With more than three billion cups drunk daily worldwide, tea endures as one of the most ancient and cherished beverages.
It is also a great source of interest for scientists, but with a plethora of seemingly contradictory research, tea lovers may be left confused about whether that daily cuppa is doing more harm than good.
Marking the UN's International Tea Day, The National draws from expert sources to explore its diverse health benefits and potential risks.
Unravelling the health benefits: When and how is tea good for you?
Anti-clotting properties
A College of Life Science and Biotechnology study published in the journal Biomedicine & Pharmacotherapy found the tea plant contains a natural compound, which may have anti-clotting effects.
Dr Tim Bond from the Tea Advisory Panel told The National: “The scientists found that the newly discovered tea compound, called cystatin, helps to thin the blood naturally by relaxing blood vessels and inhibiting platelets from sticking together.
“This is significant because clotting, or thrombosis, is a major risk factor for heart attacks and strokes. Both black and green tea leaves are derived from the tea plant, which means that this natural compound could potentially be found in both types of tea.”
Blood pressure
A 2013 review published in the European Journal of Nutrition suggested daily consumption of green or black tea could potentially lower cholesterol and blood pressure. However, its authors said longer-term trials were necessary for confirmation.
A data analysis a year later revealed a minor decrease in blood pressure among hypertensive people who consumed green tea. This study, published in Scientific Reports, analysed 13 randomised controlled trials. However, it noted the exact effect on heart problem or stroke risk remains unclear.
A noteworthy reduction in LDL cholesterol levels due to green tea consumption was reported in the American Journal of Clinical Nutrition.
Heart health
Numerous studies have underscored the potential heart health benefits of tea consumption, particularly green tea. Evidence suggests that tea can help mitigate the risk of heart disease and stroke, a finding supported by Penn Medicine at the University of Pennsylvania.
Research has shown an association between tea consumption and a decreased risk of atherosclerotic cardiovascular disease and death related to it. This was outlined in the China-PAR project, published in the European Journal of Preventive Cardiology.
The British Heart Foundation has also referenced studies indicating catechins, compounds found in green tea, could be beneficial for heart health. A review in 2011 in the Cochrane Database of Systematic Reviews found a slight reduction in cholesterol associated with green tea consumption.
High consumption of black tea is linked with a 10 per cent reduction in the risk of all-cause mortality, according to one study. A daily increment of one cup of black tea is associated with a 3 per cent lower risk of all-cause mortality. However, the relationship between black tea consumption and all-cause mortality is found to be U-shaped, indicating complex, non-linear associations.
Additionally, Harvard School of Public Health highlights tea's high flavonoid content as a significant factor contributing to its cardiovascular benefits.
A comprehensive review conducted by the University of L'Aquila, Italy, and published in the journal Nutrients, found that tea flavonoids have numerous beneficial effects on the cardiovascular system.
These include improving endothelial function (the inner lining of blood vessels), reducing inflammation and oxidative stress, lowering blood pressure, and reducing LDL, or bad, cholesterol. All these factors are often associated with heart disease and establish flavonoids as significant contributors to cardiovascular health.
Cancer prevention
The relationship between tea consumption, particularly green tea, and the risk of various types of cancer has been the subject of numerous studies. Although the findings have shown promise, most of the existing research is observational and based on correlations.
In terms of specific types of cancer, the following have been studied:
Breast cancer research has revealed some interesting findings. Meta-analysis published in the International Journal of Cancer in 2017 indicated a protective effect of green tea on breast cancer risk. This effect was particularly pronounced in women who consumed more than three cups of green tea daily. The effect was not observed with black tea.
Turning to ovarian cancer, a population-based study in Sweden and published in the Archives of Internal Medicine found that women who consumed two or more cups a day had a 46 per cent lower risk of ovarian cancer compared to those who didn't drink tea.
