Tea and Cortisol: What Science Really Shows When You Look Beyond the Claims

“Tea lowers cortisol.”
It’s an appealing idea: simple, comforting, reassuring. Yet as soon as we look more closely at the biology, the story becomes more nuanced — and far more interesting.

An essential hormone, sitting just above the kidneys

Cortisol is produced by two small, discreet glands — the adrenal glands — located on top of each kidney. Despite its reputation as a “stress hormone,” cortisol is not inherently harmful. In situations of acute stress, it actually plays a protective role:

• mobilising energy rapidly,

• supporting alertness,

• modulating inflammation,

• helping the body cope with demanding situations.

After the stressful event, cortisol levels normally decrease. That’s how the system is meant to work.

Stress does not have a single face

1. Acute stress: a normal response

An exam, public speaking, an unexpected event: cortisol rises briefly, helps us cope, then returns to baseline.

2. Allostatic load: repeated stress

When several acute stressors accumulate within the same day, with little or no recovery time, cortisol does not fully return to baseline.

Signs of wear begin to appear: lighter sleep, irritability, persistent fatigue. This is not yet chronic stress, but the system starts to lose efficiency.

3. Chronic stress: long-term dysregulation

When this pattern persists for weeks or months, cortisol rhythms become irregular — sometimes too high, sometimes poorly synchronised. Possible consequences include:

• ongoing fatigue,

• disruption of the sleep–wake cycle,

• abdominal weight gain,

• weakened immune responses.

It is within this complex context that tea is often presented as a “natural regulator.” But what do the data actually show?

What studies really measure

Most human studies focus exclusively on acute stress, using highly controlled protocols:
30 to 40 participants (sometimes fewer), short interventions (a single dose or a few weeks), an experimentally induced stressor (mental task, public speaking), and immediate measurement of salivary cortisol and perceived anxiety.

Under these conditions, black tea or a beverage providing 200 mg of L-theanine may slightly attenuate the cortisol response to acute stress.

However, these effects appear modest and tell us nothing about chronic stress or allostatic load. These studies do not follow participants long term, nor do they assess circadian rhythms or immune markers.

And what about L-theanine?

Many trials use a dose of 200 mg of L-theanine.
A standard cup of tea contains about 10–20 mg, meaning roughly ten cups consumed at once — a level that may approach or even exceed the World Health Organization’s recommended upper daily intake for caffeine (≈ 400 mg/day for adults), depending on the tea and brewing method.

The proposed mechanisms remain hypothetical (modulation of certain neural pathways, possible influence on stress axes), with no robust evidence supporting a sustained, real-world effect on everyday cortisol regulation.

In short

Tea remains an excellent pause ritual, but not a hormonal correction tool. And that is precisely what makes the topic worth explaining properly.

Key takeaways

• Cortisol helps us respond to acute stress (energy, alertness).
• Acute stress ≠ repeated stress ≠ chronic stress.
• Most studies only test acute, laboratory stress.
• Black tea or 200 mg L-theanine: slight effect on the cortisol spike.
• 200 mg ≈ 10 cups of tea, close to WHO caffeine limits.
• No evidence yet for effects on chronic stress or allostatic load.

Key figures – Tea & cortisol

• 30–40: typical sample size in human trials.
• 200 mg: L-theanine dose usually tested.
• 10–20 mg: L-theanine per cup of tea.
• ≈ 10 cups to reach 200 mg.
• ≈ 400 mg/day: recommended caffeine upper limit (WHO).

What we know / What we don’t know

• Possible small effect on acute stress responses.
• Tested doses ≠ normal everyday tea drinking.
• Unknown impact on repeated stress and chronic stress.

Bibliography

1. Steptoe, A. et al. The effects of tea on psychophysiological stress responsivity and post-stress recovery: a randomised double-blind trial. Psychopharmacology, 2007; 190(1): 81-89.

2. Hidese, S., Ogawa, S., ocia et al. Effects of L-theanine administration on stress-related symptoms and cognitive functions in healthy adults: a randomized placebo-controlled study. Nutrients, 2019; 11(10): 2362.

3. White, D.J. et al. Anti-stress, behavioural and magnetoencephalography effects of an L-theanine-based nutrient drink: a randomized, placebo-controlled study. Nutrients, 2016; 8(1): 53.

4. Kimura, K. et al. L-Theanine reduces psychological and physiological stress responses. Biological Psychology, 2007; 74(1): 39-45.

5. McEwen, B.S. Protective and damaging effects of stress mediators: allostasis and allostatic load. New England Journal of Medicine, 1998; 338(3): 171-179.

6. World Health Organization. Caffeine intake and health recommendations. WHO Guidelines, 2021.