Busting Myths About Water Intake

Drinking water is essential. But how much water is best? There are many commonly held beliefs and ‘facts’ about fluid intake that have no basis in truth.  The most common myth? You should drink 8 glasses of water a day. 
 

Myth #1. You should drink 8 glasses of water a day

It is frequently recommended that we should drink 8 glasses or 2 litres of water a day. However, there is no scientific evidence to support this idea in a healthy person1.  Many researchers have been unable to find where this myth originated, however it is continually perpetuated by the health and wellness industry.  The Institute of Medicine gives guidelines about fluid intake, but many people don’t realise that this includes fluid gained from food2.  
 
There is no scientific evidence that high fluid intake is beneficial for heart disease, constipation, venous thromboembolism (DVT), headaches, or clear thinking3.  However, dehydration can worsen constipation and headaches3
 

Myth #2. If you wait until you are thirsty you are already dehydrated

The theory that if you are thirsty you are already dehydrated is the same as saying if you are hungry you are already malnourished. This is not true! Our bodies have an effective system of maintaining water balance and thirst is a vital part of this system1. For the average person, thirst is likely a good indicator of fluid requirements. 
 
The exceptions to this are excessive thirst as a symptom of diabetes, the elderly whose thirst response may not be working well, if you are unwell with vomiting or diarrhoea,  or if you are exercising hard in hot conditions.
 

Myth #3. More water is better; you can’t drink too much

High fluid intake can be harmful. Drinking too much water leads to dilution of sodium in the blood, causing hyponatremia or water intoxication. This causes the body’s salt levels to drop dangerously, increasing the risk of seizures and death. In the medical literature there are documented fatalities due to excessive fluid consumption in athletes4.
 

Myth #4. Tea and coffee don’t count towards fluid intake

Whilst large doses of caffeine have an immediate dieuretic effect, single doses have little or no diuretic action. In addition, regular caffeine drinkers develop a tolerance to the diuretic effects of caffeine5.  In some people caffeine can irritate the bladder.
 
Scientific research has found no effect on hydration when including caffeine drinks as part of daily fluid intake6.
 

Myth #5. Urine should be very pale yellow or clear.

Not true! Normal urine has a moderately yellow colour1.  The colour may also be affected by what we eat or drink.  For example, Berocca will turn urine orange.
 

So how much should we drink?

Unfortunately, there is no magic answer to this question. 
 
Bladder and kidney doctors tell us we should urinate 24mls for every kilogram of our body weight each day. So a 70kg person should urinate approximately 1680ml a day. 
 
A common sense approach for a healthy person is to have a drink with meals, have a drink between meals, and more if you are thirsty. If you are playing sport or the weather is very hot you may need more. Enjoy caffeinated drinks in moderation.
 

References

Valtin, H. (2002). "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, 283, R993. 
Institute of Medicine: Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. Washington, D.C.: The National Academies Press 2005. 
Wood, L., Markowitz, M., Parameshwar, P., Hannemann, A., Ogawa, S., Anger, J., & Eilber, K. (2018). Is it safe to reduce water intake in the overactive bladder population? A systematic review. The Journal of Urology, 200, 375-381. 
Noakes, T. (2003). Overconsumption of fluids by athletes. BMJ, 327, 113-114.
Maughan, R. & Griffin, J. (2003). Caffeine ingestion and fluid balance: a review. J Hum Nutr Dietet, 16, 411-420.
Grandjean et al. (2000). The effect of caffeinated, non-caffeinated, caloic and non-caloric beverages on hydration. J Am Coll Nutr, 19, 591-600.