Water in the body works like oil in an engine: as long as there's enough, all the parts move smoothly, but when the oil runs low, the parts grind and wear out. Scientists wanted to find out how much faster a person ages if they constantly fall short on fluids, and whether a routine blood test can reveal it.
They took data from 363,286 people in Israel and 24,611 people in the US. Everyone had their blood sodium level measured, a number that rises when water is in short supply. Then the researchers calculated biological age and compared people with low and high sodium, though all of them were within the normal range.
It turned out that women with high sodium had a biological age 2.3 years older, and men in the US, 1.9 years older. The effect is especially noticeable in the young: among women under 30, the gap reached three years. That means even mild dehydration, the kind a doctor wouldn't call a disease, speeds up aging. Sodium testing is already routine for everyone, so a doctor can spot the risk right away and suggest drinking more water.
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.