Maintaining fluid balance in the body is one of the fundamental conditions for human life. The equilibrium between fluid loss and intake is necessary for adequate hydration of the body’s tissues. The importance of maintaining optimal fluid balance and the impact of caffeinated beverages on hydration are well-known in scientific literature and various guidelines. However, there are no clear individual recommendations for fluid consumption. One study shows that the amount of fluid consumed varies from 0.416 to 4.316 liters per day. The current reference values from EFSA for an adult male are 2.5 liters per day. However, available guidelines for adult males vary from 1.5 liters per day to 3.7 liters per day. It is claimed that caffeinated drinks should not be included in daily fluid requirement calculations and that a glass of water should be consumed with each cup of coffee or tea to maintain hydration.
Caffeine is a widely used substance found in coffee, tea, and chocolate. When consumed in large doses (≥500 mg), caffeine exhibits a diuretic effect. The diuretic properties of caffeine have been known for many years, starting with the first scientific report published over 80 years ago. The authors noted the acute diuretic effect of caffeine, which developed tolerance with regular consumption. Later, it was found that tolerance to caffeine is lost after abstaining for at least 4 days. Since the first publication, the diuretic effect of caffeine has been studied multiple times. Despite some differences in data, the general conclusion of the studies is that in individuals who have not consumed caffeine for ≥4 days, a high dose of caffeine causes an acute increase in urine output, while low and moderate doses do not exhibit a diuretic effect.
Coffee contains many bioactive substances that can interact with each other, so it is incorrect to compare coffee consumption with pure caffeine intake. Only two studies have examined the effect of caffeine in coffee on the body’s hydration status. One study found a 2.7% reduction in body weight and a 41% increase in urination, with increased sodium (66%) and potassium (28%) excretion. Caution should be exercised with the results, as participants consumed 6 cups of coffee per day (624 mg caffeine) after a 5-day abstention from coffee. Another experiment did not find any difference in the hydration status of people who regularly consumed coffee when drinking an equivalent amount of water containing caffeinated cola (1.4 mg/kg caffeine per day) or coffee (3.1 mg/kg caffeine per day). A limitation of this experiment was the lack of control over total body fluid.
It is estimated that 1.6 billion cups of coffee are consumed daily worldwide, making it important to understand whether coffee can be included in daily fluid consumption calculations and whether coffee consumption leads to mild chronic dehydration.
Study Procedure Overview
Fifty-two healthy men aged 18-46 participated in the study after screening 100 volunteers. Inclusion criteria: a) stable body weight; b) positive answers to a health questionnaire; c) no medication containing caffeine or affecting body weight/fluid balance; d) living and working in conditions without significant fluctuations in humidity or temperature; e) consuming food without unusual products, drinks, or supplements; f) no chronic diseases; g) moderate coffee consumption of 3-6 cups per day (300-600 mg caffeine). Women were excluded due to potential effects of the menstrual cycle on hydration. Two participants did not complete the study.
The experiment lasted about a month. A 3-day food and fluid intake monitoring period was conducted before the first test to refine inclusion criteria, coffee consumption, and usual diet. Participants were randomly divided into two groups. One group drank four 200 ml cups of black coffee per day (4 mg/kg body weight caffeine), while the other group consumed four 200 ml cups of water. During this period and for 5 days before, participants refrained from alcohol and physical activity. The protocol was repeated after 10 days, with participants returning to their normal diet. All food and beverages were weighed to the nearest 0.1 g. The diet consisted of 50% carbohydrates, 35% fats, and 15% proteins. Participants visited the lab every morning between 7 and 9 a.m. after fasting for weight measurements. After the test, they had breakfast from 7:30-9:00, a second breakfast at 10:30, lunch at 13:30, afternoon snack at 16:30, and dinner at 19:30.
Blood and urine samples were collected on days 1-3 of each 4-day period. Fluid distribution was controlled using doubly labeled water.
Discussion of Results
For decades, publications and media have emphasized the negative effects of caffeine-containing products on hydration status. However, there were few studies evaluating this effect in people with moderate coffee consumption in real-life conditions.
According to the authors, this is the first study to directly compare the long-term effects of coffee and water consumption using several methods to assess hydration. The results indicate that moderate coffee consumption (4 mg/kg body weight caffeine per day) does not significantly affect body hydration. No significant differences were found in hematological markers such as blood urea nitrogen or serum creatinine, suggesting normal kidney function. Urine analysis also revealed no significant differences in 24-hour volume, residual volume, osmolarity, or specific gravity. Body weight fluctuations were minor and did not reach significant levels. Recently, a study on the effect of caffeine capsules (5 mg/kg/day) on body weight in people with low caffeine consumption (<100 mg/day) also showed no difference in total body weight compared to the control (non-consuming) group.
An increase in sodium and potassium excretion with urine was observed. Sodium is an important, but not the only element determining urine volume. Some amount of water in the urine is “osmotically free,” particularly when a relatively large volume of diluted urine is excreted, as was observed in the experiment. The increased potassium excretion is due to its presence in soluble coffee (200 ml = ~2g of coffee), containing approximately 80 mg of potassium. Therefore, the participants consumed about 320 mg of potassium additionally from coffee compared to the water group. No differences were observed on the second day, suggesting adaptive potassium retention by the kidneys.
Despite no changes in total body weight, a small but significant decrease in body weight was observed in both groups, averaging 190 g/day (0.2% of body weight). This decrease is too small to be considered clinical dehydration, where body weight loss is typically 1-3%. The weight loss is likely multifactorial and related to changes in drinking habits. Furthermore, stool condition was only monitored through a questionnaire, and stool mass was not weighed.
Limitations of the study include insufficient laboratory control. On the other hand, to understand the effect of coffee on individuals in real-life conditions, reduced control is inevitable. The influence of other coffee components should also be excluded, but considering the minor differences between caffeinated coffee and water, it is unlikely that the response to decaffeinated coffee would differ.
Conclusion
Moderate coffee consumption (4 cups per day; 4 mg/kg/day caffeine) does not negatively affect the body’s fluid balance and can be counted as regular fluid intake.