Tea (mixed).

Adding vitamin C to the same meal partly rescues the iron that tea tannins would otherwise lock up, because ascorbate keeps iron in the ferrous state that polyphenols bind less tightly. Lemon in tea has a real biochemical basis, not just a culinary one. It offsets rather than eliminates the interaction.

Theanine occurs naturally in Camellia sinensis and crosses the blood brain barrier, where it raises alpha wave activity and is consistently reported to smooth the edge off caffeine's arousal. This is why tea caffeine and coffee caffeine feel different at equal doses. Shade-grown teas such as gyokuro carry more theanine than sun-grown leaf.

A cup of brewed tea carries roughly a third to a half the caffeine of the same volume of coffee, and that adds directly to any caffeine taken separately. People counting caffeine intake routinely forget the tea. The relevant total is the daily sum across all sources, not per beverage.

Quercetin inhibits catechol-O-methyltransferase, the enzyme that methylates and clears catechins, so it can raise circulating EGCG levels when taken alongside tea. Both compounds also compete for the same conjugation enzymes. The direction is toward higher catechin exposure, which is relevant given that concentrated green tea extract has a documented liver injury signal at high doses.

Casein forms complexes with galloylated catechins, which is why milky tea tastes less astringent. Whether this meaningfully lowers catechin absorption is genuinely contested, with studies pointing in both directions. What is not contested is that milk protein largely abolishes the tea tannin effect on iron, because the protein binds the polyphenols instead.

Brewed tea and a green tea extract capsule are not different ingredients, they are different doses of the same catechins. Stacking a high-dose extract on habitual tea drinking pushes total catechin exposure well past dietary levels. That distinction matters because the hepatotoxicity signal sits with concentrated extracts taken fasted, not with brewed tea.

EGCG inhibits dihydrofolate reductase in cell-free and cell culture work, the same enzyme that converts folate into its usable reduced forms. Whether brewed tea intake meaningfully shifts folate status in people is not established. It is a reason to keep the two apart in anyone already marginal on folate rather than a demonstrated deficiency risk.

COMT degrades dopamine, noradrenaline and adrenaline, and tea catechins inhibit it. Pairing catechins with a catecholamine precursor is the reasoning behind some stimulant-free energy formulas. The mechanism is real in vitro. The size of the effect at ordinary tea intake is not characterised.

Catechin absorption is low, in the single-digit percentage range, and food matrix strongly affects it. Lipid present in the same meal improves micellar solubilisation of the more lipophilic galloylated catechins. Taking a concentrated extract with food rather than fasted also matters for tolerability.