Dear Doctor: There is an ongoing debate in my house about fluid/water intake to keep hydrated. Doesn't it make sense to drink water primarily when you feel thirsty? If not, how much do you need?

Dear Reader: Many of my patients ask how much water they should drink per day, and I give different estimations based upon their individual kidney and heart function. But I feel somewhat at a loss on what to tell them in a broad sense.

Current medical belief holds that an individual should drink eight 8-ounce glasses of water per day for a daily total of a half-gallon, or roughly 2 liters. Although thirst might logically seem to be a good indicator, some people are less likely to get thirsty due to illness, age or simply because they've grown accustomed to drinking little water. So it's better to keep an eye on total intake.

But the science on that intake is based on very few studies.

One 2002 study asked 20,297 Seventh-day Adventists -- average age of 58 with no history of diabetes -- about daily fluid intake, then followed them for six years. Men who drank five or more glasses of water per day had a 64 percent reduction in fatal heart disease compared to men who drank two glasses or fewer. Women who drank five or more glasses of water per day had a 41 percent reduction compared to those who drank two glasses or fewer.

When the researchers assessed intake of other fluids (coffee, tea, milk, juice, soft drinks), they found an opposite correlation. Men who drank more than five glasses had a 46 percent increased risk of fatal heart disease compared to men who drank two glasses or fewer. In women, this increase was a dramatic 247 percent.

A 2010 study from the Netherlands asked 120,852 people about their fluid intake, then followed them for 10 years. Here, neither increased water intake nor total fluid intake was correlated with a risk of fatal heart attacks or with strokes.

Then there was a 2012 study of 465 stroke patients who were followed for 17 months. It found a 27 percent decreased risk of stroke, heart attack or death among people who drank 2 or more liters of total fluid per day versus those who drank less.

As for older adults, a 2017 study assessed fluid intake among 1,055 Australian women over the age of 70, then followed them for 10 years. No association was found between death rates and either water intake or total intake of fluids.

Notably, in various other studies, higher water intake has been linked to a lower rate of kidney disease. Because kidney disease has been associated with higher rates of cardiovascular disease, this connection may be why there is less death from heart disease among people with higher fluid intake. This may be more applicable to an older population.

Of course, the amount of water you need to drink depends on losses of water from sweating or from illness and from fluid obtained via food (soups, vegetables, fruits and so on). Although the data are hardly ironclad, it seems to me that five glasses of water per day is healthy.

Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. 

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