A decreased risk of liver cancer has also been linked to green tea consumption. Analysis published in Nutrition and Cancer in 2017 made the link. The study suggested that consuming an additional two cups of green tea daily could decrease the risk of liver cancer by 22 per cent.
For colorectal cancer, a meta-analysis of prospective cohort studies in 2014, published in Nutrition and Cancer, discovered a lower risk associated with increased green tea consumption.
In the realm of prostate cancer, a 2006 study by researchers at the University of Wisconsin suggested that green tea polyphenols, particularly EGCG, could inhibit the growth of prostate cancer cells.
Lung cancer risk appears to be inversely related to green tea consumption, according to a 2010 study by researchers at the University of Chicago published in Endocrinology. This study found that people who drank at least one cup of green tea a day had a lower risk of lung cancer.
Weight loss
Several studies have linked green tea to weight loss and management, largely owing to its fat-burning and fat-oxidising properties.
The Nutrition and Toxicology Research Institute Maastricht in the Netherlands carried out a study, published in the International Journal of Obesity in 2009. This research found that green tea extract, rich in catechins, resulted in a decrease in body weight and aided in maintaining body weight following a period of weight loss.
The institute also conducted a review in 2013, which found that the catechins or an epigallocatechin gallate (EGCG)-caffeine mixture present in green tea exhibited a modest positive impact on both weight loss and weight maintenance.
The Faculty of Medicine at the University of Geneva published a study in The American Journal of Clinical Nutrition in 1999. This research underscored that green tea has thermogenic, or fat-burning, properties and enhances fat oxidation, an effect that extends beyond what can be attributed to its caffeine content alone.
A 2011 study by the Department of Nutritional Sciences at the University of Connecticut revealed that EGCG could inhibit pancreatic lipase. This enzyme is responsible for breaking down dietary fats for absorption in the intestines. By inhibiting this enzyme, EGCG may reduce fat absorption, thus contributing to weight management.
Digestive health
Tea, particularly certain herbal varieties, has been used for centuries as a remedy for digestive ailments.
Ginger tea is known for its possible effects on digestive health. One study by scientists at the Chang Gung University College of Medicine in Taiwan found that ginger can speed up gastric emptying and relieve symptoms of indigestion.
Another study – published in Nutrition Journal – showed that ginger can help reduce nausea and vomiting, especially in pregnant women and those undergoing chemotherapy.
Peppermint tea has also been studied for its beneficial effects on the digestive system. One review – published in the Journal of Clinical Gastroenterology – highlighted that peppermint oil, which contains the active compounds found in peppermint tea, can relieve symptoms of irritable bowel syndrome, such as abdominal pain and bloating.
Relaxation and stress reduction
Tea contains an amino acid called L-theanine that can help promote relaxation and reduce stress levels. L-theanine has been shown to increase the production of alpha waves in the brain, which are associated with relaxation and calmness.
In 2022, a review of 33 different studies, conducted by a team of researchers from the Tea Advisory Panel and published in the journal Phytotherapy Research, found that drinking two or three cups of tea, including black tea, green tea and herbal teas such as German camomile, lavender, rose, jasmine, and passionflower, can help improve sleep quality and reduce stress.
Immune system support
Tea's ability to bolster the immune system is another notable benefit. Ashtead Hospital highlights that tea contains antioxidants, such as polyphenols and flavonoids, which can help protect the body from free radicals and oxidative stress. These antioxidants play a vital role in supporting the immune system and may even contribute to a reduced risk of developing certain chronic diseases, including cancer.
Penn Medicine says the polyphenols in tea can also help fight off harmful bacteria and viruses. Black tea, in particular, contains theaflavins and thearubigins, two types of antioxidants that may contribute to improved immunity, as noted by Prevention health magazine.
Understanding the risks: When can tea consumption be detrimental to your health?
While tea offers numerous health benefits, it is essential to consider the potential risks associated with its consumption.
Caffeine addiction
Most types of tea, including green, black, and oolong, naturally contain caffeine.
