In developed countries, the percentage of adults aged ≥65 continues to increase, as does the number of people with age-related dementia. In the U.S., the number of people aged ≥65 with Alzheimer’s disease (AD) is projected to reach 7.2 million by 2025, which would be a 16% increase from 2021’s 6.2 million.
Stroke, another major cause of disability, shares some modifiable risk factors with dementia, and an estimated 90% of strokes and 35% of dementia cases are preventable. Furthermore, because a stroke doubles the risk of developing dementia and because strokes are more common than dementia, more than a third of dementia cases could be prevented by preventing strokes. These data, combined with the limited efficacy of the drugs used to treat dementia, have led to a growing interest in lifestyle strategies to prevent stroke and dementia.
Coffee and tea are among the most widely consumed beverages in the world. Observational studies have reported inverse associations between coffee or tea and the incidence of stroke and dementia, but what if you drink both?
This prospective cohort study looked for associations between (i) coffee and/or tea and (ii) the risks of stroke and dementia. It followed 365,682 adults aged 50–74 (at baseline) over a median 11.4 years. Coffee and tea intakes were self-reported at baseline.
The primary outcomes were incident stroke (and its two major component endpoints: ischemic stroke and hemorrhagic stroke) and dementia (and its two major component endpoints: AD and vascular dementia). The analyses were adjusted for a variety of potential confounders, including age, sex, smoking status, physical activity, BMI, disease history (e.g., hypertension, diabetes), and dietary factors (e.g., intakes of whole grain and processed red meat).
Over the course of the study, there were 10,053 cases of stroke (2.8% of the participants) and 5,079 cases of dementia (1.4%). The following associations were found:
Risk of stroke:
Coffee (0.5 to ≥4 cups/day): −8% to −12%
Tea (2–3 and ≥4 cups/day): −16%
Risk of ischemic stroke:
Coffee (0.5–3 cups/day): −12% to −13%
Tea (2–3 and ≥4 cups/day): −15%
Risk of dementia:
Coffee (2–3 cups/day): −7%
Tea (≥4 cups/day): −11%
Risk of vascular dementia:
Coffee (≥4 cups/day): −22%
Tea (2–3 and ≥4 cups/day): −25% to −26%
Compared to drinking only coffee or only tea, drinking both was associated with lower risks of stroke (−11%), ischemic stroke (−11%), dementia (−8%), and vascular dementia (−18%).
This study had several limitations, of which we’ll mention three:
Coffee and tea intakes were self-reported. This is a (common) problem, because people often misjudge their own intakes.
Coffee and tea intakes were reported only at baseline. Changes over time were not accounted for.
Since it was an observational study, its results were possibly influenced by unmeasured or residual confounders, such as sleep habits or genetic predisposition.
The big picture
Many of the health benefits of coffee and tea can be attributed to the antioxidant and anti-inflammatory properties of their polyphenols. Caffeine is a polyphenol present is both coffee and tea; in addition, coffee is rich in chlorogenic acid, trigonelline, diterpenes (notably cafestol and kahweol), and melanoidins, whereas tea is rich in catechins (notably EGCG). The catechin content per gram of dry weight is 150–200 mg for green tea and 40–60 mg for black tea. Green tea is unfermented tea, whereas black tea is fully fermented; as a result of fermentation, catechins combine to form theaflavins and thearubigins.
Oxidative stress plays a crucial role in the development of various diseases, including dementia and CVD. Polyphenols can mitigate oxidative stress by scavenging free radicals, which helps explain why coffee and tea have been associated with a lower risk of dementia and CVD. Polyphenols may also promote nitric oxide production and endothelial function, thus reducing stroke risk.
In line with the findings of the present study, a 2014 meta-analysis of prospective cohort studies linked a higher intake of tea to a lower risk of CVD mortality: compared to the lowest category of green-tea intake, the highest category was associated with a 33% lower risk of CVD mortality; similarly, compared to the lowest category of black-tea intake, the highest category was associated with a nonsignificant 12% lower risk of CVD mortality. The same meta-analysis found that each 375 mL increase in tea intake was associated with an 18% lower risk of stroke, with green tea having a greater effect than black tea. More recently, a 2020 cohort study of 487,377 Chinese adults associated daily green-tea intake with a lower risk of stroke.
Likewise, meta-analyses of prospective cohort studies reported that, compared to no coffee, 2.5 cups/day was associated with a 17% lower risk of CVD mortality, whereas 3.5 cups/day was associated with a 15% lower risk of incident CVD. Previously, a 2011 meta-analysis of prospective studies had found an association between 3–4 cups/day and a 17% lower risk of stroke.
The evidence is weaker for dementia. Some studies have found no association between coffee and dementia risk, while others have found a reduction in risk from 1–2 cups/day. For tea, the evidence is more positive, but it is also limited. Of two recent Japanese prospective cohort studies, one found a reduction in risk from ≥500 mL of green tea, whereas the other found a reduction in risk from any amount of green tea, but not from black tea. In contrast, a more recent German prospective cohort study found no association between green tea and AD risk; however, 68% of the participants didn’t drink green tea, and only 6.4% drank some daily.
The few studies that looked at both coffee and tea suggest that drinking both is better than drinking either for reducing the risk of CVD, the risk of stroke, and the risk of all-cause mortality. This may be due to different polyphenols activating different patterns of cell-protective genes, which is also one of the reasons eating a variety of fruits and vegetables is beneficial.
A mild biological stress response follows the ingestion of polyphenols (be they from coffee, tea, fruits, or vegetables). Many pathways get activated to increase the expression of hundreds of cell-protective genes, stimulating antioxidant and anti-inflammatory activity, mitochondrial biogenesis, DNA repair, autophagy, and cell rejuvenation or apoptosis. The pathways involved appear to be centrally regulated by the nuclear factor erythroid 2-related factor-2 (NRf2) system.
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