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A complete guide to caffeine

Whether you enjoy a cup of coffee in the morning, or you use it as a pick-me-up during the day, one thing’s for sure: caffeine is here to stay. This is supported by recent statistics, revealing that over one billion people worldwide drink coffee every single day. However, it’s not just coffee lovers that thrive on caffeine; this natural stimulant is supposedly one of the most commonly used dietary ingredients in the world – appearing in energy drinks, brewed tea, and even chocolate milk.

So, when you consume caffeine, what’s really going on? Are there any health benefits? Or should people act with caution? Find out everything you need to know right here.

What is caffeine?

Caffeine is a natural substance that stimulates the brain and central nervous system – which is why you feel more awake and alert once consuming it. Caffeine occurs naturally in over 60 plants – including coffee beans, tea leaves, kola nuts and cacao pods – but there’s also a synthetic (man-made) variety, which can be added to certain foods, drinks, and medicines.

What are the effects of caffeine?

Once consumed, caffeine is absorbed within about 45 minutes, peaking in the blood between 15 minutes and two hours. After reaching the liver, where caffeine is mainly broken down, it remains in the blood for between 1.5 to 9.5 hours.

Caffeine affects the brain the most, with one of the major side effects of consumption being an increased inability to sleep. This happens because caffeine blocks the effects of adenosine – the neurotransmitter that’s responsible for making you feel tired.

Variety of cups of coffee and coffee beans in burlap sack on black background.

Which foods and drinks contain caffeine?

The main sources of caffeine include:

  • Coffee: One cup of brewed coffee contains about 95 mg of caffeine, a cup of instant coffee contains around 60 mg of caffeine, and a cup of decaffeinated coffee contains around 4 mg of caffeine.
  • Tea: One cup of black tea contains about 47 mg of caffeine, green tea contains about 28 mg, decaffeinated tea contains 2 mg, and herbal tea contains no caffeine.
  • Soft drinks: A 12-ounce can of regular or diet dark coke contains roughly 40 mg of caffeine.
  • Chocolate: One ounce of dark chocolate – roughly one square – contains about 24 mg of caffeine, and milk chocolate contains a quarter of that.
  • Energy drinks: One cup of an energy drink contains around 85 mg of caffeine
  • Espresso: One shot contains roughly 65 mg of caffeine.

Other sources of caffeine are:

  • Caffeine supplements: One tablet contains around 200 mg of caffeine – roughly the same amount that’s in two cups of coffee.
  • Medicines: Caffeine can also be found in certain prescription or over the counter drugs, such as cold, pain, and allergy medications.

What are the potential benefits of caffeine?

For years, many have debated the effects of caffeine – could it really carry some health benefits? Or should it be consumed with caution?

Certain studies suggest the potential benefits of caffeine, which include:

  • May boost mood: Caffeine could possibly act as an antidepressant, as revealed in a meta-analysis of observational studies; there was found to be an 8% decreased risk of depression for each cup/day increment in coffee intake.
  • May improve brain function: Caffeine blocks the brain-signalling molecule adenosine, which allows the release of other signalling molecules. With adenosine out of the way, these brain-sparking chemicals can flow more freely – potentially giving you a surge of energy and potentially improving mental performance and slowing age-related mental decline. Supporting this theory, one review revealed that caffeine improved alertness, and reaction time.
  • May improve exercise performance: Caffeine supplementation has been shown to slightly enhance exercise performance – in doses of 3–6 mg/kg body mass – in some (but not all) studies. This includes improvements to muscular endurance, movement velocity, muscular strength, sprinting, jumping and throwing, to name a few. It’s also thought to reduce ratings on perceived exertion during exercise.
  • May help prevent diabetes: Some findings have shown that caffeine can be a potential contributor to reducing risk factors involved in the metabolic syndrome, including type 2 diabetes and obesity.

Chemical formula for caffeine next to a cup of coffee.

What are the possible risks of caffeine?

As one of the most-researched substances in the food supply, caffeine has a long history of safe use and overwhelming scientific evidence maintains that, when consumed in moderation, caffeine has no adverse health effects. Certain individuals may be more sensitive to caffeine than others, and excessive intake has been linked to difficulty sleeping, anxiety, restlessness, tremors, and an irregular heartbeat.

Not only this, caffeine consumption can cause a short, but dramatic, spike in blood pressure, so if you have high blood pressure, you should always consult your doctor about whether you should either stop or limit your intake.

Caffeine can also interact with certain medications – such as the muscle relaxant Tizanidine, and the antidepressant Fluvoxamine – as their effects may be increased. Caffeine can also interact with Ephedrine – as it may increase your risk of high blood pressure, stroke, seizure, or heart attack – Theophylline – which might cause nausea and heart palpitations – and Echinacea – which might increase the concentration of caffeine in your blood, thus heightening caffeine’s associated negative effects. If you’re ever worried about caffeine interfering with your medication, always ask for advice from your doctor or pharmacist.

For those with cardiac arrhythmias (abnormal heart rhythms), which is common in both young and older people, it’s recommended that any caffeine consumed is done so cautiously. Overall, researchers have found no evidence to support the link between caffeine and increased abnormal heart rhythms, but it’s important to remember that certain people may be more sensitive to the effects of caffeine than others.

How much caffeine should you drink a day?

Caffeine can be incorporated into a healthy diet, but that doesn’t mean that those who consume it shouldn’t act with caution. For healthy adults, 400 mg a day – which is roughly four or five cups of coffee – isn’t generally associated with dangerous side effects. However, this is not the case for those who are pregnant, and for those with certain pre-existing health conditions. Always chat to your doctor to see whether you should limit caffeine consumption.

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Picture of Dr Brian Fisher MBBCh MBE MSc FRSA

Dr Brian Fisher MBBCh MBE MSc FRSA

Dr Brian Fisher MBBCh MBE MSc FRSA is Clinical Director at Evergreen Life, and a Medical Expert with more than 42 years’ experience as a GP. Brian supports people in staying as fit and well as possible by having more control over their health and healthcare.