Moderate caffeine intake, around 400mg per day, is generally considered safe for most adults. However, consuming more than this can lead to symptoms of caffeine addiction.
The American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders, 5th Edition, includes a classification for caffeine withdrawal, characterised by symptoms such as irritability, fatigue, and depressed mood.
Penn Medicine says excessive caffeine intake can cause restlessness, insomnia, and rapid heart rate.
To avoid these effects, it's important to be aware of your caffeine intake, which includes monitoring the amount of tea you consume.
Dehydration
While tea can contribute to your daily fluid intake, it also has diuretic properties, particularly if it contains caffeine.
Research suggests that this effect may be overstated.
According to a study conducted by researchers at the School of Sport and Exercise Sciences, University of Birmingham and published in the British Journal of Nutrition, consuming up to 400mg of caffeine a day does not have a significant diuretic effect or lead to dehydration.
Ashtead Hospital – part of the Ramsay Health Care network, one of the largest private healthcare providers in the world – advises balancing tea consumption with other hydrating beverages to maintain optimal hydration levels.
Sleep quality
Caffeine is a natural stimulant found in tea, and its consumption can impact sleep quality.
A study conducted by researchers at the Henry Ford Hospital and Wayne State College of Medicine in Detroit – and published in the Journal of Clinical Sleep Medicine – found that consuming caffeine up to six hours before bedtime can significantly disrupt sleep.
A review by the Institute of Pharmacology & Toxicology at the University of Zurich, had similar findings.
The studies show that caffeine can delay the onset of sleep, decrease total sleep time and efficiency, and negatively impact perceived sleep quality. It tends to reduce slow-wave sleep and related brain activity, while increasing stage-1 sleep, wakefulness and sleep disruption.
If you enjoy tea in the evening, consider opting for decaffeinated varieties to help maintain good sleep hygiene.
Reduced iron absorption
Tea, particularly black tea, is known to contain polyphenols known as tannins. Tannins have a high affinity for binding with non-heme iron (plant-based iron found in whole grains, nuts, seeds and leafy greens), making it less available for absorption by the body.
According to a study by scientists at the Laboratory for Human Nutrition, Swiss Federal Institute of Technology Zurich, published in the American Journal of Clinical Nutrition, the consumption of tea with meals reduced the absorption of non-heme iron by 62 per cent.
This could be particularly concerning for those with iron deficiency or anaemia.
However, consuming foods rich in vitamin C with tea can counteract this effect, as it aids iron absorption.
Kidney stones
Tea contains oxalates, natural compounds found in many foods.
High concentrations of oxalates can promote the formation of kidney stones.
A study conducted by Brigham and Women's Hospital in Boston and published in the New England Journal of Medicine found that men who drank the most iced tea had a higher risk of developing kidney stones, which was attributed to its high oxalate content.
However, moderate consumption is generally safe for those who aren't predisposed to kidney stone formation.
Gastrointestinal problems
The tannins in tea can also cause digestive issues in some people. These compounds can increase stomach acidity, which can lead to nausea, vomiting, and diarrhoea.
A study published in the World Journal of Gastroenterology found that patients with irritable bowel syndrome reported symptoms worsening after the consumption of black tea.
Such effects vary greatly among individuals and are dose-dependent.
Allergies
While it is not common, some people can have allergic reactions to tea.
These reactions can range from mild symptoms like itching and hives to more severe ones like swelling and difficulty breathing.
A case report published in Allergology International highlighted a case where a patient experienced anaphylaxis after consuming green tea.
Anyone who suspects they might be allergic to tea should consult a healthcare professional and avoid consumption until it's safe.
Medication interactions
Tea can interact with certain medications, potentially decreasing their efficacy or increasing their side effects.
For instance, the caffeine in tea can interfere with the action of sedative medications, while the tannins can interfere with the absorption of certain antibiotics like tetracycline.
