Effects of Habitual Water Intake on Thirst in Healthy Young Adults Explained by Osmoadaptive Metabolism, Brain and Kidney Function (Adapt-Thirst) (Adapt-Thirst)

Studie Zum Habituellen Durst Und Adapt Durst Studie: Habitual Thirst Study and Adapt Thirst Study

Social relevance:

For 30 years, people have been confused about how much plain water to drink. Over 30 years, health professionals have criticized media advice to drink 8 glasses of water per day, citing lack of evidence (Valtin et al, 2002; Yamada et al, 2022). Health authorities have not set drinking water-specific recommendations, assuming 1) that any or all types of drinks hydrate equivalently, i.e. that people do not need to drink plain water to replace lost body water, and 2) the average healthy person can rely on thirst as guide for water intake.

The lack of drinking water-specific recommendations significantly impacts daily lives because it translates into limited or no support for drinking water in public health services, laws, and retail options.

Scientific relevance:

Thirst is considered the primary driver of water intake and main defense against body water deficit in healthy young adults (IOM, 2005). Health authorities set total water intake recommendations for the average healthy man and woman (e.g. 2.5 L/d for men and 2.0 L/d for women in Europe) but, additionally, advise people to use thirst as a guide for water intake, recognizing that individual water requirements vary widely (EFSA, 2010; IOM, 2005).

Although thirst can be satiated by water intake, it can also be ignored per custom (Greenleaf, 1992) or suppressed by an upward-shifted thirst threshold. The thirst threshold, the set-point where osmoreceptor cells shrink and release their neural or hormonal signal, is a function of the solute concentration or osmolality inside and outside the osmoreceptor cells (Nose et al, 1988a,b). Cells with higher intracellular solute content require a higher external osmolality to shrink.

Specific Aims The ultimate goal of this study is to address gaps in the literature about drinking water and check assumptions that limit the development of drinking water-specific recommendations. The study will examine if osmoadaptation to chronic hypertonicity, due to daily intake of hypertonic fluid sources, can explain suppressed thirst in healthy individuals under conditions of daily life. To facilitate causal inference about drinking water effects for long-term health, this study was designed to link experimental data about osmoadaptation at the cellular level with clinical data relevant for conditions of daily life in Salzburg Austria with population-based data about water intake and chronic disease risk in Salzburg Austria.

This study will test effects of drinking enough plain water to dilute urine everyday for 4 weeks (about 500 mL 4 times per day in summer). The study will include healthy, normal weight, young, men and women, who all usually meet European adequate intake recommendations for total water intake (TWI), but usually consume less than 1L/d PWI, and have biomarkers of chronic hypertonic stress (concentrated urine and saliva) for 4 consecutive weeks before starting the randomized study.

Study Overview

• Status: Not yet recruiting
• Conditions: Healthy; Dysglycemia; Brain Function; Warburg Effect; Thirst; Due to Deprivation of Water; Protein Breakdown; Chronic Underhydration; Aestivation Metabolism; Osmoadaptation
• Intervention / Treatment: Other: Drinking water to dilute urine

Detailed Description

Osmoreceptor cells in the brain and periphery, which are collectively responsible for perceived thirst, accumulate intracellular solute (Bourque, 2008), such as amino acids, to adapt to chronic hypertonic stress. Increased protein breakdown is a well-established strategy for coping with chronic hypertonic stress, observed across species with drought/aestivation (Yancey et al, 1982). Evidence of increased protein breakdown is observed in patients with excess body water loss due to skin or renal damage (Kovarik et al, 2021) and with less than 2L/d usual total water intake in healthy young men (Stookey et al, 2023). Higher concentrations of intracellular solute allow cells to tolerate chronic hypertonic stress by creating an osmotic gradient that favors retaining water inside the cells.

Patients with chronic hypertonicity due to uncontrolled diabetes are known to develop suppressed thirst. Healthy athletes (Casa et al, 2000) and individuals exposed to heat (Rosinger et al, 2022) are known to experience 'involuntary dehydration' or incomplete rehydration after dehydration, when given ad-libitum fluids and allowed to drink following thirst. Thirst is significantly reduced in older adults (Phillips et al, 1984). It is plausible that decreased thirst is a function of intracellular osmolyte accumulation, resulting from altered metabolism in response to chronic hypertonicity.

