Oolong Carried the Least of a Compound Class in a Five-Tea Study

The lede

A 2026 study measured advanced glycation end-products (AGEs), compounds tied to inflammation, across five tea types and found oolong lowest of the set, below green tea, white tea, black tea, and dark tea (Niu, Sun, Jiang and Zhu, 2026¹). The same study reports that high catechin content did not inhibit AGE formation, and that catechins may be one contributor to elevated AGEs.

What an AGE is

Advanced glycation end-products form when sugars react with proteins or lipids under heat, the Maillard reaction that browns a roast, and they also form inside the body over time. A 2016 systematic review of randomised controlled trials found that a diet high in AGEs raised circulating TNF-alpha and AGE levels in both healthy people and those with chronic disease. In healthy adults, such a diet promoted markers of oxidative stress; in diabetic patients, it raised cardiovascular markers (Clarke, Dordevic, Tan, Ryan and Coughlan, 2016²). The reviewers state that, for lack of high-quality randomised trials, more research is required.

What the study measured

The 2026 paper used ultra-high-performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) to profile 13 AGEs derived from lysine and arginine across green, white, oolong, black, and dark tea. Nine of the 13 were detected in every sample. The predominant class across all five was the arginine-derived methylglyoxal-hydroimidazolones, the MG-Hs. On total AGE content the ranking was statistically significant at P below 0.05: green tea and white tea highest and level with each other, then dark tea, then black tea, then oolong at the bottom.

The paper leaves oolong's rank without a stated cause.

The heat writes AGEs into the leaf

A 2020 study with Hongkai Zhu, a co-author of the 2026 comparison, dried black tea at 90°C (about 194°F), 120°C (about 248°F), and 150°C (about 302°F) for one hour and measured AGE formation directly (Zhang, Zhang, Ye and Zhu, 2020³). MG-H1/3 was the major AGE formed at the higher temperatures, reaching 39.66 micrograms per gram at 120°C and 58.88 micrograms per gram at 150°C, above CML, Nε-carboxymethyl-lysine, at the same settings, which sat at 19.86 and 23.71 micrograms per gram. Every AGE was significantly higher at 150°C than at 90°C or 120°C. Argpyrimidine appeared only at 150°C. The drying step is a real, measured source of these compounds, at least for black tea, at least for this class.