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How caffeine affects your body

Niamh Hennessy
Lead Dietitian, Cromwell Hospital
04 May 2021

Tea and coffee are some of the most popular drinks in the world. So you’re certainly not alone if you start your day with a mug or two to help you wake up and feel more alert. In fact, in the UK we drink nearly 123 million cups of tea and 90 million cups of coffee a day!

One of the reasons these drinks are so popular is because of their caffeine content. Caffeine is found naturally in around 60 different plants, including coffee beans, tea leaves and cacao beans (which are used to make chocolate).

What are the physical effects of caffeine?

Stimulate your brain and nervous system

Caffeine acts as a stimulant, which means it increases the activity of your body’s systems. It’s well-known for its effect on your brain and central nervous system (CNS), helping to keep you alert and stopping you from feeling tired. So it’s no surprise that you might turn to a cup of coffee to get you through an afternoon slump or a long night shift.

Increase your blood pressure

Caffeine can increase your blood pressure by a small amount for a few hours after you eat or drink it. But, if you regularly drink a moderate amount of caffeine, the effect is reduced. A moderate amount is around 400mg of caffeine or four cups of instant coffee.

Make you urinate (pee) more

Caffeine stimulates your bladder to produce pee (urine) more quickly than usual. You might find yourself going to the loo more often, but there’s no evidence to suggest it actually dehydrates you. In fact, tea and coffee contribute towards your daily fluid intake.

Trigger certain medical conditions

If you have certain medical conditions, you may be particularly sensitive to the effects of caffeine. For example, some people with irritable bowel syndrome (IBS) report that caffeine is a trigger, leading to symptoms such as diarrhoea.

Too much can lead to unpleasant side-effects

Too much caffeine – either over the course of the day or in one large dose (like in some energy drinks) – can sometimes cause unwanted side effects. These could include anxiety, restlessness, an irregular heartbeat, headaches or insomnia.

May influence your risk of disease

You might have heard questions about whether caffeine can affect your heart health. But research shows that for most people, a moderate amount of caffeine (around four or five cups of tea or coffee per day) isn’t likely to influence your heart rhythm, cholesterol levels and overall heart health.

Researchers have also investigated the connection between coffee and cancer. In 2016, the International Agency for Research on Cancer (IARC) confirmed that based on the current evidence available, caffeine is not thought to cause cancer.

Some studies looking at tea and coffee intake (not caffeine specifically) have shown that drinking one to three cups of coffee a day may be linked to a lower risk of certain diseases. These include type 2 diabetes and Parkinson’s disease. But more research is needed before doctors can be sure if caffeine lowers your risk of disease.

How much caffeine should you have in a day?

The recommended daily amount of caffeine depends on a few things. If you’re an adult over the age of 18, and not pregnant, then the safe daily amount is up to 400mg per day (about four mugs of instant coffee or five mugs of tea). But, if you’re pregnant or breastfeeding, then the safe daily amount is reduced to 200mg per day.

Caffeine might affect children and teenagers more, or differently, than adults. In the UK, the recommended safe limits of caffeine in children and teenagers is 3mg/kg body weight, and it should only be consumed in moderation. For example, the safe limit for a healthy 10-year-old weighing 32kg would be 96mg caffeine per day (two cups of weak tea).

Caffeine levels of caffeine-containing food and drinks

The amount of caffeine found in different food and drinks can vary greatly between products. For example, how much caffeine is in your favourite cappuccino will depend on things like the manufacturing process, type of beans used and how it’s made by your barista. So it’s difficult to know the exact amount of caffeine for sure. But as a rough idea, the average caffeine content of some common foods and drinks could be approximately:

  • 60ml espresso = 80mg caffeine
  • 200 to 400ml filter coffee = 90 to 180mg caffeine
  • 240ml instant coffee = 60 to 100mg caffeine
  • 220ml brewed black tea = 40 to 80mg caffeine
  • 220ml brewed green tea = 20 to 40mg caffeine
  • 240ml decaffeinated coffee = 3 to 12mg caffeine
  • 240 to 330ml fizzy drink = 30 to 50mg caffeine
  • 240ml cocoa beverage = 2 to 7mg caffeine
  • 240ml chocolate milk = 2 to 7mg caffeine
  • 28g (2 squares) milk chocolate = 6mg caffeine
  • 28g (2 squares) dark chocolate = 24mg caffeine

How does caffeine affect exercise?

Studies have shown that drinking caffeine before exercise can lead to a small improvement in performance across different types of training. This includes endurance, high-intensity and power sports.

It’s thought that around 3 to 6mg/kg bodyweight 60 minutes before exercise is enough. This is about one or two mugs of filter coffee for an adult. Although caffeine supplements are widely available online and from supplement shops, you can get the same benefits from a simple cup of coffee.

How does caffeine affect your sleep?

Everyone responds to caffeine differently. You might be able to have a mid-afternoon cup of coffee without it interfering with your sleep. Or you might find you struggle to get to sleep, or have a disturbed night‘s sleep if you have caffeine too close to bedtime.

There isn’t an agreed ‘cut-off’ time to stop consuming caffeine. But some recommendations suggest having your last caffeine-containing food or drink around six hours before you normally go to bed. This is around mid-afternoon for most people.

Tips for cutting back on caffeine

You may decide to cut back on caffeine for health reasons, or even to reduce your spending on takeaway coffees.

Try these tips to help you cut back.

  1. Cut back gradually, over a number of days or weeks. Stopping too quickly can result in withdrawal symptoms, such as headaches, fatigue and irritability.
  2. If you have insomnia or struggle to sleep well, avoid drinking caffeinated beverages late in the day or switch to decaf options.
  3. Remember that caffeine isn’t just found in tea and coffee. It’s also found in lots of foods like chocolate and some fizzy drinks, as well as in some medicines.
  4. Stay hydrated with other low or non-caffeinated drinks, such as water or green tea.


Are you interested in learning more about your health? Discover more about our range of health assessments.

Niamh Hennessy
Niamh Hennessy
Lead Dietitian, Cromwell Hospital

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