In older adults, reduced thirst is attributed to higher baseline extracellular osmolality and higher osmotic set-point for thirst sensation (Kenney & Chiu, 2001). In older people, reduced thirst or faster thirst satiation after drinking water is related to a larger drop in activation of the anterior midcingulate cortex (aMCC) in the brain (Farrell et al, 2008).

With respect to young adults, while evidence indicates that 'involuntary dehydration' depends on cations lost or excreted from the intracellular and/or extracellular space (Nose et al, 1988), roles for osmolytes other than cations remain to be explored. There are gaps in the clinical literature regarding effects of chronic extracellular hypertonicity on osmoadaptation, shifted osmoreceptor set-point, suppressed thirst, and hydration biomarkers.

Chronic extracellular hypertonicity and suppressed thirst are conceivable in daily life, because people frequently consume foods and fluids that are more concentrated than blood (beverage osmolality >280 mmol/kg). Most commercially available beverages including milk and juice have an osmolality above 300 mmol/kg.

Given that adaptation to chronic hypertonicity carries metabolic cost (Pena-Villalobos et al, 2016) and favors chronic disease risk factors in healthy young adults (Stookey et al, 2023), including micronutrient (e.g. Zn) excretion (Zorbas et al, 1993; Zorbas et al, 1995), oxidative stress, protein breakdown, and altered immune function, low thirst in young adults may not be a reliable guide for water intake - if thirst is 'suppressed' as opposed to 'satiated'. On the contrary, low thirst in young adults may signal chronic suboptimal cell hydration and unmet need for hypotonic water.

Hypotheses

Holding constant usual intake of food and other beverages and physical activity levels over 10 weeks, this study hypothesizes that participants who are randomly assigned to drink water to dilute afternoon urine to USPG<1.013 daily (PWI of about 20 mL/kg or 500 mL 3x/d in Spring and Autumn; 4x/d in Summer) for 4 weeks will have a:

Primary outcome

• significantly greater increase in the mean overnight water restricted thirst rating between Week 5 and Week 10 compared to participants assigned to the control group.

Secondary outcomes

• significantly greater decrease between Week 5 and Week 10 in the acute decrease in regional cerebral blood flow seen by functional MRI in brain regions of interest (S1/M1, prefrontal cortex, anterior midcingulate cortex, premotor cortex, and superior temporal gyrus) from maximum thirst after overnight water restriction to immediately following 500 mL drinking water, compared to the control group.
• significantly different metabolomic profile in Week 10, with greater shift away from the aestivation- and Warburg-like patterns, including significantly greater reduction in protein breakdown between Week 5 and Week 10, compared to the control group.
• significantly greater decrease in urine excretion of zinc between Week 5 and Week 10, compared to the control group.

• Study Type: Interventional
• Enrollment (Estimated): 120
• Phase: Not Applicable

Contacts and Locations

• Study Contact: Name: Jodi D Stookey, PhD; Phone Number: 1-415-312-0237; Email: jodidstookey@gmail.com
• Study Contact Backup: Name: Markus Ritter, MD; Phone Number: 43-662-2420-80500; Email: markus.ritter@pmu.ac.at

Participation Criteria

Eligibility Criteria

• Ages Eligible for Study: Adult
• Accepts Healthy Volunteers: No

Description

All randomized participants must have complete data from 4 weeks of screening. All inclusion criteria below must be met:

• 4 weeks of 3-day mean total water intake of 35 ml/kg body weight
• 4 weeks of <1L/d drinking water
• 4 weeks of <1 hr/d moderate or vigorous physical activity given daily life conditions in Salzburg, Austria
• No history of chronic disease
• Normal blood pressure,
• Normal blood chemistry and complete blood count (CBC) test results
• Normal overnight urine concentration, and normal acute urine dilution after 500mL drinking water
• Normal weight and height between 160-170 cm for women and 175-185 cm for men)
• After overnight food and water restriction urine osmolality above 800 mmol/kg, salivary osmolality above 100 mmol/kg, and a thirst score below 70 mm on a 100 mm visual analog scale.

