Daily tea
Black or green tea. Small effects on blood pressure and glucose in the pooled data.
Order of magnitude
Two to three cups a day, unsweetened.
Acute tea intake does nothing to blood pressure; only long-term intake does, and the effect is small.
Effects in detail
Each row is a directed edge from our database, with range, latency, evidence grade and the condition under which it applies at all.
| Marker | Effect | Latency | Mode of action | Outcome | Evidence | Condition | Sources |
|---|---|---|---|---|---|---|---|
| Diastolic blood pressure | helps. Lowers -1.70 mmHg confidence interval of the estimate, not the range one person can expect Twenty-five randomised trials in 1476 subjects give a systolic reduction of 2.1 mmHg (95 % CI -2.9 to -1.2) and a diastolic reduction of 1.7 mmHg for long-term green tea, and smaller reductions for black tea. The offset for this dial is not on blood pressure at all: the same polyphenols cut non-haem iron absorption by 50 to 90 % when the tea is drunk with a meal, which helps someone with high ferritin and harms someone with low ferritin. | 12–12 weeks | chronic | unclear | Evidence grade A | applies regardless of the starting value | |
| Systolic blood pressure | helps. Lowers -2.10 mmHg confidence interval of the estimate, not the range one person can expect Twenty-five randomised trials in 1476 subjects give a systolic reduction of 2.1 mmHg (95 % CI -2.9 to -1.2) and a diastolic reduction of 1.7 mmHg for long-term green tea, and smaller reductions for black tea. The offset for this dial is not on blood pressure at all: the same polyphenols cut non-haem iron absorption by 50 to 90 % when the tea is drunk with a meal, which helps someone with high ferritin and harms someone with low ferritin. | 12–12 weeks | chronic | unclear | Evidence grade A | applies regardless of the starting value | |
| Fasting glucose | helps. Lowers Direction known, size not established A network meta-analysis of 27 randomised trials in 1898 participants, with fasting blood glucose as the primary endpoint, found that green tea may reduce it. Grade C: a subgroup of a subgroup, significant only in younger or Asian populations, and the lever as defined in this database is not green tea specifically. | not specified | chronic | unclear | Evidence grade C | applies regardless of the starting value |
What this measure costs
Counteracting effects of the same measure.
No counteracting effect is on file for this measure. That does not mean there is none, only that we found no defensible edge.
Sources
Full details of every study the rows above rely on.
Inhibition of non-haem iron absorption in man by polyphenolic-containing beverages
Hurrell et al. · British Journal of Nutrition · 1999 · Randomised trial
Effects of Coffee and Tea Consumption on Glucose Metabolism: A Systematic Review and Network Meta-Analysis
Kondo et al. · Nutrients · 2018 · Meta-analysis
Effects of tea intake on blood pressure: a meta-analysis of randomised controlled trials
Liu et al. · British Journal of Nutrition · 2014 · Meta-analysis