September 25, 2025, 2:16 pm | Read time: 6 minutes

Low urine output or concentrated urine (dark yellow to amber) is a sign of suboptimal hydration. A phenomenon many are familiar with from their morning bathroom routine. Many adults consistently drink less water than officially recommended (2.5 liters for men, 2 liters for women daily).¹ Such fluid deficits are associated with an increased risk of kidney, cardiovascular, and metabolic diseases. Additionally, dehydration can also increase physical stress and lead to heightened release of the stress hormone cortisol.

While a moderate cortisol increase during stress is normal, an excessive or misdirected response is considered a risk factor for numerous diseases. Previous studies have already shown that extreme dehydration (such as from exercise or heat) raises cortisol levels.^2,3 However, whether consistently low fluid intake and mild everyday dehydration have this effect—and whether this leads to stronger reactions to everyday stress situations—was previously unclear.

The new study fills this gap. It examined whether people with consistently low fluid intake and suboptimal hydration react more strongly to acute psychosocial stress—measured by cortisol concentration in saliva. In other words, does the color of morning urine indicate a person’s stress level?

Fluid Intake, Urine Color, and Stress

The investigation is a prospective intervention study. It was conducted on 32 healthy, physically active adults aged 18 to 35.⁴

Based on objective drinking logs with data over seven days and national comparison data, the scientists divided the participants into two groups:

• LOW: low fluid intake (men drank less than 1.6 liters per day, women less than 1.5 liters)
• HIGH: high fluid intake (men drank 2.9 liters daily or more, women 2.5 liters or more)

To monitor fluid status, multiple urine samples (to determine osmolality and color), blood samples (to measure copeptin and plasma osmolality), and standardized questionnaires were collected. Using a recognized laboratory stress test, the Trier Social Stress Test (TSST), researchers induced a stress reaction in study participants. The TSST is a procedure involving mental stress in an interview situation. Saliva samples for cortisol measurement were taken at eight time points around the test.

The analysis focused on so-called “cortisol responders,” or individuals whose cortisol response exceeded the usual daily fluctuation. Additionally, subjective stress perception (state anxiety) was measured. Statistical evaluation was conducted using linear mixed models and correlation analyses between hydration status (state of hydration) and cortisol response.

The scientists also ensured that certain factors that could influence the physical stress response were excluded or controlled. Participants with generally high caffeine and alcohol consumption were not allowed to participate in the study. Later, participants were also instructed to avoid caffeine and alcohol around the stress test, as well as general food intake and tooth brushing. In the case of women, only those taking prescription oral combination contraceptives could participate. The previously mentioned seven-day assessment of their fluid intake (drinking log) was conducted only during their 21-day pill cycle. This reduced the influence of hormones on the study results.

Less Water = Darker Urine and More Cortisol

Participants with low daily fluid intake (LOW) and poor hydration showed a significantly stronger cortisol response to acute psychosocial stress than well-hydrated participants (HIGH).

The stress test (TSST) caused heart rate and subjective stress feelings to rise similarly in both groups. In the LOW group, salivary cortisol also increased noticeably 10, 20, and 30 minutes after the stress test, but not in the HIGH group.

Urine samples revealed that a darker color on the morning of the stress test correlated with an increased cortisol response to stress. There was also a correlation between urine osmolality and cortisol response. If this hydration marker indicated poor fluid supply over several days, the cortisol response to the stress test was also greater.

Overall, the majority of participants (81 percent) reacted biologically measurably to the stress test. The strongest reactions were measured in participants who drank little (LOW group).

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Consistently Low Fluid Intake Increases Stress Response

The results suggest a connection between poor hydration and increased physiological stress response. The observed cortisol increase in LOW participants shows that even mild but chronic fluid deficiency can enhance the activity of the body’s central stress system.

This is relevant because elevated cortisol levels are long-term associated with increased risk for cardiovascular diseases, diabetes, depression, and chronic inflammation.

For everyday life, this means: Even mild dehydration—measurable through dark urine or increased urine osmolality—can affect stress processing on a biological level. Those who regularly drink too little may react disproportionately strongly to everyday stress—with potentially harmful long-term consequences.

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Study Assessment and Possible Limitations

The study is methodologically well-structured, uses established measurement methods (TSST, salivary cortisol, copeptin), and considers important confounding factors such as sleep, caffeine, alcohol, and hormonal cycle. The division of participants into LOW and HIGH was based on objective drinking data over a week and allowed a differentiated view of hydration status.

However, the study also has limitations:

It is a cross-sectional study with a relatively small sample size, so no causal statements can be made. Additionally, the middle group (participants with moderate drinking behavior) was deliberately excluded, which could limit transferability. Possible gender differences were also not examined, although men and women were equally represented.

The study was partially funded by Danone Research & Innovation, which must be noted; however, the lead study author was independent and received no fees. The methodology, particularly the collection of hydration markers and cortisol values, was scientifically standardized and robust.

Conclusion

The study provides compelling evidence that consistently low fluid intake and suboptimal hydration lead to an enhanced hormonal stress response—specifically to increased cortisol rise during psychosocial stress. Thus, fluid deficiency could be an underestimated risk factor for stress-related health problems. For research and practice, this means: Hydration status should be considered when assessing stress responses. For everyday life, it means: Regular, adequate drinking is a simple but potentially effective protective factor against excessive stress responses. Furthermore, if you frequently notice that your urine is dark in the morning, you should pay attention to your daily fluid intake for the sake of your health and to be more resilient to stress in everyday life.