Tea Bioactive Compounds in Obesity Prevention and Management: Processing-Dependent Composition, Molecular Mechanisms, Human Evidence, and Translational Challenges

Obesity is a heterogeneous chronic disease for which safe, scalable adjuncts to lifestyle and clinical care remain needed. Tea derived from Camellia sinensis contains catechins, caffeine, theaflavins, thearubigins, theabrownins, polysaccharides, and other constituents whose abundance is shaped by withering, fixation, partial oxidation, full oxidation, and post-fermentation. This review integrates processing-dependent composition with molecular mechanisms, gut–liver signaling, human evidence, safety, and real-world preparation. Evidence is strongest for modest effects of green-tea catechin–caffeine preparations on energy metabolism and selected anthropometric or lipid outcomes, whereas inhibition of adipogenesis, activation of AMP-activated protein kinase, browning of white adipose tissue, and many appetite-related pathways remain supported mainly by cell and rodent studies. A recent meta-analysis in women with overweight or obesity estimated mean reductions of −1.23 kg in body weight and −3.46 cm in waist circumference, with intervention durations across the included trials typically ranging from 4 to 24 weeks, though most of the evidence derives from short- to medium-term interventions (generally ≤12 weeks); heterogeneity was moderate to high and the average weight effect remained below conventional clinical thresholds. Partially oxidized oolong tea and fully oxidized or post-fermented teas provide distinct profiles of caffeine, oxidized polyphenols, and microbial metabolites; their metabolic effects are promising but are less consistently tested in adequately powered human trials. Across tea types, convergent mechanisms include reduced digestive-enzyme activity, increased fatty-acid oxidation, modulation of thermogenesis, reinforcement of the intestinal barrier, and microbiota-dependent production of short-chain fatty acids and bile-acid signals. Translation is constrained by low systemic polyphenol exposure (i.e., limited bioavailability of intact catechins and their metabolites in circulation due to poor intestinal absorption, extensive phase-II metabolism, and rapid clearance), non-standardized doses and products, short intervention periods, interindividual variability, and limited direct comparisons among tea types. Available clinical data do not support tea as a primary treatment for obesity; rather, unsweetened tea or standardized preparations may serve as adjuncts to evidence-based dietary, physical activity, behavioral, and medical management. Priority areas include physiologically relevant dosing, standardized reporting of brewed-tea composition, long-term trials, and microbiome-informed personalization.