Wednesday, July 29, 2009

Insensible Losses

I weigh about 190 pounds. That's equal to about 85 kilograms.

To figure out the appropriate hourly fluid replenishment for a (basically) healthy 85 kg patient just lounging around in the hospital we follow the 4-2-1 rule, which is that for the first 10 kg we supply 4 cc per kg per hour, the next 10 is 2 cc per kg per hour and then 1 cc per kg per hour for the rest of the weight. So 40 + 20 + 65 = 125 cc per hour.

This means that a basically healthy 85 kg guy (like me) loses around 125 mL of fluid from urine and insensible losses (think sweating, respiration, etc) per hour.

Over a 25 hour period that amounts to a nice figure of 3,125 mL or 3.125 Liters. Think about it - that's a huge water debt. A big bottle of soda is only 2 Liters of fluid.

People don't drink enough before a fast and it's dehydration far more than hunger that makes them feel like crap.

So drink well and have a meaningful fast.


Anonymous said...

Your mistake is in equating hydration formulas with loss. Those formulas err far on the side of caution; you do not normally lose that much water in that time. Your advice ("drink!") is right, but your justification is off.

Orthoprax said...

True enough. Average daily losses are more like 2-3 liters. The point is that if you were fasting in the hospital you'd [I'd] be getting 125 cc/hr by IV.

George said...

To what extend can one store up water? Are humans like camels?

The Leader, Garnel Ironheart said...

Not only do you need to drink water, but you need to do it over the preceding 12 hours, not just guzzle a jug before the fast.

Also, don't forget caffeine. When I was in med school, I got a wicked migraine by 2 pm of every fast day and for 3 years I tried everything, including overloading on water the day before. Finally one of my profs asked me the magic question: How much coffee do you drink?

E-Man said...

Can anyone explain to me why I get cramps in my mouth when I try to eat after the fast?

Orthoprax said...


"To what extend can one store up water? Are humans like camels?"

Minimum healthy obligatory water losses are in the 1200 to 1600 mL range. Insensible losses are largely unavoidable unless you cover up all your skin and stop breathing. And you need to get rid of certain toxins every day while your kidneys can only concentrate your urine so much. Over time if you get seriously dehydrated, kidney failure can begin.

Humans don't really store water physiologically but we typically happen to have an abundance in the healthy state. The human body is something like 60% water by weight, so sitting here I hold about 50 Liters of water in me - though health problems will start far before you get even close to that number.


"Can anyone explain to me why I get cramps in my mouth when I try to eat after the fast?"

I've never experienced that myself but I've heard of it from other people. I'm not sure of the reason but I'd bet it has something to do with your salivary glands kicking in.

Michael said...

Isn't alot of the water stored together with glycogen, so that when we fast and use up glycogen, water is released in the process? Therefore making dehydration less of a problem?

Orthoprax said...


The liver stores about 100 grams of glycogen after a big meal. With about 3 grams of water being locked with each gram of glycogen this amounts to only a replenishment of some 300mL.

Though basic metabolism via the Krebs cycle and oxidation will net ~16.7 mL of water per 100 calories burned. This can also net more than 300mL of water on fasting calories burned.

This does not make up for the amount being lost.

Michael said...

Thanks for explaining,

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