NutritionLogic

Hassle-Free Hydration

We reviewed the concept of water balance last week and saw that the quick advice was "drink 2 L of water per day".  So what is the scientific basis for this?  Google searches give many hits but a concrete answer is elusive.

One line you'll see in the search results is "metabolism requires 1 mL water/kcal energy produced".  Since average caloric intake is 2000 kcal, this seems to be where the 2 L rule comes from.  This makes some sense because the water you need varies based on your size and activity level, but I'm not sure what "requires" is supposed to mean here.  As we discussed last week, metabolic reactions produce water, so you gain water this way rather than using it in metabolism.  

Another article maps water losses to standard values and standard activity levels.  The 1 mL/kcal rule is estimated using drinking and metabolic water balanced against loses and intakes for the day (see eat to stay fit and healthy).  This makes more sense, but deriving the water you need after assuming all the other inputs are "typical" isn't rigorous.

A half day's research didn't lead me to an obvious first principles answer.  It did suggest simplistic "drink x L/day" guidelines are useful for narrowing the target to somewhere in the liter range per day, rather than, say, milliliters or tens of liters.

So lets go beyond the simple rule and consider why you want to drink water.  Getting "enough" water means avoiding chronic dehydration.  When you lack water, your body rations how it's used, which in turn prevents optimal cellular function.  If you search for "chronic dehydration health effects" you'll turn up many issues:
  • fatigue
  • constipation
  • high blood pressure
  • respiratory issues
  • eczema
  • high cholesterol
  • acid body pH
  • digestive problems
  • asthma
  • weight gain
  • skin disorders
  • joint pain
  • urinary problems
  • premature aging
Some of the reasons given as root causes for these problems make little sense to me (the weight gain descriptions are baffling) but if even a few items are relevant to you, some basic steps to avoid chronic dehydration makes sense.

Since your body varies urine concentration to balance out highs and lows in water intake, the first sign of a problem is concentrated urine.  If concentrated urine is a clear signal, then you need to monitor it and drink enough to keep it dilute.  No need for chemical analysis here, you can just watch to see if it is a light yellow.

The way I go about balancing water intake is by:
  • drinking a glass of water when I get up; this addresses sleeping respiration losses and gets the day started
  • drinking a glass when I feel thirsty
  • drinking some water with meals
  • if I see my urine is darker than a very light yellow, drinking a glass when I finish in the bathroom
Those four rules keep things ticking over fine for me.  When you're getting started it is useful to follow these rules as well:
  • because dehydration causes headaches, drink a glass when you feel one coming on rather than reaching for a painkiller.  This will help your liver and your wallet as well.
  • drink a glass whenever you notice you have cracked or dry lips
Note: a "glass" for me is two thirds an Imperial pint glass, or 375 mL (1 1/2 cups).

That's it.  No need for timers or a personalized bathroom stall, just a little body awareness and establishing a habit when you get up.

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