Exclusion Criteria:

• Individuals will be excluded from participation if the gender-specific target numbers have already been reached

• Perceived stress score of > 20 at any time during Weeks 1-4

  • Schedule that does not allow continuous study participation
  • Intention to spend more than a day outside the Salzburg area during the study period (to reduce exposure to ground water with different background D2O as potential source of error)
• Headache within the last six months

• Pregnant, planning a pregnancy, or irregular or unknown menstrual cycle

  • Active use of tobacco or e-cigarettes
  • Daily consumption of alcohol
  • Regular use of medication
  • Agoraphobia or claustrophobia that would prevent brain fMRI testing
  • Metal-containing intrauterine device (IUD) that would prevent fMRI testing
  • Doubtful or unwilling to complete all study procedures (including drinking alcohol; drinking plain tap water, blood tests, 24-hour urine collection, fMRI scans, and stable isotope tests)
  • Unwilling to be randomly assigned to intervention or control
  • Incomplete baseline data in Weeks 1-4 (including blood tests, 24-hour urine collection, and MRI scans).

Study Plan

How is the study designed?

Design Details

• Primary Purpose: Prevention
• Allocation: Randomized
• Interventional Model: Parallel Assignment
• Masking: Single

Number of Arms

2

Arms and Interventions

Participant Group / Arm	Intervention / Treatment
No Intervention: Control; After completing 4 weeks of baseline data collection and random assignment in week 5, the control group will continue to maintain their same usual conditions for the remainder of the study.
Experimental: Intervention; After completing 4 weeks of baseline data collection and random assignment in week 5, the intervention group will be instructed to increase intake of drinking water everyday in weeks 6-9 by a volume adequate to dilute urine specific gravity below 1.013 everyday, approximately 500 mL 4x/day.	Other: Drinking water to dilute urine; After Weeks 1-4 baseline data collection, people randomized in Week 5 will be instructed to increase drinking water to approx. 4 times 500 mL/d in Weeks 6-9 to reach a volume that is enough to dilute urine specific gravity below 1.013 everyday. The PWI exposure is approx. 20 mL/kg PWI for men, 25 mL/kg PWI for women. Addition of PWI to the diet may increase TWI and minimally displace some other beverage intake. Based on Adapt Study data, the hypo-osmotic share of TWI is expected to increase from below 50% to 50% or higher. The intervention dose is also aligned with observational data from the Paracelsus 10000 study in Salzburg, which found that healthy adults who met hydration criteria reported at least 1L/d PWI. The intervention will be communicated to participants in terms of L/d instead of mL/kg, because L/d are easier to translate into portion size and drinking behavior.

What is the study measuring?

Primary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Increase in thirst rating after overnight food and water restriction	Thirst rating on a visual analog scale ranging from 0 to 100 is expected to be significantly higher in Week 9, by +10 or more (out of 100) units, compared to a baseline rating below 70 (ensured by screening criteria). Participants will be asked to rate their thirst at the same time in the morning, each week, after food and water restriction at the weekly in-person study site visit.	For each study participant, change in thirst rating will be calculated as rating in Week 9 minus rating in Week 4. The mean absolute change in ratings and % of participants who increase ratings >70 will be described for intervention and control groups.

