Coffee is best known, and perhaps most valued, for its caffeine and its ability to make us feel more awake and alert.
Author
- Emma Beckett
Senior Lecturer, Nutrition and Food Science, Australian Catholic University
But coffee is much more than caffeine dissolved in hot water.
Coffee is a chemically complicated plant extract containing hundreds of bioactive compounds . These can have all kinds of weird and wonderful effects in your body, even when you’re drinking decaf.
Here are seven of the strangest.
1. Coffee can make you poo
One study shows about three in ten people say they get the urge to poo shortly after drinking coffee.
This happens quickly, and with both regular and decaf. So it’s down to more than just the caffeine. But it’s not clear exactly which coffee compounds cause this.
Your colon can also be more active in the morning , and this is when most people drink their first coffee.
What you add to your coffee can also affect your bowels. The lactose in milk or some sugar-free sweeteners can also get the bowels moving, particularly if you consume a lot.
2. Coffee can affect your reflux, eyes and ears
Coffee can worsen reflux symptoms for some people. Reflux, when your stomach acid flows back up into your food pipe, isn’t always just felt as heartburn. It can contribute to coughing, wheezing and other respiratory symptoms when reflux affects the throat and airways.
Caffeine can temporarily increase pressure inside the eye in some people with glaucoma or ocular hypertension , where pressure in the eye can already be high. Controlling this pressure is an important part of protecting the major nerve of the eye from damage. So some people with these conditions might be advised to limit their coffee and caffeine intake.
There’s also an ear condition where the tube connecting the middle ear to the back of the nose stays abnormally open, which can make you hear your own voice, or your breathing, unusually loudly. People with this condition, known as patulous Eustachian tube dysfunction, are sometimes advised to drink fewer caffeinated drinks and stay well hydrated , because dehydration can worsen symptoms. However, there is little direct evidence coffee itself causes the condition.
Caffeine’s relationship with migraine is complicated : caffeine can help relieve a migraine, but too much, or suddenly having less than usual, can trigger one in some people.
3. Coffee can interact with your medicines
Coffee can change the way some medicines behave in the body. For instance, it can reduce absorption of the thyroid medication levothyroxine and the osteoporosis drug alendronate .
Caffeine can slow the metabolism of the antipsychotic clozapine , increasing its concentration in the blood.
Sometimes medicines change the way your coffee behaves. For example, the antibiotic ciprofloxacin slows your breakdown of caffeine. So, your usual coffee may stay in your system for longer.
4. Coffee can affect your cholesterol
Coffee contains compounds called diterpenes. Two of these, cafestol and kahweol , can increase total and LDL (“bad”) cholesterol.
But in short-term trials, the same compounds lower lipoprotein(a) , which may indicate a lower risk of a heart attack or stroke.
Other coffee compounds, including chlorogenic acids, may modestly lower blood pressure and improve blood vessel function .
So overall, it’s unclear what coffee means for markers of heart health.
5. Coffee may feed your gut microbes
You’re not the only one getting something from your coffee. Laboratory experiments suggest your gut microbes do too.
Some of coffee’s chlorogenic acids aren’t absorbed in the small intestine and reach the colon. There, microbes break them down into other compounds .
Coffee contains complex carbohydrates and roasting products called melanoidins that can reach the colon, where gut microbes can ferment them.
Small human studies suggest drinking coffee can also change the composition of the gut microbiome . But the evidence is still developing, so it’s too early to call coffee a prebiotic.
6. Coffee can influence your iron levels
Coffee can also limit how much iron you get from your food. Drinking coffee with a meal can substantially reduce the absorption of non-haem iron – the form found mainly in plant foods.
This isn’t primarily a caffeine effect. Polyphenols in coffee, including chlorogenic acids, can bind with iron in the digestive tract , making it harder to absorb.
This matters most for people who already have low iron stores or rely heavily on plant sources of iron, rather than being a reason for everyone to give up coffee with breakfast.
7. Coffee can kick-start the gut
Coffee can kick parts of your digestive system into action even when there’s no food to digest. It can stimulate the pancreas to release trypsin, an enzyme involved in digesting protein.
This happens with both regular and decaffeinated coffee, suggesting other compounds in your cup are talking to your digestive system.
Whether this digestive “heads-up” changes how hungry you feel isn’t clear.
Coffee can also affect gut hormones involved in appetite and fullness. But studies haven’t consistently shown whether this translates into eating more or less .
So, what does all this mean?
Caffeine and feeling more alert might be coffee’s most popular feature, but this is far from the whole story. Caffeine can do much more than that.
A cup of coffee contains hundreds of compounds that can interact with our digestive system, microbes, medicines, nutrients and more – sometimes in confusing and unexpected ways.
