Caffeine in tea and other stimulants

Recently I have found numerous articles and been asked about the levels of caffeine in tea. This is most likely due to the general consensus that caffeine is bad for you. In this blog I am going to debunk some of the myths of caffeine levels in tea.

Owing to an ancient misconception that the active constituent in tea and coffee is different, caffeine is called “theine” when found in tea. Originally, theophylline was thought to be the active constituent but is actually only found in trace amounts. There are four stimulants found in tea: caffeine, theobromine, theophylline and the amino acid L-theanine (in respective order below).

Theine and caffeine are the same alkaloid, and were recognised as being identical in 1838. The caffeine in tea is nonetheless distinguishable from that found in coffee because it forms different bonds with other substances, which in turn change how it affects the body. When tea leaves are infused, the caffeine combines with tannins, which attenuate and stabilise the caffeine’s effects on the body. Tannins prevent caffeine being released too rapidly, so it can be absorbed over a long period of time; the effect is therefore more regular, as well as lasting longer. In tea, caffeine stimulates the central nervous system and the cardiovascular system by enlarging the diameter of the vessels in the cerebral cortex. On the other hand, when ingested with coffee, caffeine has a direct effect on blood circulation through the coronary system, stimulating an acceleration of the heart rate. In other words, tea is more of a stimulant than an excitant. It sharpens the mind, increases concentration, eliminates fatigue and enhances intellectual acuity. For many the effects of caffeine in tea are much milder and more harmonious with their metabolism than the caffeine in coffee.

Both theobromine and theophylline are structurally related to caffeine and belong to a class of organic compounds called xanthines that have two, rather than three, methyl groups.

Theobromine is considerably weaker than its related molecules, as it has about one tenth the stimulating effect of either. It can be found in cocoa products in large quantities, kola nuts and tea in small amounts; however, it is not found in coffee, despite various claims. Dogs metabolise theobromine more slowly than humans and this may cause chocolate products to be toxic or lethal to them, by affecting the heart, central nervous system and kidneys.  Theobromine can stimulate the heart, but it does have a mild diuretic effect (it stimulates the bladder to remove water) and improves blood flow around the body, leading to a net reduction in blood pressure.

Theophylline has a stronger effect on the heart and breathing than either related molecule, including caffeine. The molecule stimulates both the rate and force of the heart’s contraction and for this reason it is considered more toxic, less speedy, as well as more diuretic than caffeine. Owing to the fact that theophylline can relax the smooth muscles in the airway, thereby making breathing easier, it is often used in home remedies for treating asthma, bronchitis and emphysema. In medicine the theophylline found is from tea or coffee extract.

Theobromine and theophylline are present only in trace amounts in a cup of tea, so their net effect on the body is negligible.

L-theanine is a unique non-essential amino acid found almost exclusively in the tea plant (Camellia sinensis) and has a structural similarity to glutamine. An amino acid is a simple organic compound and is the building block for proteins, whilst non-essential are amino acids that can be produced by converting other amino acids into it. Both neurotransmitters that are produced from the amino acid L-theanine (GABA and glutamate, which both mediate brain activation by counteracting one another) are able to cross the blood brain barrier following oral ingestion (taken through the mouth). Alpha waves are associated with alert relaxation and the L-theanine found in tea increases the generation of alpha waves in humans. Therefore, L-theanine is known as a relaxant without being a sedative, unlike lemon balm which relaxes but may also sedate.

The mass of caffeine found in various beverages differs due to the nature of the plants, including where they are grown, as well as how they are processed. For these reasons, tea from the Camellia sinensis plant has a wide range of caffeine levels. In general, the less processed the tea leaf, the lower the level of caffeine. This is because a tea leaf has more caffeine than a coffee bean before it has been processed; however, a cup of tea uses less material with lots of caffeine trapped in the leaves. The figures below are a guide; they vary from product to product, as well as how they are brewed. The hotter the temperature, the more caffeine is leached from the tea leaves into the drink.

Caffeinated beverage Average mg of caffeine per 236ml or 8fl.oz of liquid
Coffee brewed from grounds

94.8

Instant coffee

57.0

Hot chocolate

5.1

Black tea

47.4

Earl Grey tea caffeine levels are the same as the base black tea used, because Earl Grey is simply black tea with added bergamot oil, and bergamot oil does not contain caffeine.
Oolong tea

37.0

Pu’erh tea

41.6

Green tea has caffeine levels unique to the way the green tea is processed and grown. General green teas (those labelled as such on supermarket shelves) contains 25.0mg of caffeine.
Sencha green tea

46.0

Matcha green tea

70.0

White tea

28.0

Herbal teas (tisanes), including Rooibos, contain zero caffeine, with the exception of Yerba Mate, which contains 85.0mg of caffeine.

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