Some central tenets of good health: more vegetables, less soda, lots of exercise. And let’s not forget water: at least eight glasses a day. Much ink is spilled over the first three of these recommendations, but the last sometimes seems to be taken for granted by all the people lugging around Nalgene bottles. Is drinking so much water necessary? Is reaching eight glasses per day crucial to good health?
The short answer — at least to the specific question of eight glasses versus, say, seven or nine — is no, there is nothing special about eight. This threshold appears to be a long-standing medical myth. It’s not even clear where it started. The best answer I can find (based on this review) is that the source was a 1945 publication by the National Food and Nutrition Board, a government advisory agency, that stated this: “A suitable allowance of water for adults is 2.5 liters daily in most instances. … Most of this quantity is contained in prepared foods.” The theory is that people read this, ignored the last sentence, and the eight glasses a day (about 2.5 liters) recommendation was born.
So let’s dispense immediately with glass-counting attempts to reach this magical threshold. If we take a more charitable view of the goals of the water lobby, however, its goal is not to get us to some specific cutoff but to increase water consumption in general. So really the question shouldn’t be so much about eight glasses versus seven, but whether there is evidence that drinking more water makes you healthier.
There is one clear benefit of water: It is calorie-free. Given the magnitude of the obesity epidemic in the United States, it would be a huge public health boon if everyone replaced their fruit juice or soda with water.
But the eight-glasses claim is not about weight, it’s about flushing toxins, avoiding dehydration, and improving various bodily functions. And when researchers study these things, they typically do not focus on water alone but on total consumption of fluid. When the Mayo Clinic gives recommendations for fluid intake, it specifically notes that these are about all beverages, not just water.
So while it’s safe to say that you are a lot better off with a bottle of water than a can of soda, the question then becomes whether it’s better to drink more fluids or not. And here, the evidence conflicts.
Many papers show no effect of fluid intake on mortality. One very large study was run in the Netherlands in the 1980s, and published in the British Journal of Nutrition in 2010. Researchers followed over 120,000 individuals for a period of 10 years, and studied the relationships between fluid intake levels and mortality from heart attack and stroke. The authors found no link between total fluid intake or water intake and either cause of mortality.
Other studies echo this. This one found no impact of fluid intake (although, oddly, the authors exclude water) on chronic kidney disease or cardiovascular mortality. And this one, which randomized people into two groups — more and less water intake — and recorded outcomes like blood sodium levels, found no effect. And lest you think the reason to drink water is to improve your outer beauty, this study found no impacts of hydration on skin quality. (These studies all focus on healthy people. For individuals with particular medical conditions — kidney stones, for example — there are likely larger benefits of hydration.)
But some studies do find effects. Most notable is a study of 20,000 Seventh-Day Adventists, who were followed for six years to look for impact of fluid intake on mortality. The researchers found that drinking more water lowered the risk of dying. For women, the risk was lowered by having three or more cups per day, versus fewer than three. For men, three to four cups was much better than fewer than three, and five or more cups was a bit better than that. And at least one study has found that more than six cups of water a day lowers bladder cancer risk relative to less than one cup.
It’s important to note that other than the study on blood sodium, none of these studies were randomized trials. It’s therefore entirely possible that the effects were driven by other differences between water-drinkers and non-water-drinkers, and are not really due to the water at all.
And here is the other thing: Even in the studies that found effects, the threshold was significantly below eight glasses. Based only on the Seventh-Day Adventist study, we would conclude that women should be having at least three cups a day for maximum effect, and men at least five. Nothing here would suggest that eight glasses are necessary.
About now, you may be wondering: If there’s any evidence at all — even possibly flawed evidence — that more water is better, isn’t that reason enough to stick with the Nalgene, given that there doesn’t seem to be any downside to hydration? The answer is probably yes. But it is worth noting that there is such a thing as too much water. It’s possible to die from water intoxication — and people have — although it’s very rare and drinking eight glasses a day (or 10, or 12) isn’t going to do it.
What you can take from the evidence is that obsessing about reaching some water goal every day is unproductive. Most of us are going to get in three to four cups without doing anything out of the ordinary, and that’s likely to deliver nearly all of the benefits of water intake (if there are any). Probably the best advice is some I got from a doctor colleague recently: “When my patients ask when is a good time to drink water, I tell them: ‘When you are thirsty.’”