Hydration and Longevity

In my video on how much water we should drink, I trace the origins and bust the myth of the “drink at least eight glasses of water a day” recommendation, as well as discuss the difficulty of establishing cause and effect in the multitude of studies correlating low water intake with a wide range of diseases. There are a lot of confounding factors. For example, who drinks a lot of water? People who exercise more! Then there’s the issue of reverse causation. Are people sick because they don’t drink enough water, or are they not drinking enough water because they’re sick?

There have been a few large prospective studies in which fluid intake is measured before disease develops. For example, dehydration is associated with a steeper decline in cognitive function in older adults over time. There are interventional studies showing acute mental and mood benefits to water consumption, but it’s hard to rule out the placebo effect. (Even just changing the color of water has been shown to affect how people feel!)

What about longevity? The Adventist Health Study found that those drinking five or more glasses of water a day had about half the risk of dying from heart disease compared to those who drank two daily glasses or less, even after controlling for other factors such as diet and exercise. Perhaps the extra hydration lowered blood viscosity (thickness) or they were benefiting from heart-friendly minerals like magnesium found in “harder” tap water. Two subsequent studies supported the Adventist finding. One in Japan echoed significantly less cardiovascular mortality, and one in the U.S. suggested underhydration may be quadrupling chronic disease mortality and contributing to more than 100,000 deaths each year. Four other studies, however, failed to replicate the findings, uncovering no differences in mortality between those who drank more water versus less, so the link between hydration and longevity remains murky.