Does coffee dehydrate you? Exploring the myth
In 2014, researchers at the University of Birmingham reported that moderate coffee consumption did not cause dehydration in the regular coffee drinkers they studied. Their hydration measurements were comparable with water. Given coffee's reputation for having a diuretic effect, the findings attracted considerable interest.
Why do we need all that water?
Our bodies are largely water, with estimates ranging from around 45–75% of body weight, depending on age, sex and body composition. Water is present in our cells and tissues, acts as a solvent, supports digestion, regulates body temperature and helps remove metabolic waste. In short, it is essential to life and needs to be supplied through food and drink.
How much water do we need?
A common everyday starting point is around 1.5–2 litres of drinks a day, with water making up a large share. Needs vary considerably between people and circumstances, so this is not a universal prescription. Drinking regularly is more practical than trying to take in everything at once.
You may need more fluid during hot weather, strenuous exercise, diarrhoea or vomiting, and with certain diets. Thirst is a useful reminder to drink. A fluid deficit of around 2–3% can affect concentration and contribute to headaches, fatigue or muscle cramps.
Is it true that coffee causes dehydration?
Dehydration develops when fluid losses exceed intake, especially when needs increase. Caffeine does have a diuretic effect, but that does not mean a moderate coffee inevitably causes a net loss of fluid. The effect is generally less pronounced in people who drink coffee regularly.
Enjoy coffee in moderation, paying attention to caffeine content rather than assuming every cup is the same. The familiar advice of up to three or four cups is only an approximate guide, and water should remain part of your routine.
Many myths have developed around coffee and the body's fluid and electrolyte balance. Let us look at the research discussed in the original article.
Can coffee contribute to hydration? A study
A 2014 University of Birmingham experiment involved 50 healthy men who were already regular coffee drinkers. Across the study conditions, participants consumed four cups of coffee or an equivalent quantity of water over a three-day period.
Researchers compared measures of hydration, including blood and urine samples. The study found no significant difference in overall hydration between the coffee and water conditions and did not show the expected harmful disruption of fluid balance.
The findings are encouraging, but they do not establish that coffee and water are interchangeable in every situation. The participants were healthy. The results may not apply to people at risk of dehydration from diarrhoea or vomiting, people with kidney disease or those taking medicines that affect fluid and electrolyte balance.
The important message is that moderate coffee intake does not automatically dehydrate healthy regular drinkers. But what about the claim that it “flushes out” magnesium?
A twitching eyelid is not automatically coffee's fault
Minerals, including magnesium, are lost in urine, and coffee is often blamed for increasing those losses. Magnesium matters: it contributes to metabolism, electrolyte balance, muscle and heart function, and healthy bones. Cramps or a twitching eyelid can have several causes, so they should not be treated as a diagnosis of deficiency.
Caffeine can increase urinary mineral losses slightly. The original guide cites around 4 mg of magnesium in a day in the context of higher coffee intake. But coffee also supplies magnesium: a cup may contain around 7 mg, depending on the brew. An unbalanced diet is a more useful place to look than automatically blaming coffee.
As so often, moderation matters. Coffee can fit into a healthy routine, but it should not be your only drink. Keep drinking water and other suitable fluids as well.
Spring water or mineral water?
Spring water may have a mineral content comparable with local tap water. It does not need a high mineral content to hydrate you effectively.
Mineral water can contribute useful minerals; the original guide highlights moderately mineralised options and concentrations around 500–1,000 mg per litre. Check the label because mineral content varies considerably.
Medicinal mineral waters can have much higher concentrations, sometimes around 25,000 mg per litre. These are not everyday substitutes for ordinary water and may require medical guidance.
Rather than relying on sweetened flavoured waters, fizzy drinks or sugary juice drinks, choose water as your everyday starting point. Tea and, in moderation, coffee can also contribute to your fluid intake.
Bibliography
- Killer S., Blannin A., Jeukendrup A., “No Evidence of Dehydration with Moderate Daily Coffee Intake: A Counterbalanced Cross-Over Study in a Free-Living Population”, full text (accessed 1 September 2020).
- Maughan R. J. et al., “A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index”, publication (accessed 1 September 2020).
- NHS: A small amount of coffee will not dehydrate you.
- Akademia Dietetyki: proper hydration in athletes.
- Woda dla Zdrowia: water and mineral balance.
- Photo: Food photo created by KamranAydinov — Freepik.