJ/min (quercetin) and 4.8 ± 0.35 kJ/min (placebo) and did not significantly change between baseline and end (t ₁₈₀) in either group (P = 0.992 for time effect in repeated measures analysis of variance; P = 0.581 for time × treatment interaction). Mean RQ was 0.78 ± 0.04 (quercetin) and 0.77 ± 0.04 (placebo). RQ values decreased slowly and to a similar extent during both treatments (P < 0.001 for time; quercetin, −0.09 ± 0.05; placebo, −0.08 ± 0.03; P = 0.877 for time × treatment interaction). Resting systolic and diastolic blood pressure, pulse pressure as well as resting pulse rate did not significantly change between baseline and end in either treatment group. No significant differences were found between the results of the baseline measurement and 24 h after treatment. In conclusion, the present pilot study provides no evidence for a thermic effect of quercetin in humans.