"The data shows that even the most experienced runners are unable to accurately estimate their sweat losses and cannot subjectively judge how much fluid to drink to prevent dehydration."
Mary Horn, M.S.- scientist at the Gatorade Sports Science Institute (GSSI).
We're fully in to summer and the importance of hydrating properly is now (or should be) at the forethought of us all. In terms of performance, dehydration will have a huge negative impact on the outcome of any race or training session. Our bodies are composed of ~65% water... we've all heard that. But did you know that our brains are ~70% water and that our lungs are nearly 90% water? Let me repeat that- our lungs are nearly 90% water. As an athlete you should really think about this. In terms of oxygen supply to our blood stream, dehydration will have an even more profound effect. And blood- plasma is also 90% water which makes makes up 55 percent of blood volume. Blood volume is a HUGE performance factor because with out adequate blood volume you simply can't supply the muscles with what they need to function. The digestive tract is also profoundly affected by inadequate hydration. As intensity increases blood supply to your GI tract is reduced (think fight or flight survival) which makes it more difficult for your stomach to empty. If you eat too many calories with out adequate dilution with fluid it will not empty and the food will simply sit. With out adequate hydration your digestive tract simply can't absorb calories as effectively
That last sentence should also stick in your head- with out adequate hydration your digestive tract simply can't absorb calories as effectively. So before you even consider the number of calories per hour you should be ingesting.. start with hydration otherwise the calories will sit in your gut and give you a bloated feeling. And if your exertion level is low enough- your body will actually take water from your blood stream in order to help digest the food, compounding the dehydration issue.
Where to start in figuring out how much to drink. First of all there tons of factors that determine how much to drink. Body weight, body composition, exercise volume and intensity and life style factors like your job and every day activity. The very first thing to do is pay attention to what your body is telling you.
Dark urine is first. That's simple to see. And if you pee clear after 4 cups of coffee that may not be accurate.
High exercising HR- this is due to increased blood viscosity, decreased muscle flexibility and decreased lung function (lack of oxygen to the blood forces your heart to compensate by beating faster).
Body weight- before going out for a run weigh yourself. Then after the run weigh yourself again. If you lost weight then you lost water. Simple. There is no way for me to say that X amount of water is ok to lose. But- it is nearly impossible to drink enough that you don't lose a little bit.. so you should be drinking as much as you can or are willing to. Every pound of body weight lost takes ~24 oz of water to replace. The funniest thing I hear is that a hydration pack is "inconvenient" or feels "bulky". Funny choice..
Nausea after a run? Dehydrated.
Loss of appetite? Dehydrated.
A rough guide for hydrating that I tell my athletes- drink 1 liter of water in the hour preceding any run. It will be a good idea to add salt to your diet in the summer. My favorite is watermelon and salt. V8 juice is nearly ideal for many reasons. One thing to consider too though is consistently adding too much sodium to your diet will cause your body to flush sodium. It senses that there is an excess and it will learn not to hold on to it. Sodium loading is another subject (try www.google.com) that is quite individual. So don't go crazy on the sodium.. a little (~1g per day) go a long way.
Immediately after every run in the heat you should drink another liter of fluid.. not necessarily water though. Gatorade is still my favorite. If you're a triathlete with ~20 hours per week of training that includes weight training then consider a recovery specific drink with a carbohydrate to protein ratio of ~4:1.
During training (I would say that a run of less than 1:00 would be ok to not hydrate if you drink 1 liter before starting) you should be trying to hydrate with 8-10oz of fluid every 2 miles. This equates to ~2 swallows every mile. For my long runs going through a 50oz Camelback even in moderate temps is still too little. Starting well hydrated is a good way to help with this so don't be afraid to start a long run with a bloated stomach of fluid. It will clear before your warm-up is over.
Still confused... then mimic your race goal as closely as possible! Simple. If there are aid stations every mile and you plan on drinking every mile then practice this. The thought that you race how you train is so understated on many levels! In the last 8 weeks you need to train as specifically as possible to your race goals.
Hyponatremia is my last thought. It occurs when you drink too much water and you flush out electrolytes. This occurs in roughly 1 in 1000 marathon runners and is most common in 4+ hour finishers. It's tough to become hyponatremic. Adding small amounts of sodium to your diet so you don't start the race already deficient will help. Don't drink just water- Powergel with 3X electrolytes is great stuff. So is Gatorade Endurance which specifically addresses this issue.