As the weather finally seems to be getting warmer, I thought maybe this would be a good time to answer a question I get fairly often related to hydration.  I’m often asked how much water is the correct amount to consume, and further I’m asked how much extra is necessary when exercising in warm or hot weather.

Let’s cover the first question.  The standard answer to this is 8-10 glasses of water per day.  But I think we all know this isn’t accurate.  Obviously my wife, who weighs half as much as me, requires a lot less water than me in a day.  So the answer, as with most things, needs to be individualized.  The calculation that I like best is simply half your body weight per day.  Body weight in pounds.  Water in Oz.  So, a 200lbs person requires 100oz of water daily.  There are 8oz per cup.  100oz divided by 8 is 12.5 cups.  There are 4 cups per litre of water, so a 200lbs person requires just over 3L of water daily. This size of person following the standard guideline of 8-10 glasses per day will be chronically dehydrated.  An 80lbs person, on the other hand, using this calculation would require 40oz per day, or 5 cups.  This person will be over hydrated at 8 glasses per day.  So, because few of us are as light as our 80lbs example, it’s likely that most of us, if following the 8-10glasses per day rule, are in a  state of constant dehydration and would do well to add at least a little more water per day.

In addition to the quantity of water, however, we need to consider the quality.  Mineralized water is essential.  The base recommendation is over 200 parts per million of total dissolved solids in your water.  Most tap water satisfies this rule.  A lot of bottled waters do no.  So be sure to read the label.  If you’re unsure, its always a good idea to put a small pinch of sea salt into a litre of water.  A pinch of sea salt will contain typically around 100mg of sodium and about 25mg of potassium.  It’s also a good source of calcium, magnesium, chloride, and phosphate.  So even if in doubt, add some salt.  It’s good for you in many ways.

Now, what about when exercising, and/or exercising in hot weather.  The general recommendation during exercise at normal/room temps is to consume double your normal hourly allotment per hour.  So, take the 200lbs person again.  He needs 100oz per day.  Divide that by 24hrs means he needs about 4oz per hour.  Double that is 8oz (1 cup).  So this person needs about 1 cup of water per hour when exercising.  But this water does not count towards the previously established minimum of 12.5 cups for this individual.  You’re going to sweat that out, so you are simply replacing it.  It doesn’t count to your daily total.  For temps above normal, add more.  The base recommendation is for temps above 24 degrees C, add another half.  So the 200lbs man now needs 12oz per hour.  At temps above 27 C you double it.  So now the 200lbs man needs 16oz (1L) per hour.  Above 30C you stop exercise.

In addition to the added water, at hot temps you’ll also need to consider replacing electrolytes.  The base recommendation for this is an additional 200-250mg of dissolved solids per 1/2L of water.  As an example, a 480mL bottle of gatorade will contain 160mg sodium, 45mg potassium, and about 45mg total of other minerals, for about 250mg per 1/2L.  It’s actually fairly well formulated.  Other options are Coconut Water, BioSteel, or plain and simple sea salts.