Does Coffee Dehydrate the Body?

Check out this answer from Consensus:

The evidence suggests that moderate coffee consumption does not lead to dehydration and provides similar hydrating qualities to water. However, higher doses of caffeine can induce an acute diuretic effect, and coffee may slightly impede fluid replacement after exercise-induced dehydration. Overall, coffee can be part of a hydrating diet when consumed in moderation.

The belief that coffee causes dehydration is widespread, often leading individuals to avoid or limit their coffee intake to maintain proper hydration. This article examines the scientific evidence regarding the effects of coffee on hydration status, drawing on multiple research studies to provide a comprehensive understanding.

Coffee and Hydration: The Evidence

No Evidence of Dehydration with Moderate Coffee Intake

A study involving 50 male coffee drinkers who habitually consumed 3-6 cups per day found no significant differences in total body water (TBW) or other hydration markers between coffee and water consumption. Participants consumed either 4×200 mL of coffee or water daily over three days. The results showed no significant changes in TBW, urinary, or haematological hydration markers between the coffee and water trials, suggesting that moderate coffee consumption provides similar hydrating qualities to water1.

High Caffeine Content and Acute Diuretic Effect

Another study investigated the acute effects of low and high caffeine levels in coffee on fluid balance. Ten healthy adults consumed water, low caffeine coffee (3 mg/kg), or high caffeine coffee (6 mg/kg) in separate sessions. The high caffeine coffee induced greater diuresis and urinary osmotic excretion compared to water and low caffeine coffee. This indicates that while moderate caffeine intake does not disturb fluid balance, higher doses can have an acute diuretic effect2.

Caffeine and Guarana in Athletes

Research on young soccer players assessed the impact of caffeine and caffeine with guarana on dehydration. Players consumed caffeine, caffeine with guarana, or a placebo before participating in a 60-minute training session. The study found no significant differences in water loss between the groups, suggesting that caffeine consumption at about 3 mg/kg body weight does not result in greater water loss compared to a placebo3.

Heat-Induced Dehydration and Coffee

A study evaluated the effect of heat-induced dehydration on urinary caffeine excretion after consuming a strong coffee solution. Participants were exposed to heat in a sauna until they lost 2.9% of lean mass. The results showed that heat dehydration did not lead to higher concentrations of caffeine in urine, indicating that dehydration did not exacerbate the diuretic effect of coffee4.

Post-Exercise Rehydration and Coffee

Research on fluid replacement after exercise-induced dehydration examined the effects of drinking plain espresso coffee compared to water. Subjects rehydrated by drinking either water or a caffeinated beverage after moderate exercise. The study found differences in urine specific gravity, volume, and color between the two conditions, suggesting that coffee may impede fluid replacement to some extent5.

Does coffee dehydrate the body?

Neil Clarke has answered Unlikely

An expert from Covenant University in Nutrition

It is often suggested that coffee causes dehydration and its consumption should be avoided or significantly reduced to maintain fluid balance. However, a number of studies e.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3886980/ suggest that coffee, when consumed in moderation i.e. around four cups per day provides similar hydrating qualities to water and that drinking coffee as part of a normal lifestyle doesn’t cause fluid loss in excess of the volume ingested. 

Does coffee dehydrate the body?

Neil Schwarz has answered Unlikely

An expert from University of South Alabama in Sports Science, Exercise Physiology, Kinesiology

Generally, coffee does not dehydrate the body. The caffeine is coffee can stimulate some additional urinary output, but this is usually offset by the fluid consumed with the coffee.