The 8x8 recommendation continued, rarely questioned, until Dr. Valtin’s seminal investigation in 2002, trying to find any scientific justification for drinking so much water. This was followed by other critical papers and the 2004 National Academy of Sciences recommendations. Still, the scientific evidence hasn’t caught up with some health writers, nutritionists and healthcare professionals…
The science of 8x8
Conventional wisdoms, despite their popularity, aren’t always true. Dr. Valtin conducted a 10-month review of the scientific literature and historic documents, and interviewed medical experts, to find any supportive evidence at all for this long-standing adage. He also looked for any evidence behind the most cited claims about the our water needs and water’s health benefits. “I’m talking about randomized trials published in peer-reviewed journals,” he said. Not opinion pieces and anecdotes. He also tried to uncover a definitive science-based origin for the recommendation.
His major undertaking found that “the universal advice that has made guzzling water a national pastime is more urban myth than medical dogma and appears to lack scientific proof.” His search results, published in the American Journal of Physiology, said:
*No scientific studies were found in support of 8 x 8. Rather, surveys of food and fluid intake on thousands of adults of both genders — analyses of which have been published in peer-reviewed journals — strongly suggest that such large amounts are not needed because the surveyed persons were presumably healthy and certainly not overtly ill. This conclusion is supported by published studies showing that caffeinated drinks (and, to a lesser extent, mild alcoholic beverages like beer) may indeed be counted toward the daily total, as well as by the large body of published experiments that attest to the precision and effectiveness of the osmoregulatory system for maintaining water balance. *
His review of lay press found that the rationale for 8x8 typically goes something like this: “Our bodies consist mostly of water and our blood, muscles, brain, and bone are made up mainly of water. Therefore, 1) we need water to function and survive and 2) we need at least eight 8-ounce glasses of water each day.” The second conclusion, he said, besides being unproven is a nonsequitur. “It is akin to arguing that our homes run on electricity, and that, therefore, every house needs at least 1,000-ampere service.”
Dr. Valtin specifically investigated the research evidence behind the most popular myths about our fluid needs.
All fluids count. The popular papers left “little doubt” that most advocates of the 8 glasses of water a day mean to convey that people should drink water per se, and specifically exclude caffeinated or sweetened drinks from the daily count. But this is a misperception, he wrote. He found strong scientific evidence that all fluids count, including water, coffee, tea, soft drinks, milk, juices and beer.
Dehydrating caffeine myth. Similarly, he found that recent experiments have “cast serious doubt on the often asserted diuretic role of caffeinated drinks,” he said. Caffeine had no significant effects on any of the variables that measure dehydration in one such study conducted at the Center for Human Nutrition in Omaha, for example, and the investigators concluded that “advising people to disregard caffeinated beverages as part of the daily fluid intake is not substantiated.” The diuretic effect of caffeine in drinks and moderate alcohol is trivial compared to the amount of water they contain.
We need less than we think. Among most adults, he found, caffeinated and alcoholic beverages constitute half or slightly more of their daily fluid intake, meaning the average adult drinks a respectable 1,700 ml and this doesn’t include the water from foods and metabolism, which also count. Yet, the medical research indicates that even 1,700 ml may be as much as a full liter more than what sedentary adults actually need to maintain physiological homeostasis, he said.
He couldn’t find any article where 8x8 was recommended on the basis of scientific evidence. The idea seemed to appear out of nowhere. He did, however, find one possible source for a misinterpretation that may have been repeated, like urban legends often are. The 1945 Food and Nutrition Board of the National Academy of Sciences had written in its Recommended Dietary Allowances:
*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. *
Dr. Valtin believes this last sentence may have been ignored, leaving the incorrect interpretation of these early dietary guidelines that eight glasses of water to be drunk each day.
Thirst doesn’t mean dehydration. The scientific evidence also debunks the popular myth that by the time you feel thirsty, you’re already dehydrated. A number of scientific studies have confirmed there is no support for this fear. Quite the opposite. Thirst hits long before we’re near risk for dehydration. Our thirst kicks in when the osmolality of our blood plasma is less than 2%, whereas dehydration begins at osmolalities of 5% and higher. In fact, our bodies even adjust fluid balances moment-to-moment by changes in the plasma concentrations of the antidiuretic hormone vasopressin, and subsequent changes in urine flow, far earlier than thirst plays a role. “Osmotic regulation of vasopressin secretion and thirst is so sensitive, quick, and accurate that it is hard to imagine that evolutionary development left us with a chronic water deficit that has to be compensated by forcing fluid intake,” he wrote…Two years after Dr. Valtrin’s famous paper, the latest 2004 Food and Nutrition Board recommendations for water were issued, reinforcing his findings and defusing the myths of 8x8…
The latest debunking
In the June issue of the Journal of the American Society of Nephrology, doctors Dan Negoianu and Stanley Goldfarb at the Renal, Electrolye and Hypertension Division at the University of Pennsylvania, further examined the science behind water myths. “There is what I call an urban myth that drinking a lot of water is a healthy thing to do and it leads to people toting around plastic water bottles all day drinking water,” said Dr. Goldfarb. “The source of this is the complementary and alternative medicine worlds.” Not science.
Water doesn’t reduce appetite. Among the additional myths they reviewed was the belief that drinking water before and during a meal will help people eat less and help manage obesity. They found surprisingly little evidence to support such rationale or even biological plausibility. “Because you absorb water so quickly and it moves through the GI tract so quickly, it probably doesn’t fill you up the way people have proposed, nor does it lead to the release of hormones which suppress appetite,” he said.
Water doesn’t detoxify. Another myth is that filling up on water flushes toxins from the body. “In fact, that is not how the kidney works,” said Dr. Goldfarb. “When you drink a lot of water you end up having a larger volume of urine but don’t necessarily increase the excretion of various constituents of the urine.”