ABC Health & Wellbeing

Fact File

Caffeine

by Heather Catchpole

Caffeine is our favourite legal drug and the world's most popular drink. What does it do to your body, and how much is too much?

Published 27/04/2006

[Image source: iStockphoto]

• Background
• How much?
• Short-term effects
• Withdrawal
• Long-term effects
• Pregnancy
• Athletes
• Benefits
• More info

Background

"I have measured out my life with coffee spoons" – T. S. Elliot

Coffee or tea, anyone? They might be two of our favourite drinks, but how many of us know exactly what caffeine; the drug behind the world's most consumed beverages (except for water), is up to in the body?

Caffeine, like nicotine, is legal, and, if not addictive, it's definitely habit-forming. Unlike nicotine, caffeine is a lot more sociably acceptable. But whether you indulge occasionally or are a seasoned user, there's a lot about caffeine that might surprise you.

Humans have used caffeine since prehistoric times. In nature it acts as an efficient pesticide and is found in a number of plants: tea, coffee beans, guarana berries and cocoa nuts (although the quantity is quite small, around 0.2– 0.4 per cent). Surprisingly, it's the leaves of the tea plant that contain the most caffeine, around five per cent compared to one to two per cent for coffee beans.

The first recorded instance of caffeine drinking in history was the consumption of tea by Chinese emperor Shen Nung in 2737 BC, apparently after some leaves accidentally fell in a pot of boiling water. China has remained a big fan of the drug, with tea production growing by around 90,000 tonnes per year (according to China's Tea Market Annual Report 2004-2005). Coffee originated in Africa around 2500 years ago, where it was used for drinking and as money.

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How much?

The scientific description of caffeine is 1,3,7-trimethylxanthine. We consume it in tea, coffee, soft drinks, energy drinks and chocolate. It's also an ingredient in cold medications, appetite suppressants, and pain relievers and is effective against headaches and drowsiness.

Green tea and black tea contain the same amount of caffeine, as both come from the Camellia sinensis plant. Red tea or Rooibus tea comes from a different plant and contains no caffeine. According to the National Drug and Alcohol Research Centre at the University of New South Wales, the caffeine content of a typical 150 ml cup of tea varies from 30 to 100 mg, depending on how long the tea is infused and whether you brew with loose leaves or tea bags (strong loose-leaf tea yields the most caffeine).

Coffee beans come in two flavours, Robusta and Arabica. Lighter-coloured Robusta coffee beans contain more caffeine (2 per cent) than Arabica (1 per cent) as the roasting process removes some of the caffeine. A 150 ml cup of drip-percolated coffee has a typical caffeine content of 100 to 150 mg; instant coffee has 60– 100mg per 150 ml, depending on the brand. An espresso comes in at around 90 mg per 150 ml.

If you ate an entire 200 g block of chocolate, you'd consume about 550 mg of combined methylxathines (caffeine and other stimulants). A 30 g bar has between 20 to 60 mg caffeine – something that's worth considering when you calculate your daily caffeine intake, or think about how much the kids are consuming.

For those under the age of 20, it's likely that soft drinks and energy drinks are the dominant source of caffeine. A 375 ml cola drink contains around 40 mg caffeine while a 250 ml energy drink contains around 80 to100 mg caffeine – about the same as a shot of espresso.

How much is too much? The effects of caffeine on the body vary greatly between individuals. The ballpark guideline is to consume less than 600 mg per day – around four cups of strong drip-percolated coffee, or five or six cups of tea, although less is recommended for pregnant or breastfeeding women. The fatal dose of caffeine is 170 mg/kg – for a person of average bodyweight this would mean consuming around 10 g caffeine, or 80 to 100 cups at once – not an easy task.

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Short-term effects

Caffeine is a central nervous system stimulant and a diuretic and acts on the body in a number of ways. Short-term effects include contraction of the muscles, which can cause twitching; an increased heart rate; slowing of blood flow to the stomach; and constriction of blood vessels on the skin, causing blood flow from cuts on the skin to slow. More sugar is released from the liver into the bloodstream, and breathing tubes open up.

If you have a cup of coffee before bed you might find it harder to sleep. You'll also find that you experience deep sleep for shorter periods and have shorter dream periods, so that you feel less rested when you wake up.

After one to one and a half hours, caffeine reaches maximum concentration in the body. After three to seven hours half of the caffeine consumed has been metabolised, passing from the blood to the urine, from which is it expelled from the body.

Caffeine is metabolised by the liver, and passes into the bloodstream, crossing the blood/brain barrier into the brain. There it binds with receptors including dopamine receptors (which is why it makes you feel good) and adenosine receptors. Adenosine when bonded to its receptors slows down nerve cells, causing drowsiness. Caffeine also binds to adenosine receptors but doesn't slow down nerve cells. Since there is less adenosine binding, the nerve cells speed up, leading to feelings of restlessness.

So can a strong cuppa sober you up after a few drinks? No such luck. Caffeine will not alter your blood alcohol level, but it will make you feel more alert, just as coffee does when you are sober.

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Withdrawal

Caffeine is not generally considered addictive, but it is habit-forming. The body develops a tolerance towards the drug, meaning more caffeine is required to produce the same sensations. So we drink more to 'top up'.

Withdrawal symptoms occur because after a day or so without caffeine the brain become oversensitive to adenosine, causing blood pressure to drop dramatically. The most familiar withdrawal symptom is the caffeine headache. Other symptoms include drowsiness, irritability and even nausea and vomiting.

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Long-term effects

Excessive caffeine intake (more than four or five cups of strong tea or coffee per day) causes tremors, a fast heart rate, increased urination, nervousness, anxiety, stomach upsets and insomnia.

In rare cases caffeine can cause death. In Western Australia a 25-year old woman with a condition affecting her heart valves died after consuming a tonic containing 10 times the amount of caffeine as coffee – not enough to kill most people but fatal with a heart condition.

Caffeine has a greater affect on children and the elderly. It can cause unborn babies to experience restlessness and withdrawal symptoms at birth, and produce hyperactivity and anxiety in kids.

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Pregnancy

The rate at which caffeine is metabolised by the body depends on the health of the liver. Women in general have more enzymes in the liver and metabolise drugs like caffeine faster than men. The same class of enzymes metabolise caffeine and the hormones of pregnancy. It takes longer to get rid of the caffeine when these hormones are elevated in the body, that is, if you're pregnant or on the oral contraceptive pill.

The effects of caffeine and pregnancy are hotly disputed. While some studies have shown that caffeine contributes to a greater risk of miscarriage or birth defects, other studies have criticised this research in failing to eliminate the possibility of other causes contributing to the miscarriage. Groups such as Motherisk and the Organisation of Teratology Information Services have reported that low to moderate consumption of caffeine (one to two cups per day) has no effect on birth weight or risk of miscarriage.

Caffeine has however been shown to reduce fertility by damaging sperm in men, and also affects whether or not the fertilised egg (embryo) successfully implants in the uterus. Caffeine is a stressor; it stimulates the heart and brain and also stimulates the release of a hormone that negatively interacts with the hormones that control fertility. As little as one cup of strong coffee a day can affect the chances of successful conception.

Caffeine can enter breast milk and significant amounts can cause wakefulness and agitation to the baby. Again, one cup per day seems to be the recommended upper level of intake.

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Athletes

Caffeine has been used in sports supplements to combat fatigue. Research has shown that it is effective in delaying the time it takes to become exhausted after strenuous exercise. Studies have shown an increase in endurance while running and cycling after even a moderate intake of caffeine.

Caffeine is also a diuretic, so it could negatively impact performance by increasing the rate of dehydration. The International Olympic Committee has banned caffeine levels of the equivalent of four strong cups per day.

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Benefits

Caffeine has been linked to a number of conditions from cancer to heart disease to fibrocystic breast disease and osteoporosis and has since been cleared of culpability. It is classified as GRAS – generally recognised as safe – by the American Medical Association.

There is more good news. Caffeine is an antioxidant, and antioxidants are thought to help prevent heart disease and some cancers. Coffee also contains flavonoids, which are also antioxidants.

Caffeine increases alertness and is thought to improve concentration. Shift workers or others suffering from fatigue in their jobs may benefit from caffeine's properties as a stimulant.

Caffeine has been used to help premature babies experiencing difficulties breathing. Apnoea happens when a newborn baby's central nervous system doesn't signal the lungs to breathe. Minute amounts of caffeine given intravenously once a day can stimulate a newborn baby's brain to tell the lungs to inflate.

Taken in moderation, caffeine is harmless and may even be beneficial. Just keep an eye on your intake – whether green tea or chocolate – and you can sleep soundly at night.