It is a common belief that you have to drink 6-8 glasses of water per day, but this myth comes from a fundamental misunderstanding of some basic physiology.
It is a common belief that you have to drink 6-8 glasses of water per day. Almost everyone has heard this recommendation at some point although if you were to ask someone why you need to drink this much water every day, they probably wouldn’t be able to tell you. There is usually some vague idea that you need to drink water to flush toxins out of your system. Perhaps someone will suggest that drinking water is good for your kidneys since they filter the blood and regulate water balance. Unfortunately, none of these ideas is quite true and the 6-8 glasses myth comes from a fundamental misunderstanding of some basic physiology.
Water is, of course, essential for life and we humans cannot go very long without it. In fact, compared to most of our animal brethren, we are some of the most water inefficient beings on the planet. The reason is that we lose quite a bit of water every day and we have no real way to store excess water or replenish our lost reserves short of simply drinking more of it. Unlike many other animals we cannot go long stretches without a supply of fresh drinking water.
Over the course of the day, we lose some water as water vapour from our breath and some water is lost through sweat. These water losses are called insensible water losses because we are not aware of them, as compared to the water lost in urine. Though many people think sweat is a consequence of exercise or of being hot, even someone living in a cold climate who is not exercising loses somewhere between 500-1000 ml of free water through sweat every day. Interestingly enough, this type of sweat is almost pure water, colourless and odourless, and is mainly used by the body for temperature control by drawing heat off the skin and allowing it to dissipate into the air. The foul-smelling sweat that most people are familiar is produced by a different type of sweat gland and is an oily substance with little water.
This water loss is essentially inevitable. You will always lose water vapour in your breath, provided you keep breathing, and you will always produce this watery-odour free sweat even if you move to the Arctic. Of course, if you move to the tropics you will produce much more sweat to compensate for the extra heat. But all told, roughly 1.5-2 litres of water loss are obligatory losses that we cannot do anything about. Those who exercise, live in hot climates or have a fever will obviously lose more water because of more sweating. Thus, a human being needs to replenish the roughly 2 litres of water they lose every day from sweating, breathing, and urination. The actual notion of 8 glasses a day originates from a 1945 US Food and Nutrition Board which recommended 2.5 litres of daily water intake. But what is generally forgotten from this recommendation is, firstly, that it was not based on any research and that secondly the recommendation stated that most of the water intake could come from food sources.
All food has some water in it, although obviously fresh juicy fruits will have more than, say, a box of raisins. Suffice it to say that by eating regular food and having coffee, juice or what have you, you will end up consuming 2 litres of water without having to go seek it out specifically. If you find yourself in a water deficit, your body has a very simple mechanism for letting you know. Put simply, you will get thirsty.
If you are thirsty, drink water. If you are not thirsty, then you do not need to go out and purposefully drink 6-8 glasses of water a day since you will probably get all the water in your regular diet. One important caveat to remember though is that on hot summer days, your water losses from sweating go up and if you plan to spend some time out doors, having water with you is important to avoid dehydration and heat stroke. While the thirst reflex is pretty reliable, it does tend to fade with age and older people are more likely to become dehydrated without realizing it. Thus, the take home message is drink water when you are thirsty, but on very hot days it might not be a bad idea to stay ahead of the curve and keep hydrated.
Drinking more water than necessary is not particularly dangerous. Drink more water and you will simply get rid of it in your urine. In fact, the main function of your kidneys is to make sure that water losses equal water intake. If they don’t and if water intake exceeds water loss, then you will start retaining water and every day you will accumulate more and more until you start to see swelling your legs (gravity drags water downwards which is why your feet swell first). This is the problem people with heart failure and kidney disease experience; they accumulate water because they cannot excrete it from their body. Thus while drinking excess water is unlikely to cause any major problems, there are some patients who will likely be told by their physicians to restrict water intake because of their heart or kidney disease.