MadSci Network: General Biology

Re: How much water does the human body need per day to

Date: Tue Mar 19 16:47:01 2002
Posted By: Peter Bosani, Grad student, Music and Science, McGill University
Area of science: General Biology
ID: 1015526085.Gb

Hoping you're not stranded as long as Gilligan was, I'll give you the 
numbers to calculate your water requirements.  This is tricky as there 
indeed are many variables such as size, activity level, ambient 
temperature, body temperature, age, gender, hormonal activity, etc.  Also 
keep in mind that eating fruits alone would lead to nutritional 
deficiencies over time, but talking hypothetically of course.
Generally for a healthy adult the recommended amount of water is about 1 
ml. per kcal so long as the diet is not too high in salt and there is 
minimal physical exertion and no sweating.
First a calculation of one`s energy needs is required, translated into 
caloric needs.  The least amount of energy one requires is known as basal 
metabolic rate (B.M.R.)  To calculate one's B.M.R.= 1 kcal.x kg. of body 
weight x  24 hrs. (for men.).  Women's formula = 0.95 kcal. x kg. of body 
weight x 24 hrs.  Sleep which requires less energy must be deducted from 
the former number.  Sleep is figured at (0.1 x kg. of body weight x hrs. 
of sleep).  So a woman that is 5'4" with an ideal body weight of (121 
lbs.) 55kg. multiply by 0.95 kcal x 24 hrs = 1254 kcals.  This is her 
B.M.R. minus sleep (0.1 kcals. x 55 kg. x 8 hrs sleep) = 44 kcals = 1210 
kcals.  Generally women's B.M.R. ranges from 1200 to 1450 kcals. while 
men's range from 1600 to 1800 kcals.  Therefore the minimum water 
requirement based on B.M.R. = 34 to 68 fluid ounces.  However, I assume 
that being stranded on an island she would not be a couch potato
(sedentary), or doing the macerena (strenuous).  More likely she'll be 
searching for food and praying to be discovered soon.  This is light 
activity which requires 50% more calories than B.M.R.  (Moderate or 
strenuous activity demands an extra 75 to 100% more calories than the 
B.M.R.).  So now we must add her activity requirements onto the B.M.R.= 
(50% of 1254)= 627 kcals plus 1210= 1837 kcals.  Now digestion requires an 
extra 10% energy so another 184 kcals. must be added to the formula 
totalling a daily requirement of 2021 kcals.  Rounding down the number for 
simplicity she needs to drink 2,000 ml. or about 68 fluid ounces.  (She 
could survive on less but this is an ideal amount to avoid problems of 
dehydration.  Although your island has no fresh water source it has 
fruits.  Lucky because fruits contain 70 to 90% water.  If she can grab 
some bananas away from the monkeys then she'd have to eat about 19 
bananas, each banana giving her 105 kcals. along with 84.7 grams (3 fl. 
oz.) of water.  An even better fruit would be the papaya, each one 
containing 117 kcals. and 270 grams of water, (about 9.5 fl.oz.)!  She'd 
only have to eat 7 papayas to fulfill her water needs.  Or lastly, she'd 
have to entice those monkeys to knock some coconuts down.  She'd only have 
to remove the husk to get at the nut, pierce a couple of the 'eyes' and 
drink the coconut water.  Ah, now if she could only find that buried 
buccaneer rum she might be tempted to stay a while longer, and that's no 
monkeyshine!    Sources:  Jean A.T. Pennington + Helen Nichols Church -
 "Food Values of Portions Commonly Used (Publishers: Harper & Row) & 
Applied Nutrtition and Diet Therapy by Burtis, Davis, + Martin (Published  
W.B. Saunderss Co.  Hope that helps. 

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