Catechins.

Catechins, and EGCG in particular, degrade quickly at the near-neutral pH of the small intestine. Ascorbate slows that autoxidation, and in vitro and animal absorption work has repeatedly shown higher recovery of intact catechin when ascorbate is present. Whether this changes any human outcome has not been established. The mechanism is a stability effect, not a transporter effect.

Catechins are rapidly methylated by COMT and pumped back into the lumen by MRP2 and P-glycoprotein, which is a large part of why plasma levels stay low. Quercetin acts on both of those routes in vitro and has raised catechin exposure in animal work. Human confirmation is thin. Both compounds also co-occur naturally in tea and fruit, so the pairing is not exotic.

Green tea delivers catechins and theanine in the same cup, and much of the human data on tea is data on that combination rather than on catechins alone. Theanine acts on glutamatergic and GABAergic signalling. Catechins act largely on redox and enzyme systems. They do not share a mechanism, and attributing a tea effect to either one alone overstates what the studies show.

Catechins inhibit COMT, which degrades noradrenaline, while caffeine antagonises adenosine receptors. The two converge on sympathetic tone from different directions, which is the standard explanation for why green tea extract trials use caffeinated preparations. Effect sizes on energy expenditure in humans are small and inconsistent between trials. The combination also stacks stimulant load, which matters for anyone sensitive to caffeine.

Ornithine feeds the urea cycle, which is the main route for ammonia disposal, while tea catechins have been proposed to affect exercise metabolism through separate routes. A 2025 trial in the European Journal of Applied Physiology examined acute and chronic co-supplementation against exercise-induced ammonia accumulation. This is a single small line of work and should be read as preliminary. The endpoint is a blood marker, not a performance outcome.

Long-chain polyunsaturated fats oxidise readily, and phenolic antioxidants slow that process in the oil phase and in the gut. This is mainly a formulation and stability argument rather than a demonstrated clinical pairing. It sits alongside, not instead of, the tocopherol normally added to fish oil products.

Once vitamin E quenches a lipid radical it becomes a tocopheroxyl radical and must be recycled to keep working. Water-soluble phenols positioned at the membrane interface can perform that reduction, which is the same role ascorbate plays. The chemistry is well described in model membrane systems. Extrapolating a measurable human antioxidant benefit from it is a longer step than the data supports.

Adding milk to tea measurably reduces the free, extractable catechin fraction because the proteins complex the polyphenol. Whether that changes net absorption in people is genuinely contested, with studies pointing both ways. Formulators should know the binding happens. Consumers should not assume it abolishes the catechin dose. The complex also blunts astringency, which is why the combination exists in food.

The same catechol chemistry that binds iron will bind other divalent metals to a lesser degree. Evidence for a meaningful effect on human zinc status from catechin intake is weak compared with the iron case. Separating high-dose catechin supplements from mineral supplements by a couple of hours is a low-cost precaution. Do not read it as a reason to avoid either.

Catechin preparations are usually standardised to EGCG content, so the two labels often describe the same material at different levels of specificity. Stacking a general catechin extract with a concentrated EGCG product raises the total EGCG dose without the label making that obvious. High cumulative EGCG intake from concentrated extracts taken fasted has been linked to liver enzyme changes in case reports, so total intake is worth tracking.