Scientists agree that tea-drinking can have positive health effects, but the ‘mechanism of action’ remains a mystery. Why is tea so good for us?

Laboratory studies suggest a possible protective role against cardiovascular diseases (CVD) due to a reduction in platelet aggregation (blood thinning/anti-clotting) and a lowering of cholesterol. We don’t know if the same effect occurs in the human body, but by increasing blood flow to the brain, this might explain why black and oolong teas are thought to have a positive neurological effect, helping to improve memory and lower the risk of dementia.

European population-based epidemiological studies suggest that long-term consumption of black tea could be providing some protection against developing Type 2 diabetes (T2D), by reducing the levels of fasting glucose in the blood.

Black oolong is popular within Chinese populations, so this could be contributing to the low levels of CVD and T2D across Asia.

Scientists can’t say for sure why tea-drinking can have such positive health effects, but they have managed to identify a lot of bioactive chemical compounds in black teas that might have a role to play.

Tea infusions are rich in polyphenols – flavonoids (flavan-3-ols, proanthocyanidins, tannins), phenolic acids, non-proteic amino acids (gamma-aminobutyric acid and L-theanine) and alkaloids such as caffeine.

Flavonoids such as quercetin rutinoside are known to have an antioxidant effect – slowing the growth of some cancers, reducing the risk of cardiovascular disease, and producing a mild anti-depressant effect.

During the processing of black and oolong teas, the oxidation process converts the primary flavonoid (flavan-3-ols) to theoflavins. While we recognise theoflavins as a biologically important component of these black teas, little is known about them. Similarly, thearubigins are phenolic compounds responsible for the dark pigment in black teas, but they remain a mystery too.

Gamma-aminobutyric acid (GABA) is the most important inhibitory and regulatory neurotransmitter in the human central nervous system (CNS), responsible for sending chemical messages through the brain and nervous system to regulate nerve cell activity. GABA helps manage our body’s response to stress, fear and anxiety, our thought patterns, and behaviours. Low levels of GABA have been linked with depression.

L-theanine is important too, providing neuroprotection, mental relaxation, regulation of systemic blood pressure, and a reduction in the stimulatory effect of caffeine – and it crosses the blood-brain barrier (BBB) much more easily than GABA.

To gain the greatest health benefit, tea should be consumed in a fasting state to improve absorption of these compounds and their metabolites.

REFERENCES

• Crowe, T. (2018). Deakin University, Superfoods or Supermyths alumni seminar, Brisbane.
• da Silva Pinto, M. (2013). Tea: A new perspective on health benefits, 1–10.
• Kim, Y., Keogh, J. B., & Clifton, P. M. (2016). Polyphenols and Glycemic Control, 2471(December 2015).