Under the Microscope: How does caffeine work?

Answered by: Professor Andrew Smith, Director, Centre for Occupational and Health Psychology, University of Cardiff

Sunday 23 October 2011 06:08


Caffeine – 1,3,7-trimethylxanthine, to give it its chemical name – is a member of a group of naturally occurring substances called methylxanthines.

These compounds are similar in structure to adenosines, naturally occurring molecules in our bodies which aid the onset of sleep. In its natural context, which is in tea and coffee plants, caffeine can kill or paralyse insects and is thus an effective natural pesticide.

The beginning of the buzz

The earliest recorded caffeine consumers were in China in the 10th century BC, when philosophers believed tea-drinking was "an indispensable ingredient to the elixir of life". Coffee-quaffing originated in Yemen in the 15th century. The exact amount of caffeine present in a drink depends on its growing conditions and preparation. While tea naturally has more caffeine gramme for gramme than coffee, there is less tea per cubic centimetre of cup, leading to its weaker stimulant properties. For the record, in a 5oz cup of filter coffee, there is between 100mg and 150mg of caffeine. The same sized serving of tea holds 35-45mg. Meanwhile, a 12oz serving of cola contains just 40mg. Doctors say at least 100mg is necessary to properly increase our alertness. (A study published by Bristol University last week argued that caffeine can't make irregular users more alert; a cup of coffee in the morning, the research suggested, only counteracts the effects of withdrawal that have built up overnight.)

How does it work?

Adenosine bonds to receptor cells in the brain to calm the activity of the central nervous system, thus triggering tiredness. There is also evidence to suggest that it decreases blood flow in the brain. Caffeine molecules bind to these receptor cells but have no active effect on the nervous system. However by doing so they take the place of adenosine molecules that could make a difference. This process is known as "competitive inhibition" and effectively delays the onset of fatigue, increases alertness and improves people's ability to sustain attention.


The amount of caffeine peaks in the bloodstream between 15 and 45 minutes after ingestion. Its half-life – the time it takes for its level to halve – is between five and six hours. In pregnant women the half-life can increase to 18 hours. This is because unborn babies can only metabolise it very slowly. Caffeine half-life in women on the pill is around 11 hours, and in smokers it decreases to three. Recent studies have linked smoking to high caffeine intake, citing this swift reduction in levels as one of the main reasons.

Regulating intake

Daily caffeine intake varies between cultures and countries. The average intake in the Western world is around 200mg a day. An excessive amount is around 500mg a day, which can cause health problems, such as anxiety. Different people have different sensitivities: for some even small amounts can cause adverse reactions. However, most people can control their caffeine consumption, limiting its interference with the natural process that tries to keep us awake. Caffeine intake is normally highest when alertness is reduced – early in the morning, after prolonged work or after lunch – and is reduced at times when high alertness is undesirable e.g. before going to sleep).

Potential ill effects

The effects of caffeine have been exhaustively studied. Over 2,000 articles have appeared in scientific journals the last two years alone. Research suggests that sensitive groups, such as children, could experience ill-effects from high-caffeine energy drinks (though much more research is needed to produce conclusive evidence). However, long-term, habitual consumption could lead to better mental functioning in old age. Indeed, ingesting significant quantities of caffeine may result in fewer errors, injuries and accidents at work and in leisure time. Researchers at London's School of Hygiene and Tropical Medicine published evidence earlier this month that showed caffeine helped improve workers' memory and concentration. For people working overnight, consuming caffeine had a similar effect to taking a power nap. Identical results were seen irrespective of whether those studied drank coffee, an energy drink, took a caffeine pill or ate food with a high caffeine content. And while the effects of caffeine on reproductive health and the metabolism – obesity, blood pressure, diabetes – have been investigated, there appears to be little evidence that moderate caffeine consumption can cause far-reaching ill effects.

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