This is one of those pieces of well intentioned health advice that never seems to die. Normally I let it go because in most cases it does no real harm, but I’m going to respond this time.
First, about me. I teach the physiology of fluid and electrolyte balance to medical students, and molecular physiology to Ph.D. students. I am also principal investigator on NIH grants to study various aspects of how the kidneys work. I also do research on pathophysiological mechanisms of kidney diseases, including renal complications of diabetes.
If you’re like me, you’ve often heard that “you’re probably not drinking enough water”. This advice often appears in magazines that are devoted to health and wellness (like the ones that I get that are devoted to my road cycling hobby). And now it has appeared in Daily Kos.
In most cases this advice is simply not true. Mammals have evolved a complex set of systems over millions of years that allow us to maintain our internal body fluid and electrolyte balance over a wide range of external conditions, including marked changes in external temperature, humidity, and sunlight exposure. We can maintain fluid and electrolyte balance in spite of substantial variations in what we eat and drink, or internal physiological changes (such as fighting off an infection, which may cause body temperature to rise). Our bodies have sensors that measure the osmolarity of extracellular fluids and when this tends to get too high, a host of responses are triggered, including thirst, as well as processes that reduce the amount of water we eliminate. You can feel thirst, and you can see water conservation mechanisms in action as urine becomes more concentrated (and darker yellow).
The fact is healthy adult humans under most conditions do not benefit by forcing themselves to drink more. You will end up producing a more dilute urine (and also more dilute sweat if you actually measure it’s conductivity). You will not “wash out more toxins” or get “softer healthier skin”, or whatever the claim du jour is.
There are exceptions to this, but generally they involve conditions where someone is not healthy, even it is a temporary condition. For example, dehydration can occur when people are vomiting or have diarrhea and large amounts of fluid are lost quickly. Dehydration can occur with certain other conditions in the GI tract (such as inflammatory bowel diseases, or in people who have had part of their intestines removed). People who are prone to constipation or kidney stones probably should drink more (the later because a dilute urine and increased flow of fluid through renal tubules can prevent stone formation). There are conditions in endurance sports, especially in the heat, where someone can lose so much water so fast that thirst reflexes are too slow to keep up with water loss, and so it is important to drink enough to stay ahead of it. Have respect for how quickly this can occur in the desert for example. It is also well documented that some elderly people have blunted thirst responses such that they do not stay in balance and can become dehydrated. There can also be issues with drugs some people take, or with people who suffer from certain cardiac arrythmias. This is not a complete list.
But again, the fact is that for adults in good health, there are no health benefits to be obtained from forcing yourself to drink more. When you are thirsty, get a drink. If not, don’t obsess.
I should even point out that there are examples of psychogenic polydipsia (excessive drinking) in which there is self-induced water intoxication. This can lead to electrolyte disturbances (such as hyponatremia) that can be life threatening.
I’d like to mention two other things. If you seem like you are thirsty all the time, this absolutely needs to be checked out. There are various reasons why this could occur. The most common would be type 2 diabetes mellitus in adults, but there are many other possible causes (including an adverse effect of lithium that some people take for bipolar disorders). Also, salt consumption in the typical American diet is much higher than is needed to maintain electrolyte balance, and there is now a mountain of evidence indicating an almost a direct correlation between long-term salt intake and blood pressure. But that is another topic, and it is complex, although it is related to this one.
Edit added: In the comments some apparently healthy people have mentioned that they feel guilty for not drinking enough water because the advice to drink more is so pervasive.
Please don't feel guilty about it!
Now as mentioned, there are some specific clinical conditions where drinking more is advisable (as noted, history of kidney stones, urinary tract infections, working or exercising in extreme heat and quite a few others). And a few where drinking less is mandated. There is a long paragraph about that above.