Secondary Outcome Measures

Outcome Measure	Measure Description	Time Frame
Change in serum, urine, saliva metabolomic profile	Methods will replicate those reported by the Adapt Study (Stookey et al, 2023): First-morning urine, post-water bolus urine, and serum specimen from Weeks 4 and 9 will be sent to the University of California Davis, West Coast Metabolomics Center (WCMC) for untargeted analysis of primary metabolism. MetaboAnalyst 6.0 software will be used to normalize and test for significantly different untargeted metabolomic profiles in Weeks 4 and 9, based on non-overlapping 95% confidence ellipses in Orthogonal Partial Least Square-Discriminant Analysis. The study hypothesizes overlap (no difference) between profiles for the intervention and control groups in Week 4; and No overlap (i.e. significant difference) between intervention and control in Week 9. For each participant, the within-person change in metabolite abundance between Weeks 4 and 9 will also be calculated. Changes in profile by intervention vs control will also be compared.	Week 4 vs. Week 9
Change in brain functional MRI (fMRI)	In Weeks 5 and 10, participants will have brain fMRI after overnight food and water restriction and also 30 min after drinking 500 mL water. Brain activation will be compared for loci considered important for the stimulation and/or inhibition of thirst (hypothalamus, medulla oblongata, midbrain, and cerebral cortex, anterior wall of the third ventricle, Brodmann area 32, pregenual anterior cingulate cortex, anterior midcingulate cortex), parahippocampal gyrus, inferior and middle frontal gyri, insula, and cerebellum). Intervention and control groups will be compared with respect to the overnight water restricted result (t=0), the 30-min post-500mL water challenge result (t+30), and the acute change from t=0 to t+30min, expressed as acute percentage change in slope. Week 10 (post-intervention) results will be compared to Week 5 results (baseline).	Week 10 compared to Week 5.
Change in zinc excretion	In Weeks 2 and 7, the stable (non-radioactive) isotopic tracer zinc-70 will be administered orally during the study center visit. The concentrations of zinc and zinc-70 will be measured in plasma and urine samples by ICP-MS using methods (Hall et al, 2006). Urinary zinc excretion over 2-weeks will be determined from 24-hour urine and first morning urine samples, and the kinetic relationship between plasma zinc exchange with tissues and urinary losses modeled using WinSAAM (University of Pennsylvania, Kennett Square, PA). The analysis will assume 70% absorption of the zinc tracer, given as less than 10 mg, in the overnight fasted state (Tran et al, 2004). Intervention and control group mean change in excretion will be compared.	Week 9 vs. Week 4
Change in total body protein breakdown	WBP turnover will be indexed by the end-product method. In weeks 4 and 9, participants will be given a single oral dose of 200mg 13C,15N-glycine, and collect all urine and record dietary intake for 24hrs. Per Hinde et al (2021), total nitrogen enrichment of urinary ammonia and urea will be measured. WBP flux (Q), protein synthesis, protein breakdown, and protein balance will be calculated as: Q(gN·kg-1·d-1) = d/(corrected tr:T)/24×body mass (BM); WBP synthesis (g·kg-1·d-1) = Q(E/24×BM)×6.25; WBP breakdown (g·kg-1·d-1) = Q(I/24×BM ×6.25; WBP balance (g·kg-1·d-1) = whole body protein synthesis-whole-body protein breakdown WBP: whole-body protein; Q = WBP flux, d = 15N oral dose (g glycine×0.1972), tr:T = ratio of tracer to tracee (corrected for background isotope enrichment), E = 24 h urinary nitrogen excretion, I = 24 h nitrogen intake, BM = body mass, 6.25 = conversion factor for nitrogen to protein.	Week 9 vs. Week 4

Collaborators and Investigators

• Sponsor: Jodi Stookey
• Collaborators: University of California, Davis; Paracelsus Medical University; University of Salzburg
• Investigators: Principal Investigator: Jodi D Stookey, PhD, Water and Hydration Translational Epidemiological Research, LLC; Principal Investigator: Markus Ritter, Paracelsus Medical University; Principal Investigator: Hubert Kerschbaum, PhD, Paris Lodron University of Salzburg; Study Director: Andrew Hall, PhD, University of California, Davis

