Somewhere more than zero and less than 20 liters*

It’s just about the easiest, safest medical advice you can give: “drink more water”.  You have a headache?  Drink more water.  Tired?  Drink more water.  Cold coming on? Drink more water. Tom Brady famously attributed his QB longevity to water drinking, among some other less… ordinary practices.

I’m a nephrologist – a kidney doctor. I think about water all the time. I can tell you how your brain senses how much water is in your body and exactly how it communicates that information to your kidneys to control how dilute your urine is. I can explain the miraculous ability of the kidney to concentrate urine across a range from 50 mosm/L to 1200 mosm/L – and the physiology that makes it all work.

But I can’t really tell you how much water you’re supposed to drink.

And believe me, I get asked all the time.

There are a couple things I’m sure of when it comes to water.  You need to drink some. 

Though some animals, like Kangaroo rats, can get virtually all the water they need from the food they eat, we are not such animals. Without water, we die.

I’m also sure that you can die from drinking too much water. Drinking excessive amounts of water dilutes the sodium in your blood which messes with the electrical system in your brain and heart. I actually had a patient go on a “water cleanse” and give herself a seizure.

But, to be fair, assuming your kidneys are working reasonably well, and you’re otherwise healthy, you’d need to drink around 20 liters of water a day to get into mortal trouble.  The dose is the poison, as they say.

Ok so somewhere more than zero and less than 20 liters of water is the amount you should be drinking a day. That much I’m sure of.

But the evidence on where in that range you should target is actually pretty skimpy.  You wouldn’t think so if you look at the wellness influencers online – with their Stanley’s and strict water intake regimens. You’d think the evidence that drinking extra water is a benefit is overwhelming.

The venerated National Academy of Medicine suggests men drink 13, 8 oz cups a day (that’s about 3 liters) and women drink 9, 8 oz cups a day (a bit more than 2 liters).  From what I can tell, this recommendation – like the old “8 cups of water per day” recommendation is pulled out of thin air.

I’m not arguing that we shouldn’t drink water. Of course water is important. I’m just wondering what data there is to really prove that drinking more water is better.

Fortunately, a team from UCSF has finally done the leg work for us. They break down the actual evidence in this paper, appearing in JAMA Network Open.

The team scoured the medical literature for randomized controlled trials of water intake. This is critical – we don’t want anecdotes about how clear your skin got after you increased your water intake.  We want icy cold, clear data.  Randomized trials take a group of people and, at random, assign some to do the intervention – in this case drinking more water – and others to… well, not.  Or in this case, to keep doing what they’d normally do.

The team reviewed nearly 1500 papers but only 18 (!) met the rigorous criteria to be included in the analysis as you can see from this… flow chart.

This is really the first important finding – there are not many high-quality studies that investigate how much water we should drink. Of course, water isn’t some prescription product – so funding is likely hard to come by. Can we do a trial of Dasani?

In any case, these 18 trials were all looking at different outcomes of interest. Four looked at the impact of drinking more water on weight loss, two on fasting blood glucose, two on headache, two on urinary tract infection, two on kidney stones, and 6 on various other outcomes.  None of the studies looked at energy, or skin tone, or overall wellness, though one did measure a quality of life score.

And if I could sum up all these studies in a word, that word would be “meh”.

For weight loss – one of the four studies showed that increasing water intake had no effect on weight loss. Two studies showed an effect but drinking extra water was combined with a low-calorie diet so that feels a bit like cheating to me. One study randomized participants to drink half a liter of water before meals and that group did have more weight loss than the control group – about a kilogram more over 12 weeks. That’s not exactly Ozempic.

For fasting blood glucose, one trial suggested increasing premeal water decreased glucose levels, the other (which looked just at increasing water overall) didn’t.

For headache – and, cards on the table here, I’m a big believer in water for headaches – one study showed nothing, and the other showed that increasing water intake by 1.5 liters per day improved migraine-related quality of life, but didn’t change the number of headache days per month.

For urinary tract infections – one positive trial, one negative one.

The best evidence for more water drinking came from the kidney stone trials. Increasing water intake to achieve more than two liters of urine a day was associated with a significant reduction in kidney stone recurrence. I consider this a positive control more or less – you would be hard pressed to find a kidney doctor who doesn’t think that people with a history of kidney stones should drink more water.

What about that quality of life study? 141 people were randomized to either drink 1.5 liters of extra water per day or not. And the scores on the quality of life survey were no different between those two groups six months later.

Thirsty yet?

So what’s going on here? There are a few possibilities.

First, I need to point out that clinical trials are really hard. All the studies in this review were relatively small, with most enrolling less than 100 people. The effect of extra water would need to be pretty potent to detect it with those small samples.

But I can’t help but point out that our bodies are actually exquisitely tuned to manage how much water we have in us. As we lose water throughout the day – from sweat, from exhalation – our blood becomes a tiny bit more concentrated – the sodium concentration goes up. Our brains detect that and create a sensation we call thirst. Thirst is one of the most powerful drives we have. Animals, including humans, when thirsty, will choose water over food, over drugs, and over sex. It is incredibly hard to resist and, assuming we have ready access to water, there is no need to resist it. We drink when we are thirsty. And that may be enough.

Of course, pushing beyond thirst is possible. We are sapient beings who can drink more than we want to. But what we can’t do, assuming our kidneys work, is hold onto that water. It passes right through us. In the case of preventing kidney stones, this is a good thing – putting more water in your body leads to more water coming out – more dilute urine – which means it’s harder for stones to form. But for all that other stuff? The wellness, the skin tone, and so on?  It just doesn’t make much sense.  If you drink an extra liter of water, you pee an extra liter of water.  Net net? Zero.

Some folks will argue that the extra pee gets rid of extra toxins or something like that, but – sorry kidney doctor Perry here again – that’s not how pee works.  The clearance of toxins from the blood happens way upstream of where your urine is diluted or concentrated.

If you drink more, the same toxins come out, just with more water around them.  In fact, one of the largest studies in this JAMA Network Open review assessed whether increasing water consumption in people with chronic kidney disease would improve kidney function.  It didn’t.

I am left then, with only a bit more confidence than when I began.  I remain certain you should drink more than zero liters and certain you should drink less than 20 (assuming you’re not losing a lot of water in some other way like working in the heat).  Beyond that, it seems reasonable to trust the millions of years of evolution that have made water homeostasis central to life itself.  Give yourself access to water. Drink when you’re thirsty. Drink a bit more if you’d like. But no need to push it. Your kidneys won’t let you anyway.

A version of this commentary first appeared on Medscape.com.

*Ok just a little note here to say that, 20 liters of intake is the amount that will kill you assuming you aren’t losing a ton of water through sweat or something. Even then, replacing sweat with only water can lead to electrolyte imbalances and get you into a world of trouble. Even Tom Brady doesn’t drink this much. I was just trying to establish an inarguable upper limit. Be reasonable, folks.