A US study published in Pharmacology suggested that high consumption of green tea – approximately 700ml per day – could affect the action of anticoagulant drugs, such as nadolol, a beta-blocker.
It's essential for people taking medication to discuss potential interactions with their healthcare provider before consuming tea.
Tea and health conditions: How its benefits and risks vary based on medical circumstances
Tea consumption can have varying effects on specific health conditions, such as diabetes, high blood pressure, and pregnancy.
Diabetes
Tea may have a positive impact on diabetes management.
According to the Academy of Nutrition and Dietetics, tea's polyphenols may help improve insulin sensitivity, potentially reducing the risk of developing type 2 diabetes.
However, it is essential to consider that some teas, especially bottled and pre-sweetened varieties, can contain added sugars, which can negatively impact blood sugar levels.
Opting for unsweetened, freshly brewed tea is the best choice for those with diabetes.
High blood pressure
Gunter Kuhnle, professor of nutrition and food science at the University of Reading, told The National: “Data show that flavonols found in tea, so called tea catechins, can improve blood lipids and reduce blood pressure, and thereby improve cardiovascular health.”
Tea, particularly green tea, may have beneficial effects on high blood pressure. The British Heart Foundation states that regular green tea consumption can help lower blood pressure, reducing the risk of heart disease and stroke.
However, it is important to note that its caffeine can cause a temporary increase in blood pressure for some. Monitoring caffeine intake and opting for decaffeinated varieties if needed can help manage blood pressure more effectively.
Pregnancy
Tea consumption during pregnancy should be approached with caution. While some studies suggest that moderate tea intake may be safe for pregnant women, excessive consumption can lead to negative effects.
Penn Medicine advises pregnant women to limit their caffeine intake, as high levels of caffeine can increase the risk of miscarriage and low birth weight.
Pregnant women should consult their healthcare provider to determine the appropriate amount of tea consumption during pregnancy.
In the UK and US, pregnant women are advised to have no more than 200mg of caffeine per day. Opting for caffeine-free alternatives, such as herbal teas, can also be a safer choice for expecting mothers.
A closer look at the different types of tea
Tea comes in various forms, each offering unique health benefits and possible risks.
Black tea
Black tea is the most commonly consumed tea globally, known for its strong flavour and dark colour. According to Prevention, black tea contains high levels of antioxidants, which may help reduce the risk of chronic diseases and improve heart health.
However, black tea also has the highest caffeine content among true teas, which can lead to potential risks such as caffeine addiction and sleep disturbances if consumed in excess.
Green tea
Green tea is known for its numerous health benefits, as it is rich in antioxidants called catechins. Cancer Research UK suggests that green tea may help lower the risk of certain cancers, while the British Heart Foundation highlights its potential to improve heart health and lower blood pressure.
Green tea has a lower caffeine content than black tea, but excessive consumption can still lead to risks, such as caffeine addiction or dehydration.
Matcha tea
Matcha is a superfood packed with antioxidants, including polyphenols, flavonoids and the catechin EGCG, which has potent disease-fighting properties and aids in weight loss.
With the highest antioxidant levels among all superfoods, Matcha surpasses others in the ORAC test, a measure of antioxidant capacity. It even outperforms antioxidant-rich foods like pomegranate and spinach, with an impressive score of 1384, exhibiting 13 times and more than 50 times the antioxidant power respectively.
White tea
White tea is the least processed of all tea varieties, which helps it retain high levels of antioxidants. MyNetDiary says white tea has a delicate flavour and may provide health benefits similar to green tea, such as improved cardiovascular health, cancer prevention, and weight management.
White tea also contains lower levels of caffeine than black and green teas, making it a more suitable option for those who are sensitive to caffeine or looking to reduce their caffeine intake.
However, it is essential to remember that white tea still contains some caffeine, and excessive consumption may lead to sleep disturbance and increased heart rate.
Oolong tea
Oolong tea is partially fermented, placing it somewhere between black and green tea in terms of oxidation and flavour.
