How much caffeine is too much? This is what coffee does to the body

How much caffeine is too much is a popular question asked by coffee-lovers around the world, here's the answer

Barista coffee machine pouring espresso with sunlight streaming over the machine
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How much caffeine is too much? If you're asking this question, you're probably prone to a few strong cups of coffee per day or you might enjoy an afternoon Red Bull. 

To do any real damage, you'd have to be drinking far more than what even the most dedicated drinkers do, but it's still important to stick to the lower thresholds of what's safe. Coffee and caffeinated drinks aren't worth the health crisis that many people make them out to be either though, with many health benefits associated with safe caffeine consumption. 

So whether you're a keen follower of the biggest coffee trends and want to try out super-strong coffee for yourself, or you're just wondering how coffee affects your skin, this is what you need to know about caffeine from two registered nutritionists and scientific advisors before you consider all the best alternatives to caffeine

How much caffeine is too much?

The Food and Drug Administration in the US has said that consuming more than 400 milligrams of caffeine per day should be avoided by everyone. This works out to be no more than five cans of Red Bull, five shots of espresso, or eight cups of tea. 

However, those who are naturally more sensitive to caffeine should consider drinking less or opting for a healthy alternative to coffee instead. Those who find that they experience more negative symptoms, such as shakes or heightened anxiety after consuming caffeine, are likely to be more sensitive to caffeine. They tend not to be able to metabolize it as fast as other people, meaning it stays in their system for longer.

Equally, Dr Greg Potter, chief science officer at Resilient Nutrition, says that's important for some people to avoid caffeine altogether. "In pregnancy, for example, as this slows the rate at which caffeine is broken down. If you're nursing and you consume caffeine, you'll pass some to your child via breastmilk, and infants degrade caffeine particularly slowly." 

Two women sat at dining room table holding cups of coffee

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Caffeine's effect on the body 

Everyone will experience caffeine differently, with genetics and your liver's ability to metabolize the caffeine both playing a significant role in this. However, there are some common ways that caffeine tends to affect us all, from improving a workout to heightening anxiety levels.

1. Increases alertness

Many of us start the day with a large cup of coffee because of the kick it gives—but not too many of us know why it has this effect. "Caffeine acutely increases alertness, largely by inhibiting the effects of a sleep-promoting chemical named adenosine that accumulates in your brain during wakefulness," says Dr Potter. 

Adenosine works as a barometer of how long you've been awake and, without caffeine, it promotes sleep. The longer you've been awake, the greater the accumulation in your brain and the sleepier you feel. 

A strong shot of espresso will perk you up if you're feeling tired because "caffeine is structurally similar to adenosine, meaning that it can occupy all of adenosine's receptors in your body and your brain, temporarily blocking the actions of adenosine," he says. So if you're looking for how to sleep better, it's worth avoiding caffeine in the hours before you want to fall asleep. 

2. Stimulates the cardiovascular system

If you've ever had a little too much caffeine, you might notice that your heart starts to beat really fast. This is because caffeine stimulates the cardiovascular system, Dr Potter explains. "It increases heart rate and blood pressure. The latter is more prominent in people who don't regularly consume caffeine, and these effects are short-lived." 

It's because of this reason that drinking two to three cups of coffee may reduce your risk of heart disease and dangerous heart rhythms, leading to a longer life span, according to a new study in association with the Baker Heart and Diabetes Institute

Over 10 years, researchers looked at varying levels of coffee consumption by half a million people, ranging from less than a cup a day to more than six cups, and their relationship with cardiovascular disorders including disease, heart failure, and stroke. They found that those who drank between two and three cups per day were 10% to 15% less likely to develop coronary heart disease, suffer from heart failure or arrhythmias, and were less likely to die prematurely for any reason. 

3. Increases calorie burn

"Caffeine can also increase activity in the fight or flight branch of the nervous system," says Dr Potter, which results in a greater production of stress hormones and a subsequent small increase in the number of calories that you burn. 

This won't make any significant difference, he adds, but it's something. When it comes to caffeine and exercise, it's the stimulating properties that will really change up your workout. "Caffeine can improve exercise performance through increasing alertness, plus increases in muscle contraction power that come from changes in how your muscles handle calcium," he says.

As a study from the University of Ulm explains, caffeine increases the release of calcium into the muscles which makes it easier to contract them—a movement essential in any strength training exercise.

But if you're thinking about incorporating more caffeine in your diet for exercise purposes, it's important to remember that how you get your caffeine matters. "Caffeine itself is a very effective stimulant for the brain and can improve our muscles' ability to break down fat for energy when exercising," explains Dr Federica Amati Ph.D., registered nutritionist and chief nutrition scientist for Indi Supplements. "But how you get your caffeine fix matters. Espresso is actually rich in fiber and has lots of beneficial polyphenols, but adding lots of milk, cream or sugary syrups will negate the benefits of coffee."

4. Impacts the digestive system

"It tends to increase gastrointestinal motility, explaining why some people have a strong urge to go to the toilet shortly after a strong coffee," Dr Potter explains, "Furthermore, caffeine is a weak diuretic, increasing urination."

But coffee is actually not the hydration killer that it's often made out to be. "You should bear in mind that if you're drinking a caffeinated drink, the hydrating effect of the water in the drink will almost certainly more than compensate for any small dehydrating effect of the caffeine," he says. 

5. Increases anxiety

After more than two cups of coffee, it wouldn't be abnormal to start to feel a little jittery. That's because caffeine acts as a stimulant to our central nervous system, explains Eve Kalinik, nutritional therapist and a gut health specialist for KÄLLA Probiotics.

"Coffee is a familiar daily stimulant for many of us, helping us feel more alert and energized throughout the day. But, as caffeine acts as a stimulant to our central nervous system, drinking too much can lead to increased symptoms of anxiety," she says. "So, for those who may experience periods of heightened anxiety, how you approach your coffee drinking habit is key. This can help you to balance levels of the stress hormone, cortisol, and optimize the connection between our mood and our microbiome."

But that's not to say that caffeine should be completely removed from our diet, Eve adds, as numerous studies highlight the health benefits of drinking coffee. "For instance, we know that coffee beans contain beneficial polyphenols and are high in fiber, with a significant amount able to pass into the beverage."

Freshly brewed espresso in black coffee mug sitting on wooden table

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What happens when you have too much caffeine? 

Because caffeine is a stimulant, Dr Potter explains, drinking too much of any caffeinated drink tends to lead to uncomfortable feelings of:

  • Overstimulation
  • Anxiety
  • Restlessness
  • Rapid and irregular heart rate
  • Muscle twitches
  • Sleep difficulties 
  • Headaches
  • Digestive issues

While these will often be disruptive, uncomfortable symptoms from consuming too much caffeine will likely fade. To cause any real damage, you'd have to drink about 10 cups of coffee, Dr Amati says. 

"Caffeine can be toxic in humans, with a fatal oral dose of about 10g, which is roughly 100 cups of coffee. So it's not something to worry about. After about 10 cups of coffee, many people will feel irritable and some may even have convulsions," she says. 

Grace Walsh
Health Channel Editor

Grace Walsh is woman&home's Health Channel Editor, working across the areas of fitness, nutrition, sleep, mental health, relationships, and sex. She is also a qualified fitness instructor. In 2024, she will be taking on her second marathon in Rome, cycling from Manchester to London (350km) for charity, and qualifying as a certified personal trainer and nutrition coach. 

A digital journalist with over six years experience as a writer and editor for UK publications, Grace has covered (almost) everything in the world of health and wellbeing with bylines in Cosmopolitan, Red, The i Paper, GoodtoKnow, and more.