Hydration: How much do you need?

waterWe have been counseled to drink eight, 8-ounce glasses of water a day (8 x 8) for such a long time that the advice has become unwritten law…and slavishly followed at that. This chant started so long ago that most people have no idea of its origin. At the same time, we are cautioned not to count alcohol and coffee as hydration elements. The science behind the recommendation is so scant that little support can be given to the exhortation, yet the possession of a water bottle is ubiquitous. It is possible that this idea is the result of misinterpretation or misreading of a notion proposed by the Food and Nutrition Board of the National Research Council that recommended one milliliter of water for every calorie consumed. The neglected fact is that there is water in our food. That would surely separate liquid intake from total dietary intake.

Dr. Heinz Valtin, a medical professor at Dartmouth, examined this mantra earlier in this century, and learned, “No scientific studies were found in support of 8 x 8.”  After reviewing surveys of food and fluid intake on thousands of adults of both genders, Dr. Valtin stated that, “…such large amounts (of water) are not needed because the surveyed persons were presumably healthy and certainly not overtly ill.”  He added that most other kinds of beverages, including soft drinks and coffee, contribute to one’s daily need for hydration, continuing that a considerable body of evidence supports the premise that the human body is fully capable of maintaining proper water balance.  But all this must be tempered with the qualifier, “in healthy persons.”  He leaves us with, “…large intakes of fluid, equal to and greater than 8 x 8, are advisable for the treatment or prevention of some diseases and certainly are called for under special circumstances, such as vigorous work and exercise, especially in hot climates.”  In the spirit of open-mindedness, Dr. Valtin asks that readers submit their own findings to him.

Including the 20% supplied by foods, the Institute of Medicine recommends a fluid intake of about 91 ounces a day for women and 125 ounces for men.  Do you know how much water is in your food?  Few of us do. The puzzling thing about this recommendation is the lack of sufficient data available on water metabolism in adults, especially those who are sedentary and living in a temperate environment.  Most of us take in more than that suggested level, when we account for comestibles, although the geriatric populace is apt to take in less of both food and liquid water, partly because of insensitivity to a thirst stimulus and partly because of a waning ability to taste foods and beverages as well as they did in their early years.  It appears that older men drink less than their younger counterparts, but excrete more urine.  Differences in women have shown to be insignificant, but contribute to the notion that, “water turnover is highly variable among individuals…”  (Raman et al. 2004)

Admittedly, older adults are at greater risk for dehydration, but water balance in this population had not been faithfully studied until Purdue University picked up the reins in 2005, and compared/contrasted water intake/output and total balance of fluids in an older population (63-81 y.o.) and a younger one (23-46 y.o.), finding that, in fat-free mass, there is little difference.  The study noted, though, that fat-free mass was lower in the elderly and that fat-free hydration was significantly higher.  Considering that the elderly have less muscle to begin with, this is simple to follow.  (Bossingham. 2005)

Many people complain that, if they increase water intake, they will spend more time in the lavatory.  While this is the case with many of us, there is a limiting factor—time.  The period of time over which a specific amount of water is consumed makes a difference in when the urge to evacuate that water will arise.  The faster you drink that glass of water, the sooner you will need to excrete it.  The longer the glass lasts, the more time there will be prior to evacuation.  “A water diuresis occurs when a large volume of water is ingested rapidly.”  (Shafiee. 2005)   Also note that water mixed with a poorly absorbed sugar (not glucose) will retard absorption and delay excretion.

The kidneys can process almost four gallons of water a day.  Too much water will make you sick because sodium stores will become depleted and electrolyte activity will be sorely jeopardized.  Drinking over a period of time can thwart this threat.  You need not measure urine output to figure out how much fluid to replace.  That is something you can eyeball.  Thirst should not be the barometer by which fluid need is determined.  While there is no absolute proof that we all need 8 x 8, have a glass of water even when you are not thirsty, working in the heat, or running a marathon.  To prevent electrolyte displacement, we might consider electrolyte replacement in at least a couple of our glasses.

References

MAIN ABSTRACT
Am J Physiol Regul Integr Comp Physiol. November 2002; vol. 283 no. 5: R993-R1004
“Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 × 8”? Heinz Valtin and (With the Technical Assistance of Sheila A. Gorman)

SUPPORTING ABSTRACTS
Am J Physiol Renal Physiol. 2004 Feb; 286(2):F394-401. Epub 2003 Nov 4.
Water turnover in 458 American adults 40-79 yr of age. Raman A, Schoeller DA, Subar AF, Troiano RP, Schatzkin A, Harris T, Bauer D, Bingham SA, Everhart JE, Newman AB, Tylavsky FA.
Department of Nutritional Sciences, University of Wisconsin-Madison, Madison, Wisconsin 53706, USA.

Am J Clin Nutr. 2005 Jun; 81(6):1342-50.
Water balance, hydration status, and fat-free mass hydration in younger and older adults. Bossingham MJ, Carnell NS, Campbell WW.
Department of Foods and Nutrition, Purdue University, West Lafayette, IN 47907, USA.

Kidney Int. 2005 Feb;67(2):613-21.
Defining conditions that lead to the retention of water: the importance of the arterial sodium concentration. Shafiee MA, Charest AF, Cheema-Dhadli S, Glick DN, Napolova O, Roozbeh J, Semenova E, Sharman A, Halperin ML.

Renal Division, St. Michael’s Hospital, University of Toronto, Toronto, Ontario, Canada.

Am J Physiol Regul Integr Comp Physiol. 2000 Sep;279(3):R966-73.
Effects of time of day, gender, and menstrual cycle phase on the human response to a water load. Claybaugh JR, Sato AK, Crosswhite LK, Hassell LH.

Department of Clinical Investigation, Tripler Army Medical Center, Tripler Army Medical Center, Hawaii 96859 – 5000. [email protected]

Eur J Clin Nutr. 2010 Feb;64(2):115-23. Epub 2009 Sep 2.
Water as an essential nutrient: the physiological basis of hydration. Jéquier E, Constant F.
Department of Physiology, University of Lausanne, Pully, Switzerland. [email protected]

J Am Soc Nephrol 19: 1041-1043, 2008
Just Add Water
Dan Negoianu and Stanley Goldfarb

Renal, Electrolyte, and Hypertension Division, University of Pennsylvania, Philadelphia, Pennsylvania

*These statements have not been evaluated by the FDA.
These products are not intended to treat, diagnose, cure, or prevent any disease.

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