Influence of ambient temperature and selected polyphenols on resting energy expenditure in humans

Resting energy expenditure (REE) constitutes the largest component of total energy expenditure and is commonly assessed by indirect calorimetry. REE is influenced by physiological factors, individual behavior, and environmental conditions, for example ambient temperature. Polyphenols may also affect energy metabolism, but their influence on REE and the bioavailability of xanthohumol, a prenylated flavonoid found primarily in hops, remains insufficiently investigated. This thesis therefore investigated the effects of ambient temperature and xanthohumol on REE and examined the bioavailability of native and micellar xanthohumol. In Study I, REE was measured by indirect calorimetry in 32 metabolically healthy adults (16 women, 16 men; 25 ± 3 years; BMI 22.4 ± 1.6 kg/m²) at ambient temperatures of 18, 22, 28, and 38 °C using a randomized crossover design. Fat-free mass and ambient temperature significantly influenced REE, with REE being significantly higher at 18 °C than at 28 °C (+96 ± 24 kcal/24 h) and 38 °C (+57 ± 23 kcal/24 h), and at 22 °C than at 28 °C (+73 ± 24 kcal/24 h). Study II investigated the plasma kinetics of native and micellar xanthohumol in 12 metabolically healthy adults (6 women, 6 men; 24 ± 3 years; BMI 21.7 ± 1.7 kg/m²) following single oral doses of 86 or 172 mg. Micellization and dose significantly increased the area under the plasma concentration–time curve and maximum plasma concentration of total xanthohumol (all P < 0.001). Apparent bioavailability was approximately ninefold higher for micellar than native xanthohumol. In Study III, the acute effects of 172 mg micellar xanthohumol on REE, blood pressure, and heart rate were assessed in 16 metabolically healthy women (23 ± 3 years; BMI 20.7 ± 1.3 kg/m²) using a placebo-controlled crossover design. Compared with placebo, acute xanthohumol administration did not significantly affect REE, blood pressure, or heart rate. In conclusion, ambient temperature significantly affected REE and should therefore be standardized during indirect calorimetry. Micellization substantially improved the bioavailability of xanthohumol, whereas a single acute dose did not appear to affect REE or cardiovascular parameters.