Hydration — Critical for Life

Water is vital for life.  Without water, humans can survive only for days.  Our bodies are even mostly water – 50-60% in women and 60-65% in men.  Water content of our bodies depends on composition – lean muscle consists of approximately 75% water, while fat tissue consists of only about 10% water.

Water balance

There are intricate mechanisms in the body to regulate water balance, mostly via the kidneys that are tasked with filtering the blood and regulating both fluid volume and electrolytes.  It’s well known that inadequate fluid balance (dehydration) is problematic and can impair our ability to function.  Similarly, excess fluid balance may be problematic in individuals who are otherwise healthy as well as in individuals with certain chronic disease states, such as kidney disease, cirrhosis, or heart failure.

Water volume can be lost through various mechanisms.  In simple terms, there are 2 types of water loss:

• Sensible water loss is what’s actually visible – mostly in the form of urine and stool, also including sweat.
• Insensible water loss is the amount of fluid lost by other processes such as respiration and evaporation from the skin.

Insensible water losses can be significant and often go unrecognized, a common issue among critically ill hospitalized individuals.

Other than water intake and the aforementioned forms of water loss, factors affecting hydration status include:

• Altitude – at higher altitudes, with lower air pressure and humidity, the evaporation of water from the skin can be accelerated
• Climate – temperature and humidity affect rates of sweating
• Level of activity – generation of sweat and utilization of water

Water intake

Most water is consumed in the form of various beverages.  The water content of food is also relevant and contributes to your total water intake.  It is estimated that 20-22% of our daily water intake comes from food.  That percentage, of course depends on the composition of one’s diet, with fruit contributing more significantly to water intake than other food items.

Rehydration: Does the choice of beverage matter?

Historically, there has been concern that caffeinated beverages (like coffee or tea) or alcoholic beverages actually work against your rehydration efforts due to their diuretic effects.  That turns out not to be true.

A study that examined the hydration potential of various beverages demonstrated that coffee, tea, and alcohol (beer, in this study) are as hydrating as plain water.  Furthermore, beverages containing solute – oral rehydration solution, orange juice, full-fat milk, and skim milk – were more hydrating than plain water.

Problems with too much water

The main concern with excessive water intake is hyponatremia – a dilution of the blood such that the concentration of sodium is too low.  Some publicized examples include a woman dying from water intoxication from a radio contest titled “Hold your wee for a Wii” and another woman who died of hyponatremia after trying to rehydrate herself with a large amount of water intake.  This problem can be mitigated by drinking fluids with electrolyte content or by rehydrating more slowly.

For those who no longer have a gallbladder, too much water intake can dilute the stomach acid, resulting in less efficient digestion of dietary fats and thereby leading to loose stools.

How much water should I drink?

The oft-cited advice of drinking 8 glasses (8 oz) of water daily is not reliable, as one’s daily requirement of water may change dramatically depending on environmental conditions and activity levels.

Given the many factors that affect hydration status, it’s impossible to have a one-size-fits-all recommendation for fluid intake.  The most useful indicator of your body’s hydration status is the appearance of your urine, as your kidneys are tasked with the complex job of regulating fluid and electrolyte status.  Your urine should be light- or pale-yellow.  If, however, your urine appears prominently yellow, that indicates that your kidneys are holding onto water and you may be dehydrated.

For proper rehydration, it is probably best to sip smaller amounts of water throughout the day rather than chugging large amounts less often.  When consuming a large bolus of water in a short time, a protective mechanism causes a larger proportion of that fluid to be eliminated as urine, in order to avoid excess dilution of electrolytes.