Ask Dr. Gary: No need to worry about water intake

By Gary Payinda, MD

It has been advocated to drink at least two to three glasses of water a day. The reason, I have been told, is "to flush out the impurities in the body". Could you please comment on this? - JVS

I usually hear about the dubious "eight glasses a day". I'm glad to see that the number has been whittled down to a more reasonable two or three.

I once actually met a lady who said she drank at least eight glasses of water every day. That's a lot of drinking. She was healthy enough, but that was probably more despite the eight glasses, than because of it. She probably would have been perfectly fine with two. And she wouldn't have had to spend so much time in the toilet.

The danger with drinking very large amounts of water is that you dilute your normally salty blood, and eventually lose too much salt in your urine. Your sodium level drops, you get weak and confused, and if you keep drinking excess water, you can eventually suffer convulsions and sometimes even die.

I've taken care of a handful of serious water intoxication cases over the past few years, so it happens, but it's not common: babies given water rather than breast milk or formula, kids with copious diarrhoea drinking plain water, long-distance runners sweating out salt but replenishing only with water, mentally ill patients drinking huge amounts, and the occasional elderly patient with an extremely poor diet low in sodium. But these are fairly rare, because the kidney is smart. It safely micro-manages our blood's sodium levels despite everything we throw at it: alcohol, toxins, medications, dehydration and even water overload.

The X-amount-of-glasses-a-day myth probably had its origins in something factual, but got twisted with each re-telling. Even the seemingly excessive eight glasses-a-day rule of thumb might not be so excessive if we apply it as it was likely intended: not just to drink, but food too. Once we learn an apple contains 70 per cent water, or roast chicken 50 per cent water, it becomes obvious that we actually eat a lot of our water each day. Most of us drink an extra few glasses of water a day, and do fine.

The accurate answer is that water intake depends on too many variables for one number to do justice to them all. The things that matter most are your activity level, the ambient temperature, the water content in your foods, your size and muscularity, and your kidney function.

The only real concern is that you drink enough water to allow your kidneys to do their job: keeping your blood salts balanced and producing enough urine to dissolve, dilute, and excrete the waste that your cells produce.

The simplest and best rule of thumb I can give readers: drink enough water to satisfy your thirst and give you a urine that is clear or light yellow, not dark and overly concentrated.

Gary Payinda, MD, is an emergency medicine consultant in Whangarei.

If you have a science or health topic question you'd like addressed, email

(This column provides general information and is not a substitute for the advice of your doctor.)

- The Aucklander

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