Q. Hey Gale,
I read to weigh myself before and after riding (or any exercise for that matter) to determine how much weight I lose during exercise.
If I lose a pound of sweat, how much fluid volume do I need to drink to replenish that lost weight? Does water weigh more or less than sweat? Does it make a difference, in terms of volume, if I'm drinking a recovery drink versus water?
Do recovery drinks weigh more per given volume than water does? In that case, should I try to drink to replace weight lost or volume of sweat lost? I know this all might sound geeky, but I'd like to know. In advance, thanks for your help. - J.K.
A. There is an old saying, "A pint's a pound, the world around." This number will get you in the ballpark, implying that one pint of water weighs one pound. This number is quite close to a gallon of water's exact weight of 8.336 pounds at 62 degrees Fahrenheit. (One pint weighs 1.04375 pounds.) This number comes from the Machinery's Handbook, Twenty-First Edition, a book that I own (which gives you some indication to my personal level of math/numbers geekiness).
To be clear, volume is the three-dimensional space that something occupies. Weight is the force of gravity on an object.
To provide some visuals for you to think about, a gallon bucket full of loose, fall leaves will weigh less than a gallon of water. Both take up the same volume, but they do not weigh the same.
Water is more dense—or has a higher density—than does the gallon bucket of leaves. You can make the leaves more dense by crushing them down into small pieces and cramming more leaves into the volume measure of a gallon until you can't cram any more leaves into the bucket. (I bet you do this each fall when you clean your yard and try to cram more stuff in a limited space.) You have now increased the density of leaves in your bucket.
The Varying Densities of Water
On another visible level, frozen water (ice) is less dense than our 62-degree water. You can see this if you freeze your gallon bucket of water. If you drew a line at the top of the water line, then froze the bucket, you would find the ice expanded above that line you drew on the bucket. The bucket still weighs the same as before you froze it, but ice takes up more volume than the water did.
An added bonus-geek-fact: Water is at its highest density at 39 degree Fahrenheit. Water, obviously, changes density depending on the temperature, which is why you will see varying weights for a liter of water on different websites.
Salt water carries a higher density than fresh water. The density and associated weight per gallon varies with the amount of salt contained in the water. The more salt and other minerals the water contains, the heavier it will be per gallon.
The lowest salinity of ocean water is found in the Baltic sea at 0.5-percent salinity. The Dead Sea is highest at 24-percent salinity. The Atlantic Ocean has a higher salinity than does the Pacific Ocean. On average, seawater has a salinity of about 3.5 percent. Though the salinity value of sweat varies by individual and condition of the athlete, let's assume for purposes of this exercise that sweat has the salinity of average seawater.
Seawater weighs about 64 pounds per cubic foot—on average. "Average" fresh water weighs about 62.2 pounds per cubic foot. Through all the conversions, a gallon of seawater weighs approximately 8.556 or rounded to 8.6 pounds—on average. There's only one-tenth of a pound difference between the two.
How Much Recovery Should You Drink?
A gallon of milk weighs 8.6 pounds, according to Colorado State University. I'm going to assume your recovery drink is about the same density as milk, perhaps slightly more.
All of this information is to let you know that yes, there are differences in the weight of the fluids you might consume post-exercise; but, they are close enough that you could drink any of them at an estimated pint-per-pound to replace lost body fluid.
That written, the nutritional values of the fluids vary considerably. Post-exercise, a homemade concoction of chocolate milk or your favorite recovery drink is your best first option. Follow that with some water and a meal eaten within two hours after exercise.
For more geeky information, try this link from The Straight Dope, "What Would Happen if You Drank Seawater"