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Does Coffee Actually Dehydrate You?
This is one of the most widespread myths in nutrition science. The short answer: not meaningfully, if you're a regular coffee drinker. A 2014 study in PLOS ONE by Killer et al. — one of the most rigorous studies on this topic — found that in men who habitually consume 3–6 cups of coffee daily, coffee is essentially equivalent to water for hydration purposes. The mild diuretic effect (increased urine output) is largely offset by the fluid in the coffee itself.
However, very high caffeine doses (400mg+ in a short window), combined with low fluid intake, hot environments, or heavy exercise, can contribute to mild dehydration. The effect is more pronounced in non-habitual caffeine users. For most regular coffee drinkers, the net hydration contribution of a cup of coffee is approximately 80% that of the equivalent amount of water.
What Actually Affects Hydration More Than Coffee
- Exercise: Sweating during moderate exercise can produce 0.5–1.5 litres of fluid loss per hour
- Hot environments: Passive sweating in heat can equal light exercise fluid losses
- Alcohol: A much stronger diuretic than caffeine — approximately 10ml of water lost per 1 unit of alcohol, beyond the fluid in the drink
- High-sodium diet: Increases obligatory urine output and water retention in fat cells
- Protein intake: High protein diets increase urea production, requiring more water for kidney filtration
The most reliable everyday indicator is urine colour. Pale yellow (like lemonade) indicates adequate hydration. Dark yellow to amber indicates dehydration. Clear urine often indicates over-hydration (also potentially problematic). A secondary indicator is urine frequency — most adequately hydrated adults urinate 6–8 times per day. Thirst is a late-stage indicator that appears only after 1–2% dehydration has already occurred.