A recent editorial in the Journal of the American Society of Nephrology reviewing all the evidence to back up the claims of “drink 8 glasses of water a day” drew my attention to the whole subject of “medical myths”. Every physician could attest to episodes of irate true believers refusing to accept any criticism or skepticism. To my astonishment, I came across many physicians who vehemently believe in those urban myths. It is a curious observation of mine. There exists an inverse correlation between the amount and quality of the evidence and temperature: the less light, the greater the heat. It is as if you attacked a central tenet of their beliefs, shaking up their view of the world. Even in scientific meetings, I have witnessed many heated arguments, where hand waving and personal insults were substitutes for solid data.

So with this in mind, I am going to put my life on the line and try to debunk as many medical myths that come to mind. Feel free to send poison mail. Or maybe, if you can think of a myth, email me and I promise to protect your identity.


The claims that drinking 8 glasses of water a day is good for you

There are numerous claims of health benefits of drinking lots of water, but the most oft-repeated ones are:

  • It increases your skin tone, keeps it hydrated, and gives it a youthful appearance.
  • It washes out toxins accumulating in the body.
  • It suppresses appetite and aids in weight-loss diets.
  • It prevents sports injuries.
  • And finally, the blanket claim: It increases longevity.


Where did those claims come from?

Here are some fascinating historical findings, some of which I found in Slate Magazine’s The Explainer column by Nina Shen Rastogi:

  • How long was this myth around? Answer: the 8×8 (eight 8 oz glasses) myth going all the way back to 1796, in a German text by Dr. Christoph Wilhelm Hufeland called Makrobiotik. The book includes an anecdote about the surgeon general to the king of Prussia, a vibrant 80-year-old man who had “contracted the habit of drinking daily from seven to eight glasses” of cold water and thus “enjoyed much better health than in his youth”. (An English translation of this book was published 1843.)
  • Fast forward to the 19th century: The hydrotherapy craze that swept through Europe and then America in the late 19th century encouraged the notion that people needed to be drinking more water.
  • By 1900, the New York Evangelist reported that a women’s association on the Lower East Side was being instructed by a Dr. Vinton that one needed to ingest “at least eight glasses of water a day” and take “four times as much water as food”. (Incidentally, the girls were also told that it was dangerous to get one’s feet wet, that it wasn’t good to “wear many skirts”, and that their brains were “soft like jelly”.) Well, what can you expect from an evangelist, especially from New York?
  • By the 1910s and 1920s, the popular press was full of exhortations to consume six to eight glasses on a daily basis. Charles Atlas, the bodybuilder, was fond of recommending the same amount.

Who can argue with such authoritative sources?

But now, for something a bit more authoritative, the U.S. government’s Food and Nutrition Board. The board’s “Recommended Dietary Allowances” from 1945 include the following advice:

“A suitable allowance of water for adults is 2.5 liters daily in most instances. An ordinary standard for diverse persons is 1 milliliter for each calorie of food. Most of this quantity is contained in prepared foods.”

What the popular, and less popular, press missed was the last sentence, which pointed out that you can get most of that water just by eating. If you actually had to drink all 2.5 liters, you’d need around 10 8-ounce glasses per day.

In more recent decades, there have been plenty of proponents of the 8×8 theory. In 1967, Dr. Irving Stillman, one of the earliest advocates of a high-protein, low-carbohydrate diet, insisted that his followers drink eight glasses of water a day in order to wash away ketones, or “ashes left in the furnace”. This doctor, his financial success notwithstanding, would have failed my biochemistry class at UCSF.

The controversial 1992 bestseller Your Body’s Many Cries for Water, which calls for a minimum of 8-10 glasses of pure water a day (not coffee, not soda), probably played a role in spreading the myth, as has the bottled-water industry, which has exploded since the 1980s.


And the scientific evidence?

In a word: None.

The key to debunking these claims is actually in that last sentence of the U.S. Food and Nutrition Board statement of 1945, namely, you can get most of your water requirements from food. We can get a bit more scientific about it and point out that animal and plant cells are made up of mostly water. Surprising? The cell cytoplasm is made of a gel, something akin to jello. Did you know, for instance, that cucumbers are over 90% water? And so are many other vegetables and fruits.


How much should we drink?

The short answer is as much as you feel you need to drink. Of course, if you exercise a lot, or live in a warm climate, you need to drink more. But don’t sweat it—your body will signal its needs for hydration through dry mucous membranes in the mouth; or if deficit is more pronounced, through orthostatic hypotension, which means a drop in blood pressure when you stand up from a sitting or lying position. You don’t have to measure your blood pressure for that—you will know it when you experience it: You’ll feel dizzy. And if these two signals fail to alert you, pay attention to your urine. If you notice a marked decline in volume and the color turning deeper yellow, it is a signal that your body is trying to conserve water. Time to replenish the tank.

And yes, coffee, tea, soda—they all count as fluids. The myth that these don’t count because they are diuretic is just that—an undocumented belief.


Take home message

First, don’t believe what you hear. Ask for the evidence; you’d be surprised by answers like “everybody knows that”, or even worse.

And second, for all you H2O guzzlers, take it easy. Drink as much as you feel like, and chalk up rest to water conservation.

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.


  1. I liked this post a lot, but why the unnecessary dig at evangelists? True, some are an embarrassment to the Christian faith, but this group appeared to be working for social justice in New York’s Lower East Side in the early 1900s. There were lots of health and gender misconceptions during that historical period, not just promoted by evangelists. The New Nuts Among the Berries notes many weird fads of the time.

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