Tom brings up some points near and dear to my heart as a nephrologist. First, i would say that the book A Step Beyond: A definitive Guide to ultrarunning has really been helpful to me as a novice ultrarunner. It can be purchased on Ultrarunning.com. Also the magazine Ultrarunning is a fantastic resource on races, times and all things ultrarunning.
As a side note, not only have I treated all of these issues I have personally experienced many of them :)
First with regards to hydration. Thirst does not start until 0-2% weight loss has occured and when thirst starts, you already starting to decline significantly in performance. Do not wait until you are thirsty to drink. This is especially important in high fluid loss states: sweating, vomiting, and diarrhea. Increased humidity and wind, even in the winter, will dramatically increase fluid loss.
That said, you can drink too much. So there are really 5 categories of electrolye/fluid imbalance:
dehydration with hypernatremia ie high blood sodium (not enough WATER): usually very thirsty, weight is down, salty foods taste BADnot urinating or concentrated dark urine so need to drink water but not take in salt tabs until back in balance.
dehydration (normal electrolytes, not enough fluid, ie sports drink): weight is down, urine dark or low, thirsty, dizzy, may be cramping if dehydrated enough, salty foods taste good, need sports drink or water and salt tabs
dehydration with hyponatremia (not enough volume and electrolytes): RARE, weight is down, high thirst, this is a difficult situation to get yourself into unless you have diarrhea and are drinking only water.
Hypernatremia ie high blood sodium (RARE)- thirsty but salty foods taste bad, desire for water high, this can occur if you take too many salt tabs without water.
Hyponatremia (common) normal weight, quesy stomach, salty foods are craved. This occurs from drinking too much water, usually seen in slower runners in a marathon or during a long race where someone is only drinking water or sodium free fluids.
The longer the race, the more likley that any or all of these can occur at some point durin the race. You have to determine you own sweat rate. This can be difficult. Once it starts getting hotter, try to run during the heat of the day. Weigh yourself before and after your run. If you drank fluids or urinated, you will need to figure that into your weight. Try different concentrations of fluids, gatorade, heed, perpetuem etc. Try taking different salts tabs and see how your stomach does with these. This will help you stay in balance during a race.
Diarrhea is almost always a sign of too much concentration of electrolyes or carbohydrates. So try cutting back on gels or salt tabs and drinking small amount of gatorade and eating small amounts of more solid food. Diarrhea can be a sign that you are going "too fast", that the bowel wont absorb or it can mean that your bowels are swollen if you have taken in too much salt tabs. Check you legs- are they swollen? If they are swollen above the ankle, you may have too much salt in your body. If you have taken in too much salt, you wont be craving it. The feet may swell at any time just from trauma or the shoes or socks being too tight so they are NOT a good indicator of your hydration. If your skin tents up when pinched ie doesnt snap back down when you pinch it together, you are dehydrated. Check this on the back of your hand of forearm gently.
Nausea or a stomach that is "sloshing around" means the stomach isnt draining. Again you may be going "too fast" and not absorbing or what you have been taking in may be too concentrated to absorb. Try to drink small amounts of normal temperature water slowly. very cold water can cause cramping. Pour the ice water over the head, but try to keep the drinking water not too cold IF you are having stomach problems. You can recover from massive vomiting. In 2007 when I ran CanLake it was in the 80s and I pushed too much gatorade. I puked from mile 25 to 35 and developed severe calf cramps and could barely walk. I started drinking small amounts of water and took a salt tab every half hour until I could run. I ran from mile 35 to the finish.
Any of these problems can be over come if you know what to look for. The best indictor though is yourself, and what has happened to you in the past. What may work for one person, wont work for the next. Dont give up. keep trying different things. I now eat a large meal (yogurt, banana and oatmeal plus 8 oz of perpetuem before my longs runs). This has pushed my bowels to be able to deal with having to digest while I run. I dont recommend trying this before a race though. Just in training.
After a race, almost as soon as I cross the finish, RAISING MY HANDS, I take a couple salt tabs to prevent cramps. Dropping you hands and placing them on your knees may cause you to throw up on the race directors shoes. This is not pretty. Sorry Tom.
I hope that this is helpful.
Matt Chaffin
manlius, ny