On a cool morning in Salzburg, a 52-year-old office worker arrives at a clinic for a routine health study visit. She hasn’t eaten for 10 hours, her urine sample is collected, and her blood is drawn. She thinks her hydration is “fine”—after all, she drinks tea, coffee, and eats fruits daily. But when her results come back, she’s surprised: she’s not technically “hydrated,” even though she feels normal.

She’s not alone.

A new population-based study from the Paracelsus 10,000 cohort suggests that most adults aged 40–70 are not drinking plain water enough to maintain healthy hydration—especially those already living with chronic health conditions. And the findings help answer a persistent public health question: How much plain drinking water does the average adult actually need?

This study, published in Frontiers in Public Health, offers rare, objective data on hydration and chronic health conditions among free-living adults in Salzburg, Austria. It shows that healthy, fully hydrated adults typically drink at least 1 liter of plain water per day, often more.

Why Hydration Matters in Midlife

Hydration isn’t just about preventing thirst or short-term fatigue. By midlife, water intake is linked to major health outcomes:

• metabolic health
• kidney function
• cardiometabolic risk
• obesity and insulin resistance
• cognitive performance

Yet most hydration guidelines only cover short-term physiological needs—not long-term health. The researchers used strict criteria:

• Serum tonicity between 285–294 mOsmol/L
• Urine specific gravity <1.013

Only participants who met these hydration cutoffs and had no chronic health conditions were classified as Healthy + Hydrated.

Out of 5,817 adults, only 15% of women and 7% of men met both health and hydration criteria. This alone highlights a public health challenge: most midlife adults are either living with chronic disease or are underhydrated or both.

Hydrated Adults Drink More Plain Water

We often assume total water from food, coffee, tea, juice, and meals is enough. This study suggests otherwise. Across all adults in the sample:

• Median plain water intake (PWI) for women: 1.1 L/day
• Median PWI for men: 0.7 L/day

But among Healthy + Hydrated adults, intake was meaningfully higher:

• Women: 1.5 L/day (22 mL/kg)
• Men: 1.3 L/day (17 mL/kg) fpubh-13-1668981

Half of these optimally hydrated adults drank more than 1 liter/day, and none reported drinking zero plain water.

In other words: Plain water—not tea, coffee, juice, or food—was key to meeting hydration thresholds.

Total Water Intake (TWI) Matters Too

Healthy + Hydrated adults had a median TWI around 2.9 L/day for women and 3.0 L/day for men. Notably, these intakes exceed many national guidelines.

A Surprising Risk Pattern

The researchers used Poisson regression to test who was most likely to have lower water intake (defined as PWI <20 mL/kg and TWI <45 mL/kg). Compared to Healthy + Hydrated adults, the risk of drinking too little water was:

• 25–40% higher among adults who were healthy but not hydrated
• 30–40% higher among those with chronic conditions
• 50%+ higher among those who were both unhealthy and underhydrated

Even after adjusting for physical activity, medications, season, kidney function, and dietary solute load, the relationship held. Plainly: people with chronic health burdens tend to drink less water—and that may make their conditions worse.

What This Means in Practice

For local health departments

• Integrate hydration screening (tonicity + USG if possible) into midlife preventive care.
• Emphasize plain water in education campaigns—separate from total fluid intake.

For clinicians

• Encourage adults with hypertension, insulin resistance, or metabolic syndrome to assess water intake as part of lifestyle counseling.
• Reinforce that coffee, alcohol, or sugary beverages do not substitute for water metabolically.

For community-based programs

• Add hydration prompts to chronic disease self-management curricula.
• Partner with workplaces and senior centers to promote access to clean, palatable water.

Why Plain Water is Different

Many public health messages claim that “all beverages hydrate.” While technically true for basic fluid balance, this study points to metabolic and physiological differences:

• Plain water is hypotonic (<20 mOsm/kg).
• Most beverages are isotonic or hypertonic (>285 mOsm/kg).

Hypertonic drinks pull water out of cells, raise plasma osmolality, and trigger water conservation mechanisms. Plain water does the opposite—moving into cells rapidly and promoting normal fluid balance.

So, How Much Water Should Midlife Adults Drink?

Based on these real-world data:

• At least 1.0 L/day of plain water appears necessary for hydration and health.
• 1.5 L/day is closer to the median among Healthy + Hydrated adults.
• Intakes above ~2.2 L/day were uncommon but still within the healthy range.

These amounts align with:

• French and Austrian hydration guidelines
• Metabolic benefits shown in U.S. clinical trials increasing water intake by 1–1.5 L/day
• Global data linking low hydration to chronic disease risks

Barriers & Open Questions

Barriers

• Older adults may struggle with thirst perception.
• Coffee and alcohol culture can crowd out water.
• Workplace access varies.
• Many believe food, tea, and juice provide adequate hydration.

Open Questions

• What is the causal impact of increasing plain water intake long-term?
• How do hydration needs shift across climates and occupations?
• Can hydration-focused interventions reduce chronic disease incidence?

Randomized trials in Austria—and globally—are needed to test 1.0, 1.5, and 2.0 L/day water interventions under real-life conditions.

Call to Action: What About You?

• How much plain water do you drink on a typical day?
• Could your organization integrate hydration checks into routine preventive care?
• What might stop your community members from drinking enough plain water—and how can we help them overcome it?

Hydration is one of the simplest, lowest-cost health interventions available. This study reminds us that plain water still matters, especially as we age.