Most of us heard this all of our lives, “Doctors recommend drinking 8 glasses of water a day” (The so-called ‘8x8’ rule. Each glass ~ 8-10 ounces or about 240-300ml.)
Back in medical school, we medical students were told the same thing in our Nutrition class. No explanation. Just a simple statement on a sheet of paper and not unlike a mathematical postulate, all of us wide-eyed students accepted it as the gospel of medicine- from the ‘Medicine Gods’…in the ‘House of Gods’.As time gone by, the students become the teachers (the word ‘doctor’ means teacher in Latin) and it’s becoming increasingly apparent that we the doctors are repeating the same water recommendation that we simply don’t have any scientific evidence to support it. Yet, it seems so right because no one has dared questioning it. After all, if the attending gods (physician supervisors) in your prestigious medical residency say it is then it must be true, right? Unfortunately, there are many examples in the House of Gods where the practice of medicine is relegated to “Monkeys see, Monkeys do”. In this case, the ‘8x8’ rule of drinking 8 glasses of water a day is all wet.
The Origin of the Myth
No one really knows where, when or how this recommendation started. Presumably, the recommendation implies that a normal healthy body looses 64-80 ounces of fluid (~1920-2400mL) per day and therefore needs to take in 8 glasses of water as replacement fluid. Or is it?
Another theory goes that one needs that much water ‘to wash the colon, and therefore, the body of poisonous intakes ’. Still another popular (but equally erroneous) belief is that the kidneys need that much fluid to ‘get rid the body of toxins’ (detoxification theory). But ask any doctor, any specialist in any medical field, and s/he would be hard pressed to give you an intelligent and scientific explanation for the magic 64-80 ounces of water requirement. Furthermore, if pressed to clarify if this means 64-80 ounces of plain water or any fluids such as soda beverages or Gatorade and these so-called experts would be even more confounded in their response. Why? Refer to the ‘Monkeys see, Monkeys do’ comment above.
One source of this urban myth can be traced back to Christoph Wilhelm Hufeland (1764-1836), a German and founder of macrobiotics. Hufeland, a believer in natural medicine and vitalism, without citing any scientific evidences or experiments, advocated 8-10 glasses of water a day in his book.
Another erroneous embrace of this daily water requirement may have been perpetuated by the mis-application of the IV (intravenous) fluid replacement regiment for post-operative (post-op) patients. In these special cases, the patients are ‘NPO’ (nothing per oral- no food/fluid by mouth) and therefore, to maintain adequate hydration, surgeons typically apply the ‘4/2/1 rule’ for IV fluid replacement in euvolumic patients (4ml/kg/hr for first 10Kg of body weight, 2mL /kg/hr of next 10Kg, and 1mL/Kg/hr for remainder). So, for a 68kg (150 lbs) person, this works out to be about 2600mL a day. (Want a simplier rule? 100-125 mL/hr IV fluid replacement and reassess in 24 hours). Note that 2600mL is more or less about 8 glasses of 8-10oz of water.
The IV fluid replacement of 2600mL reflects the fact that we know a typical adult body in temperate climate looses approximately 500- 1000mL of insensible water loss (ie evaporation loss via skin, lungs and respiratory tracts) and up to 1500-2000mL of sensible water loss via defecation and urination.
Of course, unless one is sleeping, ‘NPOs’ is but a four-letter word. A typical person drinks and eats in accordance to his thirst and appetite and sometimes even more. Most nephrologists (kidney specialists) will tell you that you need half as much water or less (2-4 glasses) to keep the kidneys and body happy. And not just water but any fluids including coffee and tea because, contrary to popular belief, as much as 50-70% of the fluids from diuretic beverages are retained by the body.
Our Water Needs
For an average healthy adult, the thirst mechanism is so powerful and evolutionarily necessary to maintain body fluid homeostasis such that one can go without solid food for weeks but only a few days without water. In fact, the urge to drink fluids is such a natural instinct that a change in blood plasma osmolarity of as little as 2% will activate a complex negative feedback and thirst mechanism of increased fluid intake and conservation of fluid loss (urine) by the kidneys- long before any ill-effects of even mild dehydration is seen. Moreover, the thirst sensation is such a basic evolutionary survival mechanism that by many estimates, a typical person, at meal time, instinctively consumes all the fluids his body requires (via beverages and yes, fluids obtaining from eating solid foods).
One special situation worth mentioning is the high dehydration rate (inadequate fluid intake) among the geriatric population. However, the compounding problems here are 1. Mental status changes, as well as aging, affecting the thirst mechanism and 2. The powerful diuretic effects of various medications that many elderly patients are prescribed.
People with high fever, illness or traumatic injury causing rapid fluid lost via vomiting, diarrhea, hemorrhages, or increased insensible water loss are also unique exceptions.
So all in all, for most of us, the take home message is:
- Eat, drink and be merry…and obey your thirst and you will be just fine.
- Another urban myth? People need 8 hours of sleep nightly. Viewers are invited to back for yet another informative and lively discussion.
Any comments/corrections, intellectual enlightenments and/or scientific evidence to the contrary are welcome and appreciative.
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