Are You “Quietly” Under‑Hydrated? New Study Shows Bigger Stress‑Hormone Spikes When You Don’t Drink Enough

Metabolism & Nutrition
Written By Cole Cruz

Healthy adults who habitually drink little fluid had a stronger cortisol response to a standardized lab stress test than high fluid drinkers. Here’s what that means for everyday training, recovery, and well‑being. [PMID: 40803748] PubMed

Why this matters

Cortisol (your primary stress hormone) rises with acute stress and then should settle. Repeatedly exaggerated spikes are linked with poorer long‑term health and recovery. A new study in Journal of Applied Physiology found that people with habitually low daily fluid intake (and more concentrated urine) had greater cortisol reactivity to psychosocial stress than those who drank more.

What the researchers did

  • Who: 32 healthy adults (16 “low” fluid intake, ~1.3 L/day; 16 “high,” ~4.4 L/day). Hydration status was tracked with urine osmolality (UOsm) and urine color (UCol)—both higher (more concentrated/darker) in the low‑intake group.

  • What: Everyone completed the Trier Social Stress Test (TSST)—the gold‑standard lab stressor that reliably raises salivary cortisol. [PMID: 8255414; 19095358] PubMed+1

  • Outcome: Salivary cortisol was sampled repeatedly before/after TSST.

Key findings

  • Bigger cortisol spike in low‑intake participants. Cortisol increased significantly only in the low‑intake group (group×time P = 0.02). Mean Δcortisol was 6.2 ± 2.9 nmol/L (low) vs 4.0 ± 1.8 nmol/L (high), P = 0.03 (Cohen’s d = 0.9).

  • Hydration status tracked with the stress response. Higher UOsm (more concentrated urine) correlated with greater cortisol reactivity (r≈0.7).

Plain‑English translation: when your day‑to‑day hydration is suboptimal (more concentrated/darker morning urine), your short‑term stress response can be amplified—even if heart rate and “nerves” feel similar.

How does this fit with the broader science?

  • Mechanism makes sense. Arginine vasopressin (AVP)—a water‑balance hormone—co‑drives ACTH and cortisol release via the V1b receptor. Animal and mechanistic human work shows V1b signaling boosts HPA‑axis responses to stress. [PMID: 14722621] PubMed

  • Hydration biology: In low drinkers, upping water intake reduces copeptin (a stable marker of AVP) within weeks—consistent with dialing down AVP drive. [PMID: 30566641; 28578535; 26603245] PubMed+2PubMed+2

  • Complementary evidence: A 2025 field study in young men found no change in day‑long cortisol patterns(awakening response/diurnal slope) across habitual intake—suggesting hydration may matter more for acute reactivity (like the TSST) than for daily circadian cortisol. [PMID: 39872007] PMC

Strengths and limitations (for the scientifically curious)

Strengths

  • Clear contrast in intake (~1.3 vs ~4.4 L/day) and objective hydration markers (UOsm, UCol).

  • Gold‑standard TSST with repeated salivary cortisol sampling. [PMID: 8255414; 19095358] PubMed+1

Limitations

  • Association, not proof of causation—intake wasn’t manipulated; RCTs are needed.

  • Young, healthy adults in a lab setting → caution generalizing to older adults, clinical groups, or hard training in heat.

Practical takeaways for everyday athletes

  1. Arrive euhydrated to stressful efforts (max lifts, metcons, big meetings).

    • Use morning urine color as your simple check: paler (roughly 1–3 on the 8‑color chart) generally indicates more dilute urine; darker suggests you likely need more fluid across the day. [PMID: 7987361] PubMed

  2. Daily intake anchors (all beverages + water in food): about 3.7 L/day (men) and 2.7 L/day (women) under temperate conditions—then adjust for heat, sweat rate, and body size. [PMID: 16028570] PubMed

  3. Before training/competition: target ~5–7 mL/kg ~4 h pre‑start; if urine is still dark/scant, add ~3–5 mL/kg ~2 hpre‑start. Include some sodium for long/hot sessions. [PMID: 17277604] PubMed

Why this is sensible (even if causality isn’t proven yet): Keeping hydration in a good range is low‑risk, aligns with sports medicine guidance, and this study suggests it may help avoid oversized cortisol spikes when life or training gets stressful. [PMID: 40803748; 17277604] PubMed+1

Who should be extra cautious?

If you have kidney, heart, or endocrine conditions, or you’ve been advised to limit fluids or sodium, ask your clinician before changing intake. For everyone else: aim for steady, moderate fluid across the day rather than chugging huge boluses.

References (PMID)

  • Kashi DS, et al. J Appl Physiol. 2025;139(3):698‑708. PMID: 40803748. PubMed

  • Kirschbaum C, et al. Neuropsychobiology. 1993;28(1–2):76‑81. PMID: 8255414. PubMed

  • Hellhammer DH, et al. Psychoneuroendocrinology. 2009;34(2):163‑171. PMID: 19095358. PubMed

  • Tanoue A, et al. J Clin Invest. 2004;113(2):302‑309. PMID: 14722621. PubMed

  • Enhörning S, et al. J Clin Endocrinol Metab. 2019;104(6):1917‑1925. PMID: 30566641. PubMed

  • Lemetais G, et al. Eur J Nutr. 2018;57(5):1883‑1893. PMID: 28578535. PubMed

  • Sontrop JM, et al. Nephrol Dial Transplant. 2015;30(10):1709‑1718. PMID: 26603245. PubMed

  • Sawka MN, et al. Med Sci Sports Exerc. 2007;39(2):377‑390. PMID: 17277604. PubMed

  • Sawka MN. Nutr Rev. 2005;63(6 Pt 2):S30‑S39. PMID: 16028570. PubMed

  • Zaplatosch ME, et al. Compr Psychoneuroendocrinol. 2025;21:100281. PMID: 39872007. PMC

  • Armstrong LE, et al. Int J Sport Nutr. 1994;4(3):265‑279. PMID: 7987361.

Cole Cruz
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