Bioactive Compounds in Camellia sinensis Tea and Herbal Infusions: A Critical Review of Health Evidence and Functional Applications

Abstract

Tea prepared from Camellia sinensis and infusions made from other botanicals are widely consumed as foods, yet their health claims often rest on evidence generated with chemically dissimilar beverages, concentrated extracts or isolated compounds. This critical narrative review integrates the chemistry, human health evidence, bioavailability, safety and functional-food applications of C. sinensis tea with selected herbal infusions for which human or translational evidence is available. Literature published from 1 January 1990 to 5 June 2026 was identified through PubMed/MEDLINE, OpenAlex, the Directory of Open Access Journals and Crossref Metadata Search, supplemented by targeted scholarly web searching and citation verification. Green and black teas provide overlapping but processing-dependent profiles of catechins, theaflavins, caffeine and l-theanine. Their biological effects are conditioned by dose, infusion practice, food matrix, intestinal transformation and host metabolism. Human evidence is most consistent for small improvements in selected cardiovascular risk markers and for short-term attention-related effects when l-theanine and caffeine are combined. Associations with lower cardiometabolic events or mortality remain vulnerable to residual confounding, while cancer-prevention evidence is heterogeneous and does not justify therapeutic claims. Among herbal infusions, Hibiscus sabdariffa has the clearest clinical signal for blood-pressure reduction, whereas evidence for chamomile, passionflower, peppermint, spearmint and rooibos is narrower, preparation-dependent or based partly on non-infusion products. Encapsulation, nano-delivery and food incorporation can improve stability or apparent bioaccessibility, but technological optimisation does not establish clinical benefit and may change exposure and safety. The principal translational problem is therefore not a shortage of plausible mechanisms, but weak alignment among botanical identity, product characterisation, delivered dose, metabolite exposure and clinically meaningful outcomes. Future studies should prioritise authenticated materials, beverage-relevant comparators, pharmacokinetic–microbiome phenotyping, longer trials and active safety surveillance. Tea and selected herbal infusions can support health-oriented diets and functional products, but claims should remain species-, preparation- and outcome-specific.