often told we should drink eight glasses of water per day. Where does this
recommendation come from, and does it apply to everyone?
How do we calculate
our water requirements?
requirements are determined by your metabolic demands and the necessity to
balance sensible and insensible water loss. Your metabolic demand is influenced
by your body size, body composition, amount of physical activity and the
presence of conditions such as fever.
You also lose fluid through perspiration
(influenced by factors such as altitude, activity, humidity and environmental temperature)
all the factors that play a role in fluid balance, it is easy to see that
individual requirements vary greatly and one size really doesn’t fit all! So, how
on earth are you supposed to ascertain how much fluid you as an individual need?
Read: Water beats energy drinks
glass a day rule stems from the need to have an easy-to-understand message
that stresses the importance of sufficient fluid intake. The body has no
provision for water storage, thus the amount of water lost every 24 hours must
be replaced. The US Institute of Medicine (IOM) recommends an
average intake of fluid (from food and drink) of 3L per day for men and
2.2L per day for women, or alternatively approximately 35ml of water per kg.
food accounts for approximately 750ml (or 3 cups) of water per day and
oxidative metabolism contributing about 250ml (or 1 cup) of water per
day, a minimum of 1.2–2.0L (or 5–8 cups) per day of additional fluid is
needed. As discussed, this value is subject to change with higher
intake needed to account for physical activity and exposure to extreme
environments. Please note that these calculations are based on the “average”
sized person, but if in doubt use the 35ml fluid per kg calculation
for a more individualised amount.
Can you drink too
people with normal kidney function are seldom at risk for water intoxication
when their needs exceed their water requirements. Although thirst is a signal
for the need to consume water, people can be at risk of chronic dehydration.
Dehydration causing as little as 2% loss of body weight can result in an
impairment of both your physical and psychological performance. In addition to
this, chronic mild dehydration has been linked to constipation, dry mouth and
kidney stones (in those who are susceptible). So, the eight glasses
a day rule is meant to be an easy average to help people to remember to drink
more fluid, but is by no means a hard and fast rule.
Which drink is the
best for hydration?
It is recommended
that most of our daily fluid intake (approximately 70%) should be plain water.
The water from additional hot and cold beverages can make up the remainder.
Read: Water myths debunked
Should you then
consume cold drinks, juices, teas and coffee, it is vital to remember that these
alternative drinks may contain additional calories (usually from added sugar),
which may result in unwanted weight gain or dental caries if they replace your
So, while you may be adequately hydrated, additional calories and
sugar may lead to unintended results like weight gain. If you do choose to
replace some of your plain water with an alternative drink, make sure that it
contains little or no added sugar for best results.
Does caffeine cause dehydration?
There is a concern
that drinking caffeine
containing beverages will cause dehydration. There is good evidence that this
is untrue, which means that caffeinated and non-caffeinated drinks are equally
What about sparkling water?
Unsweetened sparkling water can be counted as part of your fluid intake.
Sparkling water is made by dissolving carbon dioxide into the water under
pressure. By itself, carbonated water appears
to have little impact on health. While carbonated water is somewhat
acidic, this acidity is quickly neutralised by your saliva.
The bottom line
unsweetened drinks, such as plain water, sparkling water, teas (regular or
herbal) and coffees (without sugar) to thirst within the guideline of
approximately 8 glasses (or 2L) per day and you will reap the benefits of
being well hydrated.
Water and your body
Cellulite, water intake linked
Hyperglycaemics need more water
1. Charney, P. Water,
electrolytes and acid-base balance. Krause's Food & The Nutrition Care
Process. St. Louis, Mo: Elsevier/Saunders, 2012.
Practice-based Evidence in Nutrition: Healthy Lifestyle Key Practice
Points. Dietitians of Canada 2015. Last updated on 20 June 2011.
3. Institute of Medicine.
Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate.
Washington, DC: The National Academies Press. 2005.
Practice-based Evidence in Nutrition: Caffeine Knowledge Pathway.
Dietitians of Canada 2015. Last updated on 22 October 2012.
5. Ireland R. Advanced
Dental Nursing. 2nd ed. Wiley-Blackwell; 2010.