Hydration

Research summary

• Hydration's evidence base for longevity is thinner and more mixed than commonly assumed, with the most direct test of the question returning a null result.
• A prospective NHANES-based U.S. cohort of over 24,000 adults followed for a median of 11.4 years found no survival advantage associated with higher total or plain water intake in either men or women -- if anything, women in the highest intake quartile had a slightly elevated (non-significant) mortality risk the authors called possibly spurious.
• The strongest positive finding comes from a different angle: a 25-year analysis of the Atherosclerosis Risk in Communities cohort (n=15,752) used serum sodium as a proxy for long-term hydration habits and found middle-aged adults with high-normal serum sodium (>142 mmol/L) had a 39% higher risk of developing chronic disease and a 21% higher risk of premature death, alongside up to 50% higher odds of being biologically older than their actual age -- though the authors explicitly state intervention studies are still needed to confirm hydration itself is the cause.
• On cognition, results are inconsistent: a longitudinal study using the Berlin Aging Study II found objectively measured dehydration predicted steeper decline in cognitive function and wellbeing over time in older adults, while a separate study of 60 free-living elderly volunteers found no significant relationship between hydration status and cognitive test performance, though that sample was already well-hydrated overall.
• Given a direct mortality study found no benefit, a large observational proxy study found a real but indirect signal, and cognitive findings are inconsistent, hydration is best treated as a plausible but currently under-evidenced longevity factor rather than an established one.