From heart health to cognition: What tea may do for the body

Tea, the world's second-most consumed beverage after water, has been linked to a range of potential health benefits, with decades of research pointing to the effects of antioxidants, polyphenols and other plant compounds found in tea.

Researchers have described tea as "Nature's gift for promoting human health", although studies have generally found associations rather than establishing that tea directly causes better health outcomes, says The Washington Post.

Green, black, white and oolong teas all come from the same plant, Camellia sinensis. Differences in flavour, colour and appearance are largely determined by how the leaves are aged and processed. Herbal teas such as chamomile, mint, hibiscus and ginger are made from herbs, flowers or spices rather than tea leaves and generally contain no caffeine.

Tea consumption and longevity

A 2023 study using data from the UK Biobank followed about half a million people aged 40 to 69 in Britain for more than a decade, examining their diets, beverage consumption, lifestyles and health conditions.

About 90% of tea drinkers in the study consumed black tea, while around 7% drank green tea. Researchers found that tea drinkers had lower mortality rates than people who rarely or never drank tea.

The strongest association was among people who drank two to three cups a day, who had a 13% lower risk of death during the study period than those who rarely or never drank tea. The association remained after accounting for coffee consumption.

A separate meta-analysis covering about 2 million adults in North America, Europe, Asia and Australia also found an association between tea consumption and lower mortality.

Researchers caution, however, that such studies cannot establish a direct cause-and-effect relationship. Although factors including alcohol consumption, physical activity, smoking and body mass index were considered, other lifestyle and genetic factors could also influence the results.

Potential cardiovascular benefits

Research has also linked regular tea consumption with improvements in cardiovascular health.

A clinical trial published in Circulation followed 66 adults with coronary artery disease who drank about three cups of black tea daily for four weeks and, during another phase, drank only hot water.

The study found that black tea consumption significantly improved vascular function, a change associated with increased levels of polyphenols in the bloodstream.

Other clinical trials have found that drinking either green or black tea regularly for several weeks or months can produce a modest reduction in blood pressure.

Metabolic and immune effects

Tea is not considered a way to prevent Type 2 diabetes, but clinical trials suggest it may influence some aspects of metabolic and immune function.

Small trials have found that black tea can reduce blood sugar spikes after meals. In another trial involving people with Type 2 diabetes, participants drank either three cups of black tea or three cups of hot water each day for three months.

Those drinking black tea showed an increase in regulatory T cells, immune cells that help control inflammation. Many participants in the black tea group also recorded lower levels of C-reactive protein, a marker associated with infection, injury and systemic inflammation.

Effects on the gut microbiome

Polyphenols in green and black tea may also affect the gut microbiome by encouraging the growth of beneficial bacteria.

Studies have found that black tea consumption can increase bacteria that produce short-chain fatty acids. These compounds help maintain the colon lining, reduce local inflammation and stimulate the release of GLP-1, a hormone involved in appetite regulation.

Cognition, alertness and stress

Tea also contains caffeine, which can improve attention and cognitive performance, but research suggests it may interfere with sleep less than coffee.

In a study of 30 healthy adults, participants consumed either four cups of coffee or four cups of black tea daily. Both groups showed similar improvements in cognitive performance and alertness, while those drinking tea experienced significantly fewer disruptions to their sleep at night.

A 2017 review of research on green tea concluded that it could help reduce anxiety and support memory and attention.

Black, green and white teas also naturally contain L-theanine, an amino acid that research has associated with reduced stress and improved mood.