The science of tea

I've been lucky enough to work with many food categories over the past 20 years including tea, meat, eggs, fish, mycoprotein, fruit and veg, chocolate, cereal bars, breakfast cereals, fruit juice, protein drinks, oral nutrition supplements, sweeteners, yogurt, smoothies, dietary supplements, probiotics and plant stanol esters. This blog is all about tea.

Worldwide, it's the most popular drink after water yet many people still don't know that black and green teas come from the same plant, that green tea doesn't contain food dye or that tea is lower in caffeine than coffee. That's according to annual consumer surveys carried out by the Tea Advisory Panel.

However, people do know how tea makes them feel. Relaxed, energised, hug-in-a-mug, calm, focussed and destressed are some of the words used to describe the moment of sitting down with a cup of tea. But is there any physiological reason for this or is it all in the mind?

What's so special about tea?

Like several other plant products, including fruit, cocoa, olive oil and coffee, tea is bursting with bioactive compounds which have recognised effects on the body. A notable example is flavonoids – polyphenol compounds which have antimicrobial, anti-inflammatory and antioxidant properties.  Studies show that tea flavonoids – both from green and black teas – promote good vascular function explaining why tea consumption is associated with a reduced risk of cardiovascular diseases.

Tea also contains L-theanine, an amino acid, which has been shown to change brain activity according to studies using functional MRI. In one clinical trial, people were less stressed after completing intense computer tests when they were given L-theanine compared with a placebo.

Caffeine is a well-known psychoactive compound found in tea (as well as cocoa and coffee) and creates a mild stimulatory effect, boosting concentration and helping to reduce the perception of fatigue. This works in tandem with L-theanine to create a feeling of calm focus, according to intervention studies. At 40-50mg of caffeine per serving, tea contains around half the caffeine found in a cup of filter coffee. The safe upper level for caffeine is 400mg per day in Europe which equates to 8 cups of tea.

Finally, in teas grown in certain soils, fluoride is present with the highest concentrations seen in regular black tea and decaffeinated tea. Fluoride is taken up naturally from the soil and concentrates in the older leaves which is why more robust-tasting teas have a higher fluoride content. Fluoride, at levels found in tea, is beneficial for protecting teeth. A study in Nutrition Bulletin found that drinking 4-5 cups of tea daily was enough to meet European fluoride recommendations.

Tea tips

To get the most health benefits from tea, intakes should be 3-5 cups daily and we should leave the tea bag (or leaves) in the cup for 2-3 minutes to ensure maximum polyphenol levels. Caffeine content peaks at around 30 seconds so there is no advantage to dunking and removing the tea bag. Always make tea using fresh water as re-boiling affects the flavour. To save on energy (and costs!), only boil the water that you need.

Further reading

Ruxton & Hart (2011) Black tea is not significantly different from water in the maintenance of normal hydration in human subjects: results from a randomised controlled trial. Br J Nutr 106: 588-95. https://www.cambridge.org/core/journals/british-journal-of-nutrition/article/black-tea-is-not-significantly-different-from-water-in-the-maintenance-of-normal-hydration-in-human-subjects-results-from-a-randomised-controlled-trial/E5142A6CCACBB28C1018119B2CE2935D

Ruxton & Bond (2015) Fluoride content of retail tea bags and estimates of daily fluoride consumption from typical tea drinking in UK adults and children. Nutrition Bulletin 40: 268–278. https://onlinelibrary.wiley.com/doi/full/10.1111/nbu.12171

Mason & Bond (2021) Tea and Wellness throughout Life. Nutr Food Technol Open Access 7(1). https://www.sciforschenonline.org/journals/nutrition-food/NFTOA172.php