Oolong tea contains antioxidants and is known for its potential benefits in weight management and improving heart health.
Some studies have also suggested that it may help improve brain function and reduce the risk of developing type 2 diabetes. Oolong tea contains caffeine, carrying the same risks as the other varieties.
Pu-erh tea
Pu-erh tea is a fermented and aged tea variety that originates in China. It has an earthy flavour and contains antioxidants and probiotics due to its fermentation process.
Pu-erh tea is believed to aid digestion, support weight loss, and promote heart health. As with other true teas, it contains caffeine.
Herbal Teas
Herbal teas are not true teas, as they are made from plants, fruits, and spices other than the Camellia sinensis plant.
Herbal teas come in many flavours, and their health benefits vary depending on the ingredients used.
Dr Bond said: “A new study examining several women’s health issues finds compelling evidence that drinking herbal teas – German camomile, rosehip and spearmint – can boost sleep quality and in turn help memory function and low mood”.
Some common herbal teas and their benefits include:
- Camomile tea: Known for its calming properties, camomile tea may help reduce anxiety, promote sleep, and alleviate digestive issues. Camomile tea has been linked to miscarriage and premature labour, so it is essential to consult with your doctor before consuming it in significant or medicinal quantities during pregnancy.
- Peppermint tea: This tea is often used to soothe an upset stomach, reduce bloating, and relieve muscle spasms.
- Ginger tea: Known for its anti-inflammatory and antioxidant properties, which may help with digestion, nausea, and reducing pain.
- Hibiscus tea: Contains antioxidants and may help lower blood pressure and support liver health.
- Rooibos tea: Rich in antioxidants and minerals and may help improve heart health, support digestion, and reduce inflammation.
Herbal teas generally do not contain caffeine, making them an option for those who are sensitive to caffeine or looking for a caffeine-free alternative.
It is important to be aware of the potential for allergies or interactions with medications when consuming herbal teas, as some herbs can cause adverse effects or interfere with the efficacy of certain medications.
Pregnant and breastfeeding women should also consult their healthcare provider before consuming herbal teas, as some herbs may not be safe during pregnancy or while nursing. For example, camomile tea is
In general, it is crucial to research and understand the specific herbal tea you plan to consume and use it responsibly to enjoy the potential health benefits safely.
Continuing research into the health benefits of tea
According to a study published in the Annals of Internal Medicine in 2022, regular tea drinkers were found to have a mortality risk that was 9-13 per cent lower compared to those who do not consume tea. Additionally, the study revealed that they lived, on average, 18 months longer.
Dr Bond said: “Ongoing research is looking at the impact of tea on gut health (as a prebiotic) and hence its potential value in conditions associated with use of probiotics such as immune function, inflammation, brain health and eye health.”
The art of brewing tea for optimal health benefits
Tea consumption offers many health benefits. It is crucial to consider individual health conditions and requirements when choosing which kind to drink.
To extract the beneficial compounds from the tea bag or tea leaves, it is recommended to always brew your tea bag well before adding milk. Dr Bond said: “Brew your tea bag for about five minutes to allow the hot water to extract the beneficial compounds from the tea bag or tea leaves. Then add milk.
“In fact, brew all types of tea for five minutes to extract the health-giving ingredients and get the tea to a comfortable temperature to drink.”
For optimal health benefits, it is recommended to consume tea in moderation and take extra advice if you are pregnant or breastfeeding.
Tea can be a valuable addition to a healthy lifestyle when consumed responsibly and with consideration for individual health needs. By understanding the different types of tea and their associated benefits and risks, drinkers can make informed choices about tea consumption and enjoy the versatile beverage as part of a balanced diet.
As we've steeped ourselves in the world of tea, remember that every cup tells a story of tradition, labour, and sustainability.
However, if you're more inclined towards the aromatic notes of coffee, don't fret – we've got something brewing for you too!
