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Nutrition and daily life

What tea contains and the evidence: what we know and what we don’t

We hear many claims about tea: that it protects against cancer or boosts immunity. This section explains in plain words what tea contains and what science has actually shown.

Last updated: ~1 min read
A glowing tea leaf above amber tea.
Figure 1.What is in a tea leaf?

01Short answer

Tea contains water, caffeine, polyphenols (catechins, theaflavins), L-theanine, tannins and small amounts of minerals. In lab and animal studies polyphenols have shown interesting effects, but human studies have not shown that tea prevents or treats cancer. Tea is a pleasant drink and a source of fluid, not a treatment. Extracts are very different from a cup of tea.

02Simple explanation

Tea is water, caffeine and polyphenols.

A lab result is not proof in people.

Tea does not treat disease.

An extract is not a cup of tea.

03Scientific explanation

Levels of evidence: cell and animal studies give ideas; observational human studies show links; only randomised trials can prove cause.

NCI: it has not been shown in people that tea prevents or treats cancer.

MedlinePlus: about 400 mg of caffeine a day is generally considered safe for adults; during treatment watch your sleep and heart rate.

04What happens in the body?

Composition

A glass of black tea has about 40–70 mg of caffeine, polyphenols and almost no calories (without sugar).

A flask in shadow on the left, a warm cup of tea on the right.
Figure 2.The lab and real life differ.

How to read adverts

"Worked on cells" does not mean "works in people". Ask for the source and type of study.

Untitled journals and tea on a desk.
Figure 3.Evidence comes from studies.

Safe brewing

Freshly boiled water, a clean teapot and a fresh brew. During neutropenia avoid cold tea that has stood for hours.

Iron and medicines

Drink tea about an hour after meals; do not take extract capsules without your doctor.

05Brewing: four steps

Sealed pack, fresh brew, right temperature, drink hot.

Sealed pack — Dry place.1Sealed packDry place.
Fresh brew — Not old cold tea.2Fresh brewNot old cold tea.
Right water — Boiled.3Right waterBoiled.
Drink soon — Do not keep.4Drink soonDo not keep.
FlavourTradition.
FluidsA warm drink.
SafetyBoiled water.

06Key points at a glance

TopicIn brief
CompositionA glass of black tea has about 40–70 mg of caffeine, polyphenols and almost no calories (without sugar).
How to read adverts"Worked on cells" does not mean "works in people". Ask for the source and type of study.
Safe brewingFreshly boiled water, a clean teapot and a fresh brew. During neutropenia avoid cold tea that has stood for hours.
Iron and medicinesDrink tea about an hour after meals; do not take extract capsules without your doctor.

07Key terms

L-theanine
An amino acid found in tea.
Randomised trial
The most reliable study type for testing cause.
Caffeine
A stimulant.
Polyphenols
Antioxidant plant compounds.
Neutropenia
Low neutrophils.

08Key points

  • Tea’s composition is known.
  • Lab evidence is not enough.
  • Tea is not a treatment.
  • Brew with boiled water.
  • Capsules only with your doctor.

09Frequently asked questions

Does tea boost chemotherapy?

No; some extracts may even weaken it (see “Tea and medicines: when caution is needed”).

Which tea is healthiest?

The one you enjoy, in moderation.

Does this tea treat leukemia?

No. Tea is a pleasant drink, not a treatment.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. Does tea boost chemotherapy?

  2. Which tea is healthiest?

  3. Does this tea treat leukemia?

My questions

12Sources

  1. National Center for Complementary and Integrative Health (NIH). Green Tea
  2. Memorial Sloan Kettering Cancer Center — About Herbs. Green Tea
  3. MedlinePlus. Caffeine
  4. Leukemia & Lymphoma Society. Diet Guidelines for Immunosuppressed Patients
  5. National Cancer Institute. Diet and Cancer Risk
  6. Memorial Sloan Kettering Cancer Center — About Herbs. Nigella sativa
  7. Memorial Sloan Kettering Cancer Center — About Herbs. Resveratrol
  8. National Cancer Institute. Laetrile/Amygdalin (PDQ®)–Patient Version
  9. Centers for Disease Control and Prevention. Listeria (Listeriosis): People at Risk
  10. U.S. FDA. Talking About Juice Safety: What You Need to Know
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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