People who drink less than the recommended daily fluid intake experience a greater stress hormone response, which is associated with an increased risk of heart disease, diabetes and depression, according to a new study from scientists in Liverpool, U.K.
A new study, published on August 22, 2025 in the Journal of Applied Physiology, found that individuals who drank less than 1.5 liters of fluid -- or seven cups of tea -- per day had a cortisol response to stress that was more than 50% higher than those who met daily water intake recommendations.
Study lead Professor Neil Walsh, a physiologist at Liverpool John Moores University, said, "Cortisol is the body's primary stress hormone and exaggerated cortisol reactivity to stress is associated with an increased risk of heart disease, diabetes and depression."
Poor long-term health
Neil and his team divided healthy young adults into two equally sized groups, representing the lowest and highest 25% for daily fluid intake. The "low fluid" group consisted of individuals who typically drank less than 1.5 liters of fluid per day (water, hot drinks etc.). The "high fluid" group included individuals who regularly met the daily water intake recommendations -- 2 liters for women and 2.5 liters for men. Both groups were matched for key factors known to influence stress responses, such as psychological characteristics and sleep.
Participants maintained their usual drinking habits for one week, during which hydration levels were monitored in blood and urine samples. Then participants underwent the Trier Social Stress Test, widely used to simulate real-world stress through a mock job interview and a mental arithmetic task.
Dr. Daniel Kashi, a member of the study team, said, "Both groups felt equally anxious and experienced similar increases in heart rate during the stress test. However, only the 'low fluid' group showed a significant increase in saliva cortisol in response to the stress test."
He added, "Although the low fluid group did not report being thirstier than the high fluid group, they had darker and more concentrated urine, clear signs of poor hydration. An important observation was that poor hydration was associated with greater cortisol reactivity to the stress test. Exaggerated cortisol reactivity to stress has been associated with poor long-term health."
Why is dehydration harmful?
The answer lies in the body's water-regulation system, which is closely connected to the brain's stress-response center. When the body senses dehydration, whether due to inadequate fluid intake or excessive fluid loss, it triggers the release of a hormone called vasopressin. Vasopressin acts primarily on the kidneys, promoting water reabsorption to maintain blood volume and electrolyte balance.
This conservation mechanism comes at a cost. Sustained vasopressin release places additional strain on the kidneys, which must work harder to concentrate urine and manage electrolyte balance.
Vasopressin also acts on the brain's stress-response center, the hypothalamus, where it can influence the release of cortisol. This dual role for vasopressin helps maintain blood volume and electrolyte balance but also increases cortisol.
The researchers say that while further long-term studies are needed, the findings reinforce current water intake recommendations -- approximately 2 liters of fluid each day for women and 2.5 liters for men.
Hydration helps us manage stress
"Being hydrated may help your body manage stress more effectively," added Dr. Kashi. A practical way to check your hydration status is to monitor your urine color -- light yellow typically indicates good hydration.
"If you know, for example, that you have a stressful schedule, maybe a looming deadline or a speech to make, keeping a water bottle close could be a good habit with potential benefits for your long-term health."