“People need to drink extra water to compensate for dehydration caused by drinking coffee.”

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 over three days. Participants consumed either 4×200 mL of coffee or water daily. 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 water.

The data indicate that caffeine intake of 6 mg kg-1 in the form of coffee can induce an acute diuretic effect, while 3 mg kg-1 do not disturb fluid balance in healthy casual coffee drinking adults at rest. Coffee ingestion at the HCAF trial induced greater diuresis during the 3-h period (613 ± 101 mL, P < 0.05), when compared to W (356 ± 53 mL) and LCAF (316 ± 38 mL).

It is well established that caffeine has a diuretic effect. However, single doses of caffeine up to 200 mg, corresponding to about 3 mg/kg bw for a 70-kg adult are unlikely to induce clinically relevant changes in ... hydration status.

Caffeine exerted a minor diuretic effect which was negated by exercise. Concerns regarding unwanted fluid loss associated with caffeine consumption are unwarranted particularly when ingestion precedes exercise. The regression toward the mean suggests the acute diuretic effect of caffeine was small; however, this could be case to case varying in degrees of effect.

The most ecologically valid of the published studies offers no support for the suggestion that consumption of caffeine-containing beverages as part of a normal lifestyle leads to fluid loss in excess of the volume ingested or is associated with poor hydration status. Therefore, there would appear to be no clear basis for refraining from caffeine containing drinks in situations where fluid balance might be compromised.

The main finding of this study was that ingestion of highly caffeinated coffee (6 mg kg−1) resulted in an acute diuretic effect represented by higher cumulative UV 2 and 3 h after ingestion. Therefore, habitual consumers of coffee not exceeding 3 mg kg−1 of caffeine should not worry about detrimental diuretic effects. However, once consumers approach 6 mg kg−1 of caffeine potential disruptions in fluid balance should be considered.

But most research suggests that the fluid in caffeinated drinks balances the diuretic effect of typical caffeine levels. Caffeinated drinks usually won't dehydrate you, but water is still your best option. Overall, water is probably your best bet to stay hydrated.

The report went on to say that the contribution of caffeinated beverages to the daily total water intake appears to be similar to the contribution of noncaffeinated beverages. The bottom line is that most adults do not need to avoid caffeine if consumed in moderate amounts and that caffeinated beverages do contribute to the daily water requirements in a manner similar to other fluids.

A double-blind, randomised, crossover trial published in 2013 investigated the effects of a moderate dose of caffeine (5mg/kg/bw/d) on total body water, extracellular water, and intracellular water in 30 non-smoking males aged between 20–39 years. These authors found that a moderate amount of caffeine, equivalent to 5 espresso cups of coffee or 7 servings of tea, does not alter total body water and fluid distribution in healthy men, regardless of body composition, physical activity, or daily water ingestion.

It is commonly reported that drinking caffeinated coffee can lead to dehydration. Recent scientific evidence, however, does not support this commonly held belief. Coffee drinking in moderation contributes to our fluid intake and does not lead to dehydration or significant loss of body water. Whilst caffeine may have a small diuretic effect, this effect is not strong enough to counter-balance the benefits of fluid intake from coffee drinking. Black coffee contains more than 95% water.

A daily intake of 300mg of caffeine induces only a mild, short-term diuretic effect, similar to that of water, with no significant effect on overall fluid balance. Statements suggesting the avoidance of caffeinated beverages before, and during, exercise are unfounded.

A known effect of caffeine is its ability to act as a mild diuretic (causing the body to lose water). [But] for healthy adults ... total daily caffeine consumption of up to 400 mg are safe.

Though the caffeine in coffee may have a diuretic effect, it’s unlikely to dehydrate you. For caffeine to have a significant diuretic effect, studies show that you need to consume more than 500 mg per day — or the equivalent of 5 cups (40 ounces or 1.2 liters) of brewed coffee. Researchers observed that drinking the higher caffeine coffee had a short-term diuretic effect, whereas the lower caffeine coffee and water were both hydrating.

Recent scientific research showed that a caffeine dose of 5 mg/kg bw/day does not alter total body water and fluid distribution in healthy men ... caffeine induced diuresis disappears under physical exercise ... does not lead to dehydration.

Habitual coffee drinkers who drink more than two cups of strong coffee could experience caffeine-induced diuresis. In the article, titled 'Coffee with High but Not Low Caffeine Content Augments Fluid and Electrolyte Excretion at Rest,' the authors suggest that caffeine intake of about 500 milligrams in the form of coffee can induce an acute diuretic effect although ingesting 250 milligrams of caffeine a day did not disturb fluid balance in healthy, casual coffee-drinking adults at rest.

Major health organizations like the Mayo Clinic and Harvard T.H. Chan School of Public Health state that moderate coffee consumption does not cause dehydration and contributes to daily fluid intake similarly to water. The diuretic effect of caffeine is mild and tolerance develops in habitual users, negating the need for extra compensatory water intake.

The European Food Safety Authority (EFSA) has found that consuming up to 400 milligrams (mg) of caffeine in a day, or a single dose of up to 200 mg, does not present a safety concern for adults. Single doses of caffeine up to 200 mg … from all sources do not raise safety concerns for the general adult population.

Many believe excessive coffee intake leads to dehydration, but research shows that caffeine's diuretic effect is relatively weak. Consuming coffee in moderation, typically around three to four cups a day, doesn’t significantly increase the need for extra hydration in most people. A study published in the journal PLOS found that coffee drinkers did not experience different hydration levels than those who primarily consumed water.

Coffee is about 98% water, which means it does contribute to your daily hydration and can count toward your overall fluid intake. The idea that coffee “dries you out” comes from caffeine’s mild diuretic effect (it makes you pee more). But research shows that the effect is small and temporary. If you drink coffee regularly, your body adapts and you won’t lose more fluid than you take in.