Coffee on an Empty Stomach:
Harmful or Not?
5:47. Ivy's day used to start the same way: the alarm, the kettle, a strong black coffee before anything else. Breakfast was for later. Usually, for never.
Then a friend told her coffee on an empty stomach "burns a hole in it". Another said it "kills your metabolism". Ivy did what she always does. She went looking for the actual research.
Here is what she found.
Your stomach: less dramatic than the rumours
Coffee does make the stomach produce more acid. Decaf does too, so it isn't only the caffeine.
The ulcer story doesn't hold up, though. A study of more than 8,000 healthy adults in Japan found no link between coffee and stomach ulcers, duodenal ulcers or reflux.
What is real: if you already have heartburn, reflux or gastritis, coffee on an empty stomach can make the discomfort worse. Then it isn't a rule to follow. It's a signal to listen to.
After a bad night, coffee first works against you
In 2020, researchers at the University of Bath tested this on 29 healthy adults. After a night of broken sleep, a strong black coffee half an hour before breakfast raised the blood sugar response to that breakfast by about 50%.
The bad night on its own didn't do that. The coffee did.
It's a small study and one morning, so no need to panic. But on tired mornings it's worth swapping the order: eat first, then coffee.
Iron: timing matters
Coffee drunk with a meal lowers how much iron you absorb from that meal. In a classic 1983 study, by 39% on average.
Coffee an hour before the meal didn't have that effect. An hour after the meal, it still did.
If your iron tends to run low (common for women), keep coffee away from your main meals.
Cortisol: your body gets used to it
Caffeine raises cortisol, the stress hormone. But in people who drink coffee every day, the morning rise almost disappears: the body adapts.
So the strict rule "no coffee for the first 90 minutes after waking" has no strong evidence behind it.
How much is fine
The European Food Safety Authority (EFSA) considers up to 400 mg of caffeine a day safe for healthy adults, and up to 200 mg at once. In pregnancy, up to 200 mg a day.
A 200 ml cup of filter coffee has about 90 mg of caffeine, an espresso about 80 mg.
5:47. A glass of water.
6:00. Eggs and whole-grain toast.
6:20. Coffee. Slowly, by the window.
Ivy, London Diary
In short
Your stomach is fine and nothing hurts: coffee first thing is not a crime.
Heartburn, gastritis or a bad night's sleep: breakfast first, then coffee.
Low iron: keep coffee away from main meals. An hour before eating, it doesn't get in the way.
More than about four cups of filter coffee a day is already a lot.
This is not medical advice. If you have stomach problems or anaemia, are pregnant or take medication, talk to your doctor.
Sources: Smith et al., British Journal of Nutrition, 2020 (University of Bath); EFSA, Scientific Opinion on the safety of caffeine, 2015; Shimamoto et al., PLoS ONE, 2013; Morck et al., American Journal of Clinical Nutrition, 1983; Lovallo et al., Psychosomatic Medicine, 2005.
