GPER (G protein coupled estrogen receptor 1), a particular estrogen binding site, is ubiquitously present in human tissues, but its precise physiological role is still very disputed. GPER is associated with normal and abnormal estrogen-dependent proliferations in female tissues and is involved in generation of rapid estrogenic answers. A very important fact is that GPER-induced genomic effects are additive to those mediated by "classic" estrogen receptors, but regarding the rapid effects, as we prove in this study, these can be significant different or even antagonistic.

Green tea supplements (GTSs) are a favorite among the general population and, due to their methylxanthine (caffeine, theobromine, theophylline) content, might represent a potential ergogenic source in athletes’ performance. The aim of this study was to develop and validate a method that quantifies the methylxanthine content of commercially available GTSs and to evaluate their potential relevance for ergogenic use. An ultra-high-performance liquid chromatography coupled with photodiode array detection and a QDa mass detector (UHPLC–PDA–QDa) method was developed, validated and applied to determine the content of methylxanthines in seven commercially available GTSs. Method validation parameters demonstrated satisfactory performance, with rapid and efficient separation and quantification of the three methylxanthines. Considerable variability was observed among the analyzed products, with caffeine, theobromine, and theophylline contents ranging from 0.37 to 12.13, 0.07 to 3.59, and 0.05 to 0.83 mg/capsule, respectively. However, the methylxanthine amounts measured in the analyzed products were below the doses commonly associated with ergogenic effects. Overall, the developed UHPLC–PDA–QDa method provided a reliable approach for methylxanthine quantification, while the seven products analyzed did not represent meaningful ergogenic sources of these compounds.