8:52, and nothing to drink since breakfast
Tuesday, 8:52. The budget meeting starts at nine and you are the one presenting. You had a coffee at seven and nothing since. Your heart is going a little faster. Your mouth is not dry. You do not feel thirsty at all.
Most of us would say that is fine. If the body needed water, it would ask for it.
A small study from Liverpool, published in August 2025 in the Journal of Applied Physiology, suggests the body can run short without asking1. People who usually drink little felt just as tense as people who drink a lot during a staged job interview. Their hearts sped up the same way. But their main stress hormone rose more, and they were not any thirstier12.
What the researchers did
The team screened 62 healthy adults and kept the two ends of the range: 16 people who usually drank about 1.3 L of fluid a day, and 16 who drank about 4.4 L1. In the low group, only about 0.9 L of that was water1. The two groups were similar on things that change how people react to stress, such as sex, age, sleep quality and how anxious they tend to be in general1.
Everyone kept their normal drinking habits for 7 days while the team tracked them1. On day 8, each person did a classic lab test: 5 minutes to prepare, then a 5 minute mock job interview and 5 minutes of mental arithmetic, standing in front of two strangers and a camera that was not actually recording1.
Both groups said they felt equally tense, and their heart rates rose by similar amounts2. The difference showed up in saliva. The stress hormone the team measured rose significantly after the test only in the low drinkers1. Their rise was 6.2 nmol/L against 4.0 nmol/L in the high drinkers (nmol/L is a measure of how much hormone is in the saliva)1. The university summed it up as a response more than 50% higher2. The more concentrated someone's urine was, the bigger their response tended to be1.
Why thirst stayed quiet
On the morning of the test, 15 of the 16 low drinkers were below the study's line for good hydration, judged by their urine1. Their urine was darker and more concentrated. Yet they did not report being thirstier than the high group2.
Thirst mostly follows how concentrated your blood is. A 2018 analysis that pooled 12 lab studies put the trigger point for thirst at about 285 mOsm/kg (a measure of how much salt and other particles are dissolved in the blood), right next to the trigger point for the body's water-saving hormone, vasopressin, at about 2845. Both sit in the middle of the normal range5.
In the Liverpool study, blood concentration did not differ between the groups, and neither did thirst. A marker of the water-saving hormone was higher in the low drinkers1. The authors think the kidneys were doing the work: holding on to water, sending out darker urine, and keeping the blood close to normal1. If the blood barely changes, thirst has little to react to5. So thirst may simply not notice a low-water habit.
The authors think the same hormone may be the link to stress. Vasopressin also acts on the hypothalamus, the brain area that runs the stress response, where it can influence the release of the stress hormone2. That is their explanation, not something this study proved.
A normal day against the EFSA numbers
The European Food Safety Authority (EFSA) sets an adequate intake of total water at 2.0 L a day for women and 2.5 L for men34. Total means everything: plain water, coffee, tea, other drinks, and the water inside your food3. The figures assume a mild temperature and a moderate activity level3. A hot day or a run changes the maths.
Many people fall short even with food counted. In national surveys from Spain and Italy, average total water intake was 1.6 to 1.7 L a day. In France it was 2.3 L for men and 2.1 L for women4. Only French women reached the EFSA value on average4.
To see where you stand, count the glasses, mugs and bottles you actually finish on a workday, from the first coffee to the last glass at dinner. Then look at your urine. The study team's own tip is simple: light yellow usually means you are well hydrated2. And if you know a deadline or a speech is coming, they suggest keeping a water bottle within reach2.
The RYBL view
What we think
We like this study because it measures something we all live: the meeting where your voice is steady and your heart is not. It is also small. Thirty-two young adults, compared at one point in time, cannot show that drinking more would change anyone's response. The authors say so themselves and call for randomised trials1. The long-term meaning is still unknown1. The work was also partly paid for by a large food and drinks company, which the paper discloses in full1.
One more limit we find telling: by design, the study left out people in the middle, roughly 1.5 to 2.5 L a day for women and 1.6 to 2.9 L for men1. That is where many office workers sit. We do not know yet how they compare.
What we take from it is modest and practical. Do not wait for thirst to tell you how your week of drinking went. Thirst reacts to your blood, and the authors think your kidneys may keep your blood steady by holding back water1. Put a glass on the desk before the big meeting, sip through the morning, and check the colour once in a while. More is not always better either. EFSA notes that drinking far too much, too fast, is rare but can be dangerous3. Aim for the EFSA reference values, food included.
Sources
- 1Kashi DS, Hunter M, Edwards JP, et al., 2025. Habitual fluid intake and hydration status influence cortisol reactivity to acute psychosocial stress. Journal of Applied Physiology 139(3):698-708. pubmed.ncbi.nlm.nih.gov (opens in a new tab)
- 2Liverpool John Moores University, 2025. Drinking too little puts body under extra stress. University news release. ljmu.ac.uk (opens in a new tab)
- 3European Food Safety Authority (EFSA), 2017. Dietary Reference Values for nutrients: Summary report (water section, from the NDA Panel opinion of 2010, EFSA Journal 8(3):1459). efsa.europa.eu (opens in a new tab)
- 4Nissensohn M, Sánchez-Villegas A, Galan P, et al., 2017. Beverage consumption habits among the European population: association with total water and energy intakes. Nutrients 9(4):383. pmc.ncbi.nlm.nih.gov (opens in a new tab)
- 5Hughes F, Mythen M, Montgomery H, 2018. The sensitivity of the human thirst response to changes in plasma osmolality: a systematic review. Perioperative Medicine 7:1. pmc.ncbi.nlm.nih.gov (opens in a new tab)
