Water in our body works like brake fluid in a car: as long as there's enough of it, all mechanisms move smoothly, but once the level drops, every part starts wearing out faster. Scientists decided to check how much this wear accelerates if a person is chronically underhydrated, and how it can be spotted from a routine blood test.
Researchers took data from 363,286 people in the Israeli health system Leumit and 24,611 participants in the U.S. NHANES survey. All had their serum sodium levels measured, a marker that rises when the body lacks water. Then they calculated biological age for each person and compared groups with different sodium levels, all within the medical normal range.
The results were clear. Among Israeli women, the difference in biological age between those with low sodium and those with high sodium was 2.32 years. In men the effect was weaker, 0.88 years, but in the U.S. data men showed a difference of 1.92 years. The link was especially strong in people under 50: in women under 30, the difference reached three years.
Previously, it was known that severe dehydration harms health, but the link between mild water shortage and the rate of aging remained controversial. Now it's been shown that even the upper limit of normal sodium is associated with a biological age one to three years older. Comparing two large independent samples makes this conclusion reliable.
The practical benefit is obvious: a sodium test is already part of standard blood biochemistry, so doctors can immediately identify people at risk of accelerated aging and advise them to drink more water. Whether increasing water intake can reverse the wear that has already accumulated remains an open question, but as a cheap preventive measure, hydration has no competitors.
In Israel, the law prohibits medical organizations from denying membership based on health status, so the Leumit sample suffers almost no selection bias, which is common in voluntary studies.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.