Few drinks have attracted as much praise and criticism as coffee. Depending on the headline, it’s either a superfood that helps you live longer or a guilty pleasure that’s harming your health.

So which is true?

Coffee is one of the most chemically complex drinks we consume, containing well over a thousand biologically active compounds. Caffeine may be the most familiar, but it represents only a small part of the story. Coffee influences the brain, heart, liver, digestive system and metabolism in ways that are often misunderstood.

Like much of physiology, coffee refuses to fit neatly into the categories of “healthy” or “unhealthy”. Its effects are nuanced, sometimes appearing contradictory. But once you understand the science, those contradictions begin to make perfect sense.

Not Just Caffeine

When most people think of coffee, they think of caffeine. It’s the reason we reach for that morning cup after a poor night’s sleep, and why coffee has become synonymous with energy and alertness.

But coffee is far more than caffeine.

Its many compounds can influence how our bodies handle glucose, act as antioxidants, interact with cholesterol metabolism and even affect the gut microbiome.

Caffeine itself is also widely misunderstood. Many of the long-term health benefits associated with regular coffee consumption are also seen in people who drink decaffeinated coffee.

Coffee Doesn’t Create Energy

Adenosine is a naturally occurring chemical that gradually builds up in the brain the longer we’re awake. As its levels rise, it binds to receptors on nerve cells, increasing our perception of tiredness.

Caffeine has a similar chemical structure to adenosine and blocks it from binding to these receptors. It doesn’t reverse the effects of a poor night’s sleep, replenish your energy stores or replace the restorative functions of sleep. It simply prevents your brain from recognising how tired it already is.

As caffeine is gradually broken down by the liver, those receptors become available again. The adenosine that has continued to accumulate can bind, contributing to that familiar ‘afternoon crash’ as the caffeine wears off.

Temporarily reducing the perception of fatigue can improve alertness, concentration, reaction time and physical performance. But coffee doesn’t create energy. It simply delays your perception of how tired you actually are.

Why Coffee Affects Everyone Differently

Caffeine reaches peak levels in the bloodstream within 30–60 minutes. Around 95% is then broken down in the liver by an enzyme called CYP1A2.

Not everyone has the same activity of this enzyme. Some people break down caffeine quickly, while others metabolise it much more slowly. Smoking increases CYP1A2 activity, while pregnancy has the opposite effect. During the third trimester, caffeine’s half-life can increase from around 5 hours to 10–15 hours, which is one reason pregnant women are advised to limit caffeine to 200 mg per day.

Certain medications, including some antibiotics, antidepressants and the combined oral contraceptive pill, can also slow caffeine metabolism.

An average espresso contains around 60–80 mg of caffeine, a mug of instant coffee around 80–100 mg, and a large filter coffee around 150–250 mg, depending on the beans and brewing method. Up to 400 mg of caffeine per day is generally considered safe for most healthy adults, but that isn’t a target, nor does it mean everyone will tolerate that amount.

After around five hours, roughly half the caffeine from your coffee may still be circulating in your bloodstream. Five hours later, around a quarter remains.

There is no single “right” amount of coffee for everyone.

Coffee and Sleep

Caffeine doesn’t just make it harder to fall asleep; it can also reduce the amount of deep sleep you achieve. Spending less time in this restorative stage can leave you feeling tired the next morning, even if you’ve slept for the same number of hours.

If you drink 200 mg of caffeine at 3 pm, around 100 mg may still be circulating at 8 pm and 50 mg at around 1 am. In slower metabolisers, it may persist for even longer.

Caffeine isn’t found only in coffee. Tea, energy drinks, cola, pre-workout supplements and dark chocolate all contribute to your daily intake.

If you struggle to fall asleep, wake without feeling rested and then rely on coffee to overcome daytime fatigue, it can become a vicious cycle. The solution isn’t necessarily to stop drinking coffee altogether; simply bringing your last cup forward by a few hours may help.

Remember: falling asleep isn’t the same as sleeping well.

Coffee and Exercise

One of caffeine’s most consistent benefits is during exercise.

Caffeine is one of the most extensively researched supplements in sports nutrition and has been shown to improve endurance, muscular strength, power, sprint performance and reaction time.

It works largely by reducing the perception of fatigue and effort. In other words, the exercise feels easier, allowing many people to train harder or for longer before fatigue becomes limiting. Caffeine can also increase adrenaline release, improve muscle fibre recruitment and, during prolonged exercise, increase the use of fat for energy, helping to preserve limited glycogen stores.

Most studies suggest that 3–6 mg of caffeine per kilogram of body weight, taken around 30–60 minutes before exercise, can improve performance. For a 70 kg adult, that’s approximately 210–420 mg. More isn’t necessarily better, particularly if it causes tremor, palpitations, anxiety or gastrointestinal symptoms.

Coffee and the Heart

Caffeine’s effects on adrenaline and adenosine can cause temporary increases in heart rate and blood pressure. In people who rarely consume caffeine, systolic blood pressure may rise by around 5–10 mmHg, although the effect is usually modest and short-lived.

