I’m sure you have all read or seen it — drink eight glasses (about two liters) of water a day to stay healthy and hydrated. I see people carrying their Nalgene bottles of water constantly drinking and, of course, heading to the bathroom to eliminate some of that water.
This is one of those myths that keep getting repeated so that you might think was actually true, like walking those 10,000 steps every day.
As the old feathered dinosaur is meant to do, let’s take down this myth.
The eight glasses of water myth
The actual notion of eight glasses a day originates from a 1945 US Food and Nutrition Board which recommended 2.5 liters of water daily. However, there are two key points to make about this recommendation — first, it wasn’t based on any scientific research, and second, the water intake could come from food sources, which contain a lot of water.
Of course, almost no one who pushes the eight glasses claim to know anything about the origin story. The myth continues today possibly from some vague idea that you need to drink water to flush toxins out of your system. Or some may suggest that drinking water is good for your kidneys since they filter the blood and regulate water balance. Or possibly it comes from the fact that we lose two liters of water from our bodies every day (eight glasses of water is roughly two liters).
Nevertheless, I’m sure many of you subscribe to drinking lots of water, without knowing if it makes scientific sense. So, our next step is to tear down this claim.
One of my first issues with the eight glasses is that people have different sizes with different losses of water. A 50 kg woman might need to consume a lot less water than a 150 kg man. But that’s a basic issue.
But setting that basic issue aside, the most important point that we need to understand is that you can get the full two liters of water through a normal diet. You will get that from foods, like fruits and vegetables, from your coffee, tea, and soft drinks, and from that delicious burger. Drinking an additional eight glasses of water will do nothing else but make you run to the restroom very frequently.
Your kidney has a few basic functions, one of which is to maintain fluid osmolality. Thus, when you consume excess water the kidney detects that the osmolality is low, so it must pull water out of the body. And drinking an additional two liters of water is just going to give your kidney a workout.
Now, someone might say that this excess water could clean the toxins out of the kidneys or something similar. But here are some important facts — your kidney filters about 180 liters of blood every day. Two liters of water are, I’m sorry that I have to put it this way, two liters of water are a mere drop in the bucket for your kidneys. It’s not going to have a material impact on your kidneys, except to push more water into your bladder.
And here’s some science
I know you come here for published papers that support my claims or debunk other claims.
In a paper published in JAMA on 8 May 2018, researchers examined this claim in a randomized clinical trial. The researchers randomized 631 patients with kidney disease into a hydration group that was coached to drink more water and a control group that was coached to maintain their current habits. They only studied individuals with kidney disease because one would expect a change in kidney function if hydration worked.
The researchers found that drinking more water did not offer any benefit in terms of kidney function. The authors concluded:
Among adults with chronic kidney disease, coaching to increase water intake compared with coaching to maintain the same water intake did not significantly slow the decline in kidney function after 1 year.
Of course, if your diet is pure potato chips and candy bars, you will need to consume additional water. On the other hand, if your diet is nothing but potato chips and candy bars, you might have other issues to worry about.
There is no medical evidence that drinking that much water is beneficial to your health. Part of this myth is to claim that you need to keep drinking water so that your urine is crystal clear. But that’s not based on anything in medicine or science — pale yellow urine is a sign of good health.
Don’t worry about your urine and kidneys — drinking eight glasses of water a day is not going to make you any healthier. And not drinking that two liters of water are not going to make you any less healthy.
- Clark WF, Sontrop JM, Huang SH, Gallo K, Moist L, House AA, Cuerden MS, Weir MA, Bagga A, Brimble S, Burke A, Muirhead N, Pandeya S, Garg AX. Effect of Coaching to Increase Water Intake on Kidney Function Decline in Adults With Chronic Kidney Disease: The CKD WIT Randomized Clinical Trial. JAMA. 2018 May 8;319(18):1870-1879. doi: 10.1001/jama.2018.4930. PMID: 29801012; PMCID: PMC6583759.