Originating in China, the consumption and planting of tea (Camellia sinensis, Theaceae) has spread in popularity, with more than 160 countries in the world today being accustomed to tea drinking. This makes the drinking of tea one of the most widely consumed beverages globally. Tea is one of the most popular and cost effective beverages consumed after water, renowned for its health promoting properties since ancient times. Black (aerated), green (non-aerated) and oolong (partially aerated) tea are three of the most popular tea forms consumed today, all of which are produced from the leaves of the plant Camellia sinensis. Alongside this, the market and evidence-based for tea forms other than black and green tea is growing, including chamomile, sour, hibiscus, goishi and rooibos tea.
So, what is cardiovascular disease?
Cardiovascular Disease (CVD) encompasses a range of disorders of the heart and blood vessels. More people die annually from CVDs than from any other cause making this the number one cause of death globally.
Investigating the link between tea drinking and the reduction of CVD
There has been a significant upsurge in the number of scientific articles studying the role of tea in human health. A large body of observational evidence has made links between tea consumption and reduced risk of heart disease alongside other factors such as reduced stroke risk, particularly ischaemic stroke. A number of new randomised controlled trials (RCTs) have been published within this field in recent years. We collated the results of meta-analysis studies and RCTs published in the last 10 years and the effect of tea consumption on measures of CVD was considered.
- Findings from 19 meta-analysis and 23 randomised controlled trials (n=1,422 participants) were evaluated.
- Moderate evidence was of a positive effect of green tea and its associated catechins on total and low density lipoprotein (LDL) reduction.
- Growing evidence indicates that tea drinking could protect vascular health and reduce inflammation.
- A wider range of tea forms (chamomile tea, goishi tea, hibiscus tea, sour tea, rooibos tea) are increasingly being studied and also appear to have potentially favourable effects on markers of CVD.
- Regular consumption of black and green tea was associated with reductions in blood pressure which could be important for CV health at population level.
- Strong evidence from meta-analysis that long-term consumption (<12 weeks) and drinking 4-5 cups tea (green and black) daily could reduce systolic and diastolic blood pressure, especially those in pre- and hypertensive ranges.
- There is emerging meta-analytical evidence that sour tea may reduce systolic and diastolic blood pressure.
- Growing evidence from RCTs that drinking 2-3 cups black tea may also reduce systolic and diastolic blood pressure.
- Three further RCTs show that sour tea may reduce blood pressure, especially amongst type 2 diabetics.
- One RCT showed that brewed hibiscus tea daily significantly lowered blood pressure amongst pre- and mildly hypertensive adults.
Given the high prevalence of hypertension in the UK, and worldwide, these findings have important public health implications. A large proportion of the general population have blood pressure levels within the ranges included in trials, showing they are at increased risk of hypertension and hence these results are particularly applicable.
Cholesterol and lipids
- Strong meta-analytical evidence indicates that green tea and its catechins may have beneficial effects on total cholesterol and low-density lipoprotein (LDL) cholesterol reduction.
- Meta-analytical evidence for black tea is mixed with subjects with higher cardiovascular risk appearing to have most benefit.
- Longer-term RCA trials imply that black tea may reduce triglyceride levels, alter body fat distribution and increase fat excretion.
- There is emerging evidence from RCTs that chamomile and goshi tea could improve blood lipid profiles
- Strong meta-analytical evidence indicates that drinking green tea (1-3 cups daily) could reduce heart attacks and stroke risk.
- Other works show 2-3 cups of green or black tea daily could improve endothelial function.
- There is growing evidence from RCTs that black tea could improve flow-mediated dilation and levels of circulating angiogenic cells.
- The number of studies looking at tea in relation to inflammation was small but indicated a positive effect.
- Four RCTs show that patients with type 2 diabetics, coronary artery/ischemic heart disease or knee osteoarthritis could benefit from tea drinking (black, green and burdock root tested).
- Further studies on healthy populations are needed to better understand how tea drinking could affect inflammation.
So, should we be drinking more tea?
Based on the evidence collated it seems plausible that tea consumption could be recommended to the general population or to patients as a strategy to reduce cardiovascular risk. These benefits largely appear to be attributed to the synergistic effects of tea phenolics coupled with its flavonoid elements.
- Moderate intakes of tea (black and green) appear to play a significant role in blood pressure reduction at around 4 to 5 servings per day, particularly amongst those with prehypertension or hypertension.
- Green tea seems to benefit the lowering of total and LDL cholesterol.
- Daily tea consumption also appears to have generic extended benefits including improved vascular function and reduced inflammation.
The different dosages of polyphenols used in trials could have contributed to variations in study findings. It would be worth deciphering ideal ‘optimal doses’ of polyphenols that could influence specific aspects of health such as cardiovascular wellbeing. Tea polysaccharides (TPS), a group of heteropolysaccharides bound with protein are another component worthy of further investigation. TPS are thought to possess bioactive antioxidant and anti-inflammatory properties with could influence measures of CVD.
Finally, it should also be considered that whilst many biologically active compounds in tea have been identified many unidentified compounds remain which could also possess bioactive properties.
Etheridge C, Bond T & Derbyshire EJ (2018) Effects of Tea Consumption on Measures of Cardiovascular Disease: A Systematic Review of Meta-Analysis Studies and Randomised Controlled Trials. Journal of Nutrition & Food Sciences 8:5.