How much water do you really need?

It’s an old question about one of the most essential components of our diet, yet the answer is surprisingly debatable. Eight glasses a day? To satisfy thirst? So that your urine is clear?

Our body is 60 percent water, and water is essential to every organ system and every cell. Water keeps our temperature stable, clears waste through our kidneys, it pumps blood through our vessels – there’s no function in our body that doesn’t rely on water.

Since water is so critical to bodily functions our body maintains the same blood concentration over wide variations of fluid intake. It’s only when our body is missing significant amounts of water that we’ll see abnormalities in blood tests – and in our ability to function normally.

Dehydration, the state in which water is depleted to the point our body can’t maintain its normal functions is clearly dangerous.

But does being optimally hydrated make a difference? Do smaller variations in fluid intake – the kinds of variations we experience in everyday life and ones that are no way close to clinical dehydration – make a difference?

Although blood samples show little variation in concentration across wide differences of fluid intake, our urine becomes more and more concentrated the less fluid we have – that’s why urine might be a better marker of hydration. Studying urine samples suggests that half the kids in the US have highly concentrated urine, which suggests they’re not drinking much.

Mild fluid deficits, do they matter?

What are the health implications of milder fluid deficits?

A few studies suggest that mild dehydration affects short-term memory in school-aged kids, and on the other hand, when kids are given plenty of water, their memory and attention improves.

A new study, led by Naiman Khan and published in the Journal of Nutrition tests the effect of hydration on cognitive performance. 75 kids aged 9-11 years from Illinois were tested in three conditions for 4 days each: They were drinking as usual (no intervention), were prescribed low water intake (16 oz. a day or about 2 glasses), or high water intake (84 oz. a day or about 10 glasses). On the fourth day of each condition urine was collected to measure its concentration and cognitive tests were performed to assess performance, thought flexibility and inhibition.

And the results: Urine got less concentrated the more fluids the kids took in, and higher fluid intake benefited kids’ working memory and cognitive flexibility — according to the the mental tests.

These findings are consistent with findings from other studies that show better hydration improved exercise ability and cognitive flexibility.

If half of the kids have concentrated urine — signifying insufficient hydration — this study suggests that increasing fluids may be a simple way to help kids think a little better.

Just add water

Our brain is even more water than the rest of our body: it’s 70 percent water. Missing some of it may affect mood, thinking, memory and overall health.

Mood and attentiveness decline first, and while mild changes in body fluids certainly don’t put kids in danger of dropping blood pressure or shutting off their kidneys, proper hydration can help kids perform best at school.

Giving kids access to clean and safe drinking water – safe water fountains in every school! – and prompting them to drink is a really simple way to make sure studying’s a little bit easier and happier.

Optimal hydration may improve overall wellbeing and clarity of mind, and can be a cost- and calorie-free mental boost.

Hydration tips:

How much: 8 (glasses-a-day) isn’t a magic number – so no need to keep count – but is pretty good approximation for an average person in average conditions. The Institute of Medicine (IOM) doesn’t set exact intakes, but recommends about 91 ounces of total water a day for women, and 125 ounces daily for men – and no upper limit for water. Eight glasses is about 64 ounces, add to that the water in food, and you’re not far off from the IOM recommendations. Water needs depend, of course, on activity level, outdoor temperature etc.

What counts: All fluids count, including coffee, tea, soup and the plenty-of water in fruits and veggies (another thing kids are not eating enough of).

Assessing hydration: You should rarely feel thirsty, and your urine should be almost colorless. 

Drink More: Provided you’re healthy, your kidneys know how to take care of your fluid levels. Drinking too little can lead to headaches, dehydration, constipation and maybe crankiness, while extra fluids are simply excreted through your urine – it’s difficult to overdrink. Only in unusual circumstances – such as extreme endurance exertion while overdrinking water – can drinking too much water become a problem for healthy people.

Dr. Ayala

Full disclosure: I co-founded Herbal Water, which makes organic herb-infused waters that have zero calories and no sugar or artificial ingredients. I’m also a pediatrician and have been promoting good nutrition and healthy lifestyle for many years.