Lets start off with some facts:
The human body is about 60% water in adult males and 55% in adult females.
Lean muscle tissue contains about 75% water by weight.
Blood contains almost 70% water
Body fat contains 10% water
Bone has 22% water.
Skin also contains much water
1lb = 16oz/475ml of water
Normal sweat loss is equal to .6lbs/20 mins of exercise
Over a 1hr period you require 28.8oz/851ml to replenish yourself
Note: not all people sweat equally... a test that you can try in training is to hydrate well before your workout.
weigh yourself naked, and then weigh yourself post workout. After zero water added. A one hour long workout will give you better results than cutting it down to 20 mins. (if you are less efficient and need the water, drink a measured amount) take your weight before and after on a digital scale and now you have your water loss rate (this will change based on the weather)
The people that this information becomes most important to are our distance runners/cyclists and efficient swimmers (the rest of us drink enough by accident)
Now make your hydration plan for the race. Take your training times and predict your finish and do the math based on how much sweat loss you will have to recharge during the race.
eg. I plan on taking 4hrs to run my marathon.
I have done an estimate that the weather will be approximately the same as my sweat loss test, giving me a loss rate of 1.8lbs/hr (I've read about top performers and hot days getting up to 3.2L/hr)
4hrs x 1.8 = 7.2lbs (of sweat loss) aprox 4% of a 175lb athlete's mass
replacement value is: 115.2oz/3.4L of fluid
Electrolyte loss starts to effect us at 90mins of exercise. (this is highly dependent on your sweat rate and the level of performer you are)
It's recommended that you start to take in an endurance specific sport drink to help restore your depletion. Normal sport drink may not have enough to bring you back to level based on your burn rate.
There are 2 common practices that you should practice in training 1.) use the same sport drink you plan to use during the race (most races provide Gatorade but expect that sometimes it can be made very strong you may need to water it down) 2.) at water stations alternate between Gatorade and water
the body's response is not the same under extreme conditions... test it, retest it and test again
Foods that are high in potassium:
potatoes (are commonly given out on route during ultra-marathons and greater distances)
yogurt
bananas
orange juice
now put it all together:
you need : 3.4L of fluid
you also need to replace electrolytes: sport drink
How many water stands are on route:? (ie boston has 24 water stations even distributed)
how do you react to having cups of water on route, versus sips continuously
***most races have 8oz/250ml cups at water stations
that means you require 14 glasses of sport drink/water
personally I don't like to chug the water/drink back super fast, so I recommend getting an open mouth camel back water bottle that can be held in your hand and refilled on route. I have used a camel back backpack in the past and found myself with 2 liters of extra fluid at the end of the race because I took cups on route. I have also viewed race organizers helping people fumble with filling packs (test and try this out as well)
If you take a full bottle off the start line you can reduce your stops and as you see them coming up you can grab a glass and top off your bottle to minimize the extra load you are carrying. (having no more than a half liter/1lbs on you at any given time)
Why should you follow this advise? I'm not that thirsty when I run!
Sweat: Loss of >2% of body mass can decrease aerobic and mental performance. The math shows that on an average weather day, for an average person they will get into the 4% + range...
make a plan, stick to it, and practice/modify and perfect the plan well before race day!
