How much water should you drink in a day? You could ask anyone in the street, and most people would know that the recommended fluid intake is 1.5 to 2 litres or 6-8 glasses. Indeed, that is what the Eatwell Guide recommends. The NHS, although quoting the Eatwell Guide as well, suggests that we drink “plenty of water”. But how much is ‘plenty’? Is there, perhaps, an upper limit?
In 2007 seven, a 59-year-old woman nearly died after drinking too much water. Admittedly, very few people manage to drink that much, even if they try. Incidents like this are sporadic, but it happens occasionally and shows that even water can be detrimental if overconsumed. In this particular case, the woman reported having drunk more than half a pint of water every 30 minutes in an attempt to relieve a urinary tract infection.[i]
In reality, many people struggle to drink the recommended 6-8 glasses per day. But who says you have to? In 2002, researcher Heinz Valtin went in search of the source of this often-quoted recommendation. He was unable to find it.[ii] There is no scientific evidence for the advice to drink 6-8 glasses per day as far as he could see.
The absence of evidence is not surprising, considering that the water requirement changes from person to person and from day to day. How much you need to drink depends on your gender, age, weight, activity level, climate, altitude and medication … to name but a few.[iii] Some people need just S glasses of fluids a day; others need considerably more. Despite the difficulty of working out how much every one of us needs to drink, we seem to manage quite well. One paper on the subject concludes that healthy people regulate their daily water balance “with precision”.[iv] No mean feat. After all, the parameters for hydration change all the time. Our location, the weather or our activity levels – or all three – may be different today than yesterday, and so is our water need.
It appears, therefore, that there is no need to lose sleep over adequate fluid intake. Apply common sense. If you are thirsty, drink. If you sweat a lot, drink more than you normally would.
That said, thirst can be deceptive, especially in older people. Several parameters, such as blood pressure, blood volume, electrolyte levels and certain hormones, signal the brain and the kidneys when water levels are low. The kidneys will then hold back and excrete less water, while the brain triggers the thirst sensation. This works quite well until later in life. Older adults do not sense thirst as easily as younger people do.[v]
Another indicator for dehydration is the colour of your urine. If it is dark like apple juice, it is time to drink. If your urine is straw-coloured, your hydration level is about right. If your urine runs clear like water, there is no need to drink more right now.
The advice to drink 6-8 glasses a day is usually closely followed by the reminder that you must not try and hydrate by drinking alcohol or caffeinated drinks. These beverages are said to be diuretic and make you pee more, thus accelerating dehydration. However, a study looking into this found no difference in hydration levels of young men, regardless of whether they consumed caffeinated or non-caffeinated drinks.[vi] A 2015 study comparing the effects of water, cola, and juice, again, found no difference.[vii] There are, of course, many reasons to skip on soft drinks – sugar, artificial sweeteners and phosphates come to mind – but dehydration isn’t one of them.
Alcohol is a different matter. In small doses, such as half a pint, even beer can be hydrating, provided that the person drinking it is dehydrated to begin with, but more alcohol in slightly better hydrated people does lead to greater urine output.[viii]
In summary, no one can tell you how much you need to drink in a day. Perhaps your doctor could, if they took blood and urine samples, but since that is not practical on a daily basis, you’re on your own. Go by how thirsty you are and the colour of your urine.
Of course, you can drink beverages other than water to hydrate: herbal teas, fruit infusions, vegetable juices, kombucha, and tea and coffee all count. Even fruit juices and soft drinks are hydrating but are best avoided for other reasons, just like tea and coffee should be sugar-free. Don’t forget that food can be hydrating, too. Fresh fruit and vegetables, such as watermelon and cucumber, contain water, and sometimes quite a lot of it. Soups are another excellent source of fluids. For healthy people it isn’t too difficult to stay hydrated.
[ii] Valtin H. “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 x 8”? Am J Physiol Regul Integr Comp Physiol. 2002 Nov;283(5):R993-1004.
[iii] Armstrong LE, Johnson EC (2018): Water Intake, Water Balance, and the Elusive Daily Water Requirement. Nutrients. 2018 Dec 5;10(12):1928.
[iv] Sawka MN, Cheuvront SN, Carter R 3rd. Human water needs. Nutr Rev. 2005 Jun;63(6 Pt 2):S30-9.
[v] Armstrong & Johnson, 2018.
[vi] Grandjean AC, Reimers KJ, Bannick KE, Haven MC. The effect of caffeinated, non-caffeinated, caloric and non-caloric beverages on hydration. J Am Coll Nutr. 2000 Oct;19(5):591-600.
[vii] Tucker MA, Ganio MS, Adams JD, et al (2015): Hydration Status over 24-H Is Not Affected by Ingested Beverage Composition. J Am Coll Nutr. 2015;34(4):318-27.
[viii] Hobson RM, Maughan RJ. Hydration status and the diuretic action of a small dose of alcohol. Alcohol Alcohol. 2010 Jul-Aug;45(4):366-73.