Publications and helpful links

General Publications

• Greenleaf JE, Sargent F 2nd. Voluntary dehydration in man. J Appl Physiol. 1965 Jul;20(4):719-24. doi: 10.1152/jappl.1965.20.4.719. No abstract available.
• Nose H, Mack GW, Shi XR, Nadel ER. Role of osmolality and plasma volume during rehydration in humans. J Appl Physiol (1985). 1988 Jul;65(1):325-31. doi: 10.1152/jappl.1988.65.1.325.
• Kovarik JJ, Morisawa N, Wild J, Marton A, Takase-Minegishi K, Minegishi S, Daub S, Sands JM, Klein JD, Bailey JL, Kovalik JP, Rauh M, Karbach S, Hilgers KF, Luft F, Nishiyama A, Nakano D, Kitada K, Titze J. Adaptive physiological water conservation explains hypertension and muscle catabolism in experimental chronic renal failure. Acta Physiol (Oxf). 2021 May;232(1):e13629. doi: 10.1111/apha.13629. Epub 2021 Mar 7.
• Hinde KL, O'Leary TJ, Greeves JP, Wardle SL. Measuring Protein Turnover in the Field: Implications for Military Research. Adv Nutr. 2021 Jun 1;12(3):887-896. doi: 10.1093/advances/nmaa123.
• Hall AG, King JC. The Molecular Basis for Zinc Bioavailability. Int J Mol Sci. 2023 Mar 31;24(7):6561. doi: 10.3390/ijms24076561.
• Pena-Villalobos I, Narvaez C, Sabat P. Metabolic cost of osmoregulation in a hypertonic environment in the invasive African clawed frog Xenopus laevis. Biol Open. 2016 Jul 15;5(7):955-61. doi: 10.1242/bio.016543.
• Zorbas YG, Ichinose MN, Sakagamis MB. Fluid electrolyte changes in physically healthy subjects during prolonged restriction of motor activity and daily hyperhydration. Mater Med Pol. 1993 Apr-Jun;25(2):97-107.
• Kenney WL, Chiu P. Influence of age on thirst and fluid intake. Med Sci Sports Exerc. 2001 Sep;33(9):1524-32. doi: 10.1097/00005768-200109000-00016.
• Phillips PA, Rolls BJ, Ledingham JG, Forsling ML, Morton JJ, Crowe MJ, Wollner L. Reduced thirst after water deprivation in healthy elderly men. N Engl J Med. 1984 Sep 20;311(12):753-9. doi: 10.1056/NEJM198409203111202.
• Rosinger AY, Bethancourt HJ, Swanson ZS, Lopez K, Kenney WL, Huanca T, Conde E, Nzunza R, Ndiema E, Braun DR, Pontzer H. Cross-cultural variation in thirst perception in hot-humid and hot-arid environments: Evidence from two small-scale populations. Am J Hum Biol. 2022 Jun;34(6):e23715. doi: 10.1002/ajhb.23715. Epub 2021 Dec 23.
• Casa DJ, Armstrong LE, Hillman SK, Montain SJ, Reiff RV, Rich BS, Roberts WO, Stone JA. National athletic trainers' association position statement: fluid replacement for athletes. J Athl Train. 2000 Apr;35(2):212-24.
• Yancey PH, Clark ME, Hand SC, Bowlus RD, Somero GN. Living with water stress: evolution of osmolyte systems. Science. 1982 Sep 24;217(4566):1214-22. doi: 10.1126/science.7112124.
• Bourque CW. Central mechanisms of osmosensation and systemic osmoregulation. Nat Rev Neurosci. 2008 Jul;9(7):519-31. doi: 10.1038/nrn2400. Epub 2008 May 29.
• Greenleaf JE. Problem: thirst, drinking behavior, and involuntary dehydration. Med Sci Sports Exerc. 1992 Jun;24(6):645-56.
• Farrell MJ, Zamarripa F, Shade R, Phillips PA, McKinley M, Fox PT, Blair-West J, Denton DA, Egan GF. Effect of aging on regional cerebral blood flow responses associated with osmotic thirst and its satiation by water drinking: a PET study. Proc Natl Acad Sci U S A. 2008 Jan 8;105(1):382-7. doi: 10.1073/pnas.0710572105. Epub 2007 Dec 26.
• Stookey JD, Langthaler PB, Felder TK, Frey VN, van der Zee-Neuen A, Schindler K, Kedenko L, Iglseder B, Trinka E, Lang F, Haussinger D, Ritter M, Paulweber B. Hydration and health at ages 40-70 years in Salzburg Austria is associated with a median total water intake over 40 mL/kg including at least 1 L/d plain drinking water. Front Public Health. 2025 Nov 7;13:1668981. doi: 10.3389/fpubh.2025.1668981. eCollection 2025.
• Stookey JD, Paulweber B, Felder TK, Lang F, Haussinger D, Killilea DW, Kuypers FA, Ritter M. Change in Metabolomic Profile Associated with an Average Increase in Plain Water Intake of >+ 1 L/Day, Sustained Over 4 Weeks, in Healthy Young Men with Initial Total Water Intake Below 2 L/Day. Paracelsus Proc Exp Med. 2023;2(1):41-66. doi: 10.33594/000000619. Epub 2023 Apr 7.

Study record dates

Study Major Dates

• Study Start (Estimated): May 1, 2026
• Primary Completion (Estimated): August 31, 2026
• Study Completion (Estimated): December 31, 2027

Study Registration Dates

• First Submitted: March 5, 2026
• First Submitted That Met QC Criteria: March 5, 2026
• First Posted (Actual): March 11, 2026

Study Record Updates

• Last Update Posted (Actual): March 11, 2026
• Last Update Submitted That Met QC Criteria: March 5, 2026
• Last Verified: March 1, 2026

More Information

Terms related to this study

• Keywords: Randomized clinical trial; Metabolomics; Oura ring; Drinking water; Zinc isotope; Amino acid tracing (13C,15N-glycine); Deuterium elimination
• Other Study ID Numbers: PMU-EK-2025-0043

Plan for Individual participant data (IPD)

• Plan to Share Individual Participant Data (IPD)?: NO
• IPD Plan Description: Only de-identified data will be shared with research partners.

Drug and device information, study documents

• Studies a U.S. FDA-regulated drug product: No
• Studies a U.S. FDA-regulated device product: No