Have you always been told to drink at least 8 glasses of water a day? Everybody knows that that's what you must do, right? And do you find that you are struggling to meet that target? Well, you are not alone. Many of us do, and the more disciplined among us will force it down, while others don't, but then feel guilty about not reaching the target. Even though most health professionals keep repeating the "8 glasses rule", there doesn't seem to be any scientific evidence for that.
A 2002 review by Heinz Valtin, published in the American Journal of Physiology (full text here), looked at the "8 x 8" rule (8x8 ounces = 8x236 ml) and other misconceptions in connection with water intake. He could not find any solid evidence to justify the "8x8" rule, but then neither did he find proof that we do not need to drink that much. But, says Valtin, "the published data available to date strongly suggest that, with the exception of some diseases and special circumstances, such as strenuous physical activity, long airplane flights, and climate, we probably are currently drinking enough and possibly even more than enough."
Why we need water
Our bodies consist of about 60% water. It is a vital nutrient, and we would not last longer than three days without water. Water is required for temperature regulation, physical and cognitive performance, heart function and blood pressure regulation, gastro-intestinal and kidney function, detoxification, and lubrication of the joints. While good hydration appears to play a role in the prevention of urinary tract infections (e.g. cystitis), high blood pressure, fatal coronary heart disease and deep vein thrombosis, its role in the prevention of constipation, exercise asthma and even skin health may be overrated.
How do we keep hydrated?
We lose water through breathing, sweating, urination and bowel movements and this needs to be replaced, but as the right water level is critical, our body has feedback mechanisms in place to regulate it. It cannot just rely on us to do the right thing, can it? If we are over-hydrated, we will feel the urge to pee. If we are dehydrated, we will be thirsty. Simple. If we temporarily do not have access to a drink, kidney output can be reduced to retain water. This is regulated by hormones, which act very quickly and accurately, keeping our water balance in check.
Thirst is the most obvious reason for us to drink and top up our water levels, but not the only one. Think about it: Do you only drink because you are thirsty? No, we often drink, because we like the taste of what we are drinking, and that could be water or coffee or milk or a soft drink or herbal tea or beer. Now, I am not saying that all of those are equally healthy fluids (because they aren't), but they do all count towards your fluid intake.
Yes, even tea, coffee and beer: Although caffeine and alcohol have a diuretic effect (i.e. they make you pee and thus lose fluids), they are diluted in so much fluid that the net effect is still positive. Due to the higher alcohol content, however, this is not true for wine and spirits, and of course alcoholic drinks have their own health implications, not least addiction, so please don't rely on beer for hydration! Soft drinks and fruit juices may contain calories as well as sugar, sweeteners, preservatives, colourings, caffeine and more undesirable substances - good reasons not to drink them - but with regard to fluid intake, they are just fine. So, we drink a lot just for pleasure, long before we are even feeling thirsty.
In addition, food comes with water, too. This is not just true for soups or water-rich fruits and vegetables, such as oranges, watermelon, tomatoes and cucumbers, but pretty much everything else that hasn't been dehydrated, for example raisins, dried mushrooms and herbs. Food accounts for about 20% of our daily fluid intake, and this is a significant amount.
Can you drink too much?
As with everything, moderation is key, because it is indeed possible to drink too much, even if it is "just" water. Too much water can lead to hyponatraemia, a condition which occurs when too much water in the body dilutes the sodium content of the blood. Signs and symptoms of hyponatremia include frequent urination, cold hands and feet, nausea and vomiting, headache, short-term memory loss, confusion, lethargy, fatigue, loss of appetite, irritability, muscle weakness, spasms or cramps, seizures and decreased consciousness or coma.
So, if you thought you might not be drinking enough: Relax! You probably are. If anything, you may want to review your sources of fluids. Water, herbal teas, limited tea and coffee are fine, but soft drinks - including "flavoured water" - and fruit juices can be laden with sugar.
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Water, Hydration and Health, Popkin, B., D’Anci, K., Rosenberg, I. Annual Review of Nutrition, 2010 August; 68(8): 439–458.
Heinz Valtin: "Drink at least 8 glasses of water a day." Really? Is there enough evidence for "8x8"?, Am J Physiol Regul Integr Comp Physiol 283: R993–R1004, 2002. First published August 8, 2002.
British Nutrition Foundation - Healthy Hydration Guide