Underhydration may amplify the body's hormonal response to stress. Digest
Hydration is often emphasized during exercise or heat, but daily fluid intake may also quietly shape how the body responds to stress. In a new study, researchers tested whether adults who regularly drink less show a stronger spike in cortisol during challenging situations.
The scientists studied 32 healthy adults aged 18 to 35. Half habitually drank low amounts of fluid (1.3 liters per day), while the other half drank well above average (4.4 liters per day). Over seven days, participants maintained their usual fluid intake, verified using smart bottles. Afterward, they completed a simulated job interview and a mental math challenge known to raise stress levels. The team then measured cortisol in saliva and used several hydration markers.
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Although both groups showed similar increases in heart rate and self-reported anxiety, their cortisol responses diverged:
- People with low daily fluid intake had more concentrated urine, darker urine color, and higher levels of copeptin, a blood marker that reflects how hard the body is working to conserve water.
- Only the low-intake group had a clear rise in cortisol following the stress test, with hormone levels elevated at 10, 20, and 30 minutes afterward.
- Among participants whose cortisol rose meaningfully, those with lower fluid intake had a larger increase than their high‑intake counterparts.
- People with more concentrated urine across several days before the test also had stronger cortisol responses, indicating that hydration status over several days predicted reactivity.
- A simple morning urine color check helped identify who would have a stronger reaction. Those with darker urine showed larger cortisol increases.
This difference likely stems from how water-regulating hormones interact with stress systems. Vasopressin, which helps the body conserve water, also prompts the release of cortisol through a brain-to-adrenal gland signaling route called the hypothalamic-pituitary-adrenal axis. People who drink less show higher copeptin levels , consistent with stronger vasopressin activity, which may help explain their larger hormonal response under pressure.
The results suggest that hydration status shapes stress hormone responses, even when the stressful event and emotional reactions are similar. Randomized trials are now needed to test whether increasing fluid intake can reduce excessive cortisol responses and improve stress resilience.