Q. My friend and I are having an argument about how much water we need to drink to prevent brain shrinkage. My friend says ten eight-ounce glasses. I grew up being told I needed eight eight-ounce glasses. Please settle this once and for all.
A. I am very pleased you are interested in drinking enough water. You might want to stop arguing, however. That is a stressor to both the brain and the immune system.
So, you are both correct, depending on what the temperature is, what you are doing, how you are dressed, and so on. In the USA, estimates are that most people over age 50 are chronically dehydrated and drink less than 1 quart (32 oz.) or 1 liter (1,000 ml) of water per day. Thirst sensation falls over age 50 and many eat when they are actually thirsty—but do not recognize it.
In addition, Mayo Clinic estimates are that the average adult loses more than 80 oz. or 2365 ml of water every day through sweating, breathing, waste elimination, which puts them 48 ounces or 1420 ml in deficit compared to amount of water taken in. And as the water level in the brain cells falls, brain tissue can begin to shrink and pull away from the skull. Dehydration is linked with dementia. Just a one percent level of dehydration (and at the point you probably are not even thirsty) results in a five percent reduction in cognition.
Doctors I know have suggested that it is easier to just stop trying to count how many ounces or milliliters of water you are drinking each day. Instead, just drink enough so you pee one or two pale urines per day. Personally, I find that a much easier way to track my level of hydration.