What is caffeine?

Caffeine is a stimulant that acts on the brain and nervous system. It is found in coffee, most teas, cocoa, chocolate, cola, guarana and energy drinks. Caffeine's effect on your health is complex — it can cause minor health problems, but it can also reduce your chances of having others.

Pregnant or breastfeeding women and children should restrict their caffeine intake.

How do we consume caffeine?

Most adults get most of their caffeine from coffee. Other common sources are tea, chocolate bars, chocolate drinks, cola drinks and energy drinks. The amount of caffeine in each of these varies, even within 1 source. For example, espresso has more caffeine than instant coffee, and dark chocolate has more caffeine than milk chocolate.

The research is not clear, but it is likely that healthy adults can in general consume around 400mg of caffeine a day safely.

Caffeine is also found in some medications that relieve pain and treat colds and flu. Some energy supplements, appetite suppressants and weight loss products may contain caffeine too.

How does caffeine affect the body?

Caffeine affects many parts of your body, including the central nervous system, muscles, heart, lungs and kidneys.

However, it is hard to say exactly how the body responds to it. Much of the medical research in this area has been into coffee, which contains caffeine but also many other ingredients.

We do know that caffeine is addictive, and many regular coffee drinkers become dependent on it.

What are the benefits of consuming caffeine?

Because caffeine is a stimulant, it can make you feel more alert and energetic.

Also, people who drink coffee regularly might have a lower than average risk of:

• type 2 diabetes — possibly because caffeine makes you more sensitive to the insulin your body produces
• depression — possibly because some of the naturally occurring chemicals in caffeine can reduce inflammation of sensitive parts of the brain
• Parkinson's disease — possibly because it protects the brain cells at risk of damage in someone who has Parkinson's disease
• bowel cancer — possibly because it contains the naturally occurring chemicals cafestol and kahweol, which both protect against cancer
• Alzheimer's disease — in this case, it is not clear why caffeine might be beneficial

And overall, it seems that coffee drinkers live longer than people who do not drink coffee. It is not clear if that is due to the coffee or to something else.

What are the problems with consuming caffeine?

Too much caffeine at any one time — for example, more than 500mg, which is 3 espresso coffees or 6 to 7 instant coffees — can make you feel agitated and anxious. It can make it hard for you to sleep. It may also make you breathe faster and your heart beat faster.

If you drink a lot of coffee over a long period, it might increase your risk of:

• heartburn — possibly because coffee relaxes the muscle that stops acid regurgitating into the oesophagus
• osteoporosis — probably because caffeine encourages you to lose calcium through your kidneys
• insomnia — because of the stimulant effect
• miscarriage — in this case, it is not clear why there might be an increased risk
• infertility in men — possibly because caffeine damages sperm

What are the areas of uncertainty?

It used to be thought that caffeine caused high blood pressure and dehydration, but now it seems this might not be true.

Coffee has been said to reduce a woman's fertility, but that also is probably not true.

Mixing caffeine with alcohol and other drugs

When alcohol is consumed with or mixed with caffeine — as it is in certain energy drinks — the caffeine can hide some of the effects of the alcohol. This can make drinkers feel more alert. As a result, they may drink more alcohol and become more impaired than they realise, putting themselves at greater risk.

Caffeine does not affect the way the body absorbs alcohol, so it will not reduce your blood alcohol concentration or help you 'sober up' after a big night.

Caffeine can also interact with other drugs such as:

• antibiotics
• stimulants such as cocaine and methamphetamine
• some medications for mental illness
• some heart and asthma medications

Talk to your pharmacist or doctor to check if any medications you take could be affected by caffeine.

How much caffeine is it safe to consume?

The research is not clear, but it is likely that healthy adults can generally consume around 400mg of caffeine a day — that is 2 (50mL) espresso coffees or 5 (250mL) energy drinks.

Children should consume less than 3mg for each kilo of body weight a day. For children aged 9 to 13, this is about 120mg a day, or 2 (375mL) cans of cola. If you are pregnant or breastfeeding it is a good idea to limit your intake to 200mg per day — that is 1 espresso, 2 instant coffees or 4 cups of tea. See the table above for details on serving sizes.

But some people are particularly sensitive to caffeine — if you find that 1 coffee per day gives you a racing heart, then give it up.

People with health conditions should talk to their doctor about caffeine as part of any discussions about a healthy lifestyle.

Withdrawing from caffeine

Because caffeine is addictive, many people have withdrawal symptoms after a day or two without it. These symptoms can last for a few days, and include:

• headaches
• tiredness
• irritability
• nausea
• difficulty concentrating

If you want to reduce the amount of caffeine you consume, it is a good idea to do this gradually. For example, reduce your intake over a couple of weeks and avoid cutting down by more than 1 caffeinated drink between consecutive days. Another option for coffee drinkers is to gradually switch to decaffeinated coffee.

Check labels on foods and drinks. But keep in mind that only added caffeine is labelled — if caffeine is found naturally, such as in guarana, it will not be listed.