11/01/2026
People who drank too little water had a bigger stress hormone surge.
Low drinkers (about 1.3 liters a day) had a cortisol rise of 6.2 versus 4.0 in high drinkers (about 4.4 liters a day). That is about 55% bigger.
Researchers put 32 healthy adults through the same stressful test: a speech and mental math in front of a silent panel. Both groups felt equally anxious, and their heart rates rose the same. Only the stress hormone was different.
The study was small, the groups were picked by their usual habits rather than assigned at random, and it was partly funded by a bottled water company. It measured one stressful moment, not long-term health.
What this means for you: if you usually drink little, your body may react harder to stress than you can feel. Drinking enough every day is a free, easy habit worth keeping.