One of the uncomfortable but empowering lessons of going on a skeptical journey to understand the world we live in a little better is the realization that most bits of conventional wisdom floating around in the public consciousness are wrong. Essentially, if you picked up knowledge from popular culture rather than a reliable source, it is likely not true. In fact, many mostly reliable sources get fooled at times as well. I am constantly running into bits of misinformation clogging my brain that have to be flushed out.

Also, “right and wrong” are not absolute binaries. Sometimes the “wrong” information may be oversimplified or lack appropriate nuance or context. It’s hard to boil down a complex scientific question to a one-line meme that can spread through popular culture, and reality is always way more complex than you think.

Here is one meme that deserves to be flushed out – the notion that people generally should drink eight glasses of water per day. This is not a particularly harmful piece of misinformation (although it is not completely benign), but it is a good object lesson on why we should be generally skeptical of simplistic medical advice.

The origin of the myth is known – in 1945 the U.S. Food and Nutrition Board determined that adults need 84.5 ounces of water per day to remain healthy. However, this advice became distorted. First, the next sentence was the acknowledgment that “most” of this water will come from food. About 20% of water intake comes from actual food. But also, most of the things we drink are mostly water. Even coffee is mostly water, and no, it will not dehydrate you because of the caffeine. The water intake is far greater than the mild diuretic effect.

I supposes to enhance communication, the 84.5 oz. became 64, because that can be translated into the 8×8 rule – eight glasses of eight ounces of water. And that was that – a misinterpreted statement became an enduring myth. But there is another layer also. This myth has been promoted by the bottled water industry. Obviously they have a vested interest in people drinking more of their product, and they didn’t invent the myth, but they have happily promoted it.

The 8×8 myth persists, despite the fact that it is widely debunked online by numerous reasonable sources. It is a favorite myth to debunk for many medical sites. But it also remains widely spread. What is true is, of course, more complex.

First it is worth pointing out that there is zero evidence for the recommendation of eight glasses of water per day. It is based on no scientific information. There is no evidence of health benefits from following this rule, or harm from ignoring it. Also, as you probably have guessed, reality is far more complex than this one generic recommendation. As has been pointed out by numerous sources for years – hydration needs vary tremendously from person to person. They depend on the kind and amount of food consumed, the size of the person, physiological and medical parameters, the environment, and physical activity.

This was all known based upon basic understanding of human physiology. A recent study, however, sought empirical evidence to back this up. They studied what they call “water turnover”, which is essentially how much water people require from all sources. Here is their summary:

Water intake requirements largely reflect water turnover (WT), the water used by the body each day. We investigated the determinants of human WT in 5604 people from the ages of 8 days to 96 years from 23 countries using isotope-tracking (2H) methods. Age, body size, and composition were significantly associated with WT, as were physical activity, athletic status, pregnancy, socioeconomic status, and environmental characteristics (latitude, altitude, air temperature, and humidity). People who lived in countries with a low human development index (HDI) had higher WT than people in high-HDI countries.

So, how can we know how much water we need to drink to stay well-hydrated? Fortunately, evolution has perfected a method over hundreds of millions of years – it’s called thirst. Here is some nuanced advice for most people: Drink when you are thirsty. Make sure that you have access to fluid so that you can drink when you are thirsty. This is especially true if you are going to be in a particularly dry or hot environment, or engage in activity that will make you sweat. In such situations you may want to pre-hydrate a bit because you can get dehydrated very quickly when you are not acclimated to them. You don’t have to drink pure water. Electrolyte drinks are good, and may even be preferred if you will not also be eating to replace electrolytes. Other sources of fluids are fine.

There are also specific medical conditions that may require people to either limit their fluid intake or drink more water than usual. If you have a history of kidney stones, for example, you will want to keep your urine pale. If you have any questions, speak to your doctor.

Is there any potential downside to overhydrating? Yes, although it is difficult to cause direct harm. You can overhydrate to the point that you dilute out your electrolytes, but it takes a lot of water to overwhelm your body’s homeostatic mechanisms. Drinking too much water can disturb your sleep because you may need to get up at night to urinate. This is not a trivial thing – chronically poor sleep has many medical downsides. Using lots of single-use plastic water bottles is also harmful for the environment.

But perhaps the greatest harm that I see in my practice is simply useless misinformation confusing and distracting people who want to live a healthy lifestyle. When a patient tells me they are trying to lead a healthier lifestyle then rattles off a list of things they are doing, most of the list is useless. This list often includes drinking eight glasses of water per day. I would rather they focus on the things that will actually improve their health, rather than being lulled by misinformation into a false sense of being healthy.

This myth also fosters a simplistic, and wrong, understanding of human physiology. It is hard to measure the harm of such things, but it’s not helpful.

Author

  • Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.

Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.