Updated: Dec 19, 2018
Most people, when asked how much fluid they should drink in a day, will say 6 to 8 glasses. This is also the message being shouted out in most health magazines and blogs. But why then, do so few of us die when we don’t?
Well, the answer to this is quite simple… there is absolutely no scientific evidence to back this up. How much water you need heavily depends on the climate you're in, what you eat (you can get up to 1 litre of fluid through a regular diet) and what activities you're doing.
Caffeinated drinks e.g. tea and coffee have traditionally been bad mouthed since it is commonly believed to act as diuretics and to cause your body to lose fluid. Research has, however, shown that this may only be the case for people who are not used to drinking these substances.
The American College of Sports Medicine has the following to say on caffeine: “Caffeine ingestion has a modest diuretic effect in some individuals but does not affect water replacement in habitual caffeine users, so caffeinated beverages (e.g. coffee, tea and soft drinks) can be ingested during the day by athletes who are not caffeine naïve (who regularly drinks caffeine).”
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So how do you know if you are drinking enough water?
I have read that I should drink before I get thirsty...
So how do you know if you are drinking enough water?
Research has shown that there are 3 ways in which a person can tell if they are dehydrated. They usually become thirsty, their urine becomes concentrated (darker colour) and they lose more than 1% of their body weight over a short period e.g. hours or a day. It is important to understand that there are 2 types of dehydration a healthy person can experience:
1. Acute or sudden dehydration which occurs over a period of hours or days. Examples of this is when you work or do exercise in the heat. This is normal and research have found that people are very good at rectifying this by drinking according to their thirst. You may also be interested to know that muscle cramps during sports are not actually caused by dehydration.
2. Chronic dehydration where a person’s fluid intake is just a bit too low over a long period of time so that it causes mild chronic dehydration. In this case, you won't notice a drop in body weight or increased thurst as your body would be used to this dehydrated state. Urine colour is your best measure for this. Chronic low-level dehydration can cause kidney stones and may also contribute to chronic kidney disease.
3 Step test to see if you are dehydrated – daily test
You have to perform this test first thing in the morning.Any one symptom on its own does not indicate dehydration. You have to display at least 2 of the 3 symptoms before you consider yourself dehydrated.
You may for instance have lost a lot of weight due to eating less and doing a lot of exercise the previous day, but be well hydrated. Similarly you may be thirsty, but not have lost so much fluid that you are deemed to be dehydrated. You are very likely to be dehydrated if you display all 3 symptoms.
Step 1: Urine
Look at the colour of your urine after your first visit to the loo in the morning. Use the standardized urine colour chart to assess your urine’s colour. You are very likely dehydrated if you score a 4 or higher. Remember, if you have been drinking too little fluid daily for a very long time, dark urine may be the only sign that you're dehydrated.
Step 2: Weight
Weigh yourself after your first visit to the loo. If you have lost more than 1% of your body weight compared to the previous morning, you may be dehydrated.
Step 3: Thirst
Are you thirsty when you wake up? Research has shown that a combination of thirst and loss of body weight is a strong indication of dehydration.
3 Step test to see if you are dehydrated – after sport
You can use the same 3 step test described above to see if you are dehydrated after a run, long walk or even a rugby match. The only difference is that you perform it immediately after completing your sports activity and you have to make sure that you were well hydrated when you weighed yourself before the activity.
It is not necessary or possible to replace all your fluids that you lose during exercise while you are training. Research conducted on athletes in laboratories has shown that a 2% loss of body weight due to dehydration can cause a decrease in performance, but this stands in stark contrast with results from field studies where the top performing athletes have been measured to have lost between 6 and 8% of their body weight.
"Time and again, studies, even those by researchers expecting different outcomes, have shown that the runners who are the most dehydrated, as measured by percentage of body weight loss, run the fastest. As two examples, notice the results in figure 2.4, from the 2000 and 2001 South African Ironman Triathlons and the 2004 New Zealand Ironman Triathlon. The five fastest finishers in the South African Ironman all finished in less than 9 Hours and all lost 6% to 8% of their body weights during the race (arrowed in figure 2.4a). Three years later, this relationship was confirmed in finishers in the 2004 New Zealand Ironman Triathlon (figure 2.4b)." - taken from Waterlogged: The Serious Problem of Overhydration in Endurance Sports
Dehydration has, however, also been shown to affect concentration. The effects of dehydration on your sports performance may thus depend on the type of sport that you participate in. Endurance athletes may for instance benefit more from losing some body weight while racing despite the dampening effect on their concentration. One can argue that a tennis or hockey player's performance will suffer a lot more if their concentration lapses.
The American College of Sports Medicine advises that you should try and keep fluid losses below 2% of your body weight, but they warn that you should not drink too much during exercise either since this can cause dangerous electrolyte imbalances in the body (see below).
While it may not be necessary to replace all your fluids while exercising, you should aim to do so afterwards. You can read about the best methods to replace fluid losses quickly and safely by following this link.
I have read that I should drink before I get thirsty
This is simply not true and may even be dangerous advice. Research has shown that healthy humans are more than capable of maintaining healthy hydration levels during normal daily activities as well as during exercise by simply drinking when they are thirsty.
This is surely also common sense? The “drink at least 8 glasses of water” and “drink before you are thirsty” advice have only been around in the last century and the human species have very successfully managed to survive in and populate some of the harshest environments on earth.
There's also evidence that the “drink before you are thirsty" advice may have been pushed by companies to increase sales.
Drinking too much water without replacing the salts that you lose during sweating can lead to a condition called hyponatremia. Hyponatremia causes swelling on the brain and it can be fatal if not treated swiftly. This is really only a problem for people who do not eat a normal diet or who drink excessive amounts of water for prolonged periods e.g. while running a marathon.
In the video below Prof Tim Noakes explains about the dangers of drinking too much water.
The only time that a healthy person cannot trust their thirst sensation is if they have been chronically dehydrated over a very long period of time. Research have shown that your thirst detection can be less sensitive in these cases.
Let me know if you have any questions. Need more help with an injury? You can consult me online using Skype video calls. I've also created a Facebook group where you can get answers to any injury prevention or exercise related questions you may have.
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Cotter JD, Thornton SN, Lee JK, et al. Are we being drowned in hydration advice? Thirsty for more? Extreme physiology & medicine 2014;3(1):18.
Sawka M, Burke L, Eichner E, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and science in sports and exercise 2007;39(2):377.
Valtin H. “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 × 8”? American Journal of Physiology - Regulatory, Integrative and Comparative Physiology 2002;283(5):R993-R1004.