Regular coffee drinkers develop tolerance to many of these effects, so the same cup may produce much less of a response over time.

Although caffeine can trigger palpitations, moderate coffee consumption has not consistently been shown to increase the risk of common arrhythmias in healthy individuals.

For people with poorly controlled high blood pressure, troublesome palpitations or certain heart conditions, individual advice may differ. However, observational studies generally associate moderate consumption – often around 3–5 cups per day — with favourable cardiovascular outcomes.

Coffee may briefly change how the cardiovascular system behaves, but that isn’t the same as causing cardiovascular disease.

Coffee and Metabolism

Caffeine can temporarily reduce insulin sensitivity by around 15–20%, particularly when consumed shortly before a carbohydrate-rich meal.

Based on that alone, you might expect regular coffee drinkers to have a higher risk of type 2 diabetes. Yet large population studies consistently associate regular coffee consumption with a 20–30% lower risk of developing type 2 diabetes, and the association is seen with both caffeinated and decaffeinated coffee.

So how can both be true?

Coffee contains chlorogenic acids and other polyphenols that may reduce oxidative stress and inflammation, influence glucose absorption, affect liver glucose metabolism and interact with the gut microbiome. Over time, these effects may outweigh caffeine’s temporary impact on insulin sensitivity.

Exercise provides a useful comparison: during strenuous exercise, stress hormones and blood pressure rise and insulin sensitivity can temporarily change, yet regular exercise improves metabolic health over time.

The body’s immediate response doesn’t always predict its long-term adaptation.

This doesn’t mean coffee prevents diabetes, but the evidence suggests that moderate consumption is compatible with good metabolic health.

Coffee and the Liver

If there is one organ that consistently seems to benefit from coffee, it’s the liver.

Repeated liver injury from excess alcohol, obesity, viral hepatitis or metabolic disease can lead to fibrosis – the gradual accumulation of scar tissue that interferes with normal liver function.

Regular coffee consumption is consistently associated with a lower risk of chronic liver disease, cirrhosis and liver cancer. In people who already have liver disease, coffee consumption is also associated with slower progression of fibrosis.

Coffee contains antioxidant and anti-inflammatory compounds that may reduce oxidative stress within liver cells. Some compounds also appear to reduce the activity of hepatic stellate cells – cells involved in producing scar tissue following liver injury.

Importantly, some benefits are also seen with decaffeinated coffee, once again showing that coffee is about much more than caffeine.

Coffee and Cholesterol

Coffee naturally contains two oily compounds called cafestol and kahweol. Cafestol is one of the most potent naturally occurring dietary compounds known to raise LDL (“bad”) cholesterol.

But how much reaches your cup depends heavily on how your coffee is made.

Paper filters trap most of these compounds, so paper-filtered coffee generally contains much less cafestol. French press, Turkish and Scandinavian boiled coffee, however, allow substantially more to remain and can raise LDL cholesterol when consumed regularly in large amounts.

Espresso sits somewhere in the middle. It is unfiltered, but its small serving size limits the total amount consumed.

If you already have high cholesterol or familial hypercholesterolaemia, switching from unfiltered to paper-filtered coffee may therefore be worth considering.

How you brew your coffee matters.

Coffee and Your Gut

For many people, the first effect of a morning coffee isn’t feeling more awake — it’s
needing the bathroom.

Coffee stimulates the gastrocolic reflex, a normal response in which the digestive system signals the colon to increase its activity. Both caffeinated and decaffeinated coffee can produce this effect.

Coffee may also worsen heartburn or indigestion in some people with acid reflux (GORD) or sensitive stomachs.

Researchers are increasingly interested in coffee’s effects on the gut microbiome. Its polyphenols and other compounds can be metabolised by intestinal bacteria, and regular consumption has been associated with changes in the abundance of certain gut bacteria, although the significance of these changes is still being studied.

There is one other consideration: iron absorption.

Coffee’s polyphenols can bind to non-haem iron – the form found predominantly in plant foods – reducing its absorption from a meal. The effect on haem iron from meat and fish is much smaller.

For most people this is unlikely to matter. However, those with iron deficiency, heavy menstrual bleeding, pregnancy or predominantly plant-based diets may benefit from avoiding coffee with meals and leaving at least an hour between eating and drinking it.

So, Is Coffee Good For You?

For most healthy adults, moderate coffee consumption appears to be safe and may offer some health benefits. But those benefits need to be kept in perspective.

Coffee cannot compensate for a poor diet, physical inactivity, smoking, excess alcohol or consistently poor sleep. Drinking three cups a day won’t undo the effects of the lifestyle surrounding them, and no amount of caffeine can replace the fundamentals of good health.

Equally, you don’t need to start drinking coffee simply because some studies associate it with better health. If you enjoy it, tolerate it well and it isn’t interfering with your sleep or causing unwanted side effects, there is little reason for most people to avoid it.

Information and other content provided in these blogs should not be construed as medical advice and should not be considered a substitute for professional medical expertise. If you have any medical concerns, you should consult with your health care provider.