If you take in enough isotonic mixture, your body will keep what it does need and flush out what it doesn’t need, either through your bladder or your bowels.
Of course, it is not a perfect world, and a 100-mile running event, at high altitude, with monster 1000ft+ climbs, in weather flipping from cold to heat to cold again, is going to stress your system to the max. Trying to guess what your body needs exactly is nearly impossible. The aid stations are handing out completely erratically mixed beverages. It’s up to each runner to figure it out.
When in doubt, drink a known neutral mixture. This is one of the things having your own drop-bag is essential for. Your drop-bag is a reliable source for controlled-ratio mixtures. If you have a crew, then your bottles need to be marked clearly and your crew instructed clearly which-is-which, and when to hand you what. If you’re in a stupor, your crew needs to figure out if it’s from hypoxia, dehydration, hyponatremia, or low blood sugar.
(As for the latter, have any of you considered using a diabetic glucose tester in the later stages of a race to see if you're heading toward a glycemic crash?)
Many of us have finished a race during summer months and after our sweat dries, we’re covered by a comical streaking of white salt. It might be worth it, to judge the quantity of salt you’re expending, to open an electrolyte capsule and spread the powder on your clean, dry arm. See how much surface-area the contents will cover, leaving it as white and thick as you usually are at the end of these events. Then extrapolate for your body. THAT is how many electrolyte capsules you have coating your body. And that is how much more salt you need to consume (in addition to the usual urine) during a race to stay neutral.
Some of us sweat more than others because we’re all pushing at different intensities, and some of us have lower conditioning. An elite runner won’t burn as many calories to go the same distance, even at a faster pace. Fewer calories means less heat (byproduct of calorie-burn is heat) which in turn means less sweating, which means less breathing, lower rate of dehydration, and less loss of electrolytes. The worse shape you’re in, the more you hemorrhage electrolytes, water, and calories, and the more you build lactic acid. Even small reductions in weight and increases in systemic efficiency have a significant difference in the snow-balling long-term effect it has on your physiology. The lower your rates of expenditure, the easier it is to stay on top of your rates, levels, and ratios. When you end up with a deficit, it can cause serious problems, which lead to DNFs and/or much slower finish-times and unnecessary pain. Add to that, the first-place runners only have to be on their feet for half the time that last-place runners are out there.
Everyone has different strategies for maintaining optimal ratios and levels. Some prefer to keep everything separated into measured doses so they can count calories, electrolytes, and water ounces accurately. Some people can’t do math very well during a race and insist on drinking pre-mixed isotonics. Some drink isotonics without any calories, and then eat measure calories. The simplest way of all, is to mix isotonic sport drinks with adequate calories such that you don’t have to eat anything nor take any electrolyte supplements, but that tends to leave a slight mucous in your mouth. You might not be bothered by such a thing walking around town, but during a race, it’s bothersome. Most racers seem to prefer sport drink that is half-strength, and then eat some food and take some electrolyte supplements.
If you have a history of bad race experiences where you just can’t seem to nail the ratios or levels, there is something you can do.
Warning, this is gross, but it is clinically sound…
At the point of collapse, at the end of your rope, pee into a bottle – like an empty sport-drink bottle. Try to fill either a full quart, or exactly half a quart. Then let it evaporate. When all the water is gone, what is left is mostly salt, including ammonia salts and phosphate soda. Remember blood has about 2tsp of salt per quart (~10gms/qt). Pee reflects what your blood was like at the end of the race. If there’s less than 2tsp/qt, then you needed more electrolyte supplements. If there’s more than that, you took too much.
You can use a scale designed for light-accurate measurements, like a digital food scale that can flip from ounces to grams. Measure the whole bottle with the dried contents, then record the weight. Next, clean and dry the bottle thoroughly and weigh it again. After subtracting your bottle weight, you should have an accurate weight for salt. Then finish by multiplying, if you needed, to extrapolate a full quart.
Don’t ever forget the lesson from the sea – no matter how much sea water you drink, you will die of dehydration. If you pop electrolyte supplements like candy, you are