The Science of Caffeine in Tea: Effects, Metabolism, and Limits

1. Caffeine Content in Tea and Coffee
Caffeine in tea is primarily concentrated in the young shoots of the plant, making up about 2%-4% of the dry weight. For example, a cup of green tea brewed with 3 grams of leaves contains approximately 60-120 mg of caffeine. (Tea made from older leaves, such as those used for black or oolong tea, typically has lower caffeine content per gram.)
Since caffeine is more concentrated in younger leaves, green tea, which is made from tender shoots, generally contains more caffeine, explaining its energizing effects. Caffeine is absorbed by the body within about 45 minutes of consumption, and its stimulating effects are typically felt quickly.
(Note: Coffee contains 1%-2% caffeine by weight, but since more coffee grounds are used per cup compared to tea leaves, a typical cup of coffee can contain 100-200 mg of caffeine.)

2. Recommended Daily Caffeine Limits
For most adults, it’s recommended to limit daily caffeine intake to no more than 400 mg.
Can children drink tea? Children as young as six months old metabolize caffeine similarly to adults. However, many health organizations, including pediatric associations, recommend that children consume as little caffeine as possible. For example, Health Canada advises that children under 12 should consume no more than 2.5 mg of caffeine per kilogram of body weight each day.

3. How Caffeine Boosts Alertness
Caffeine (1,3,7-trimethylxanthine) shares a similar molecular structure with adenosine, a compound that naturally occurs in the brain. Both molecules are small enough to cross the blood-brain barrier and affect the synapses between neurons. Adenosine binds to specific receptors in the brain, promoting relaxation and sleep. Caffeine, however, competes for these receptors, blocking adenosine’s calming effects and stimulating the brain instead. As a result, caffeine enhances alertness and improves mental clarity.

4. Individual Differences in Caffeine Metabolism
Caffeine is primarily metabolized in the liver by the enzyme CYP1A2. However, the genetic variations in the CYP1A2 gene mean that caffeine metabolism varies from person to person. Individuals who metabolize caffeine quickly tend to experience fewer side effects, such as sleep disturbances or nervousness, while those with a slower metabolism may find caffeine stays in their system longer, potentially causing insomnia.
This explains why the idea that frequent tea or coffee drinkers become “immune” to caffeine is inaccurate. Caffeine metabolism is genetically determined. For instance, despite drinking tea regularly for years, I still experience insomnia if I consume it late in the evening.

Conclusion
Caffeine can improve alertness and work efficiency, but it’s important to understand how your body processes it. If you have a slower caffeine metabolism, avoid consuming tea or coffee after 4 PM, especially in the evening. If you accidentally consume too much caffeine, eating food and engaging in physical activity can help mitigate its effects and reduce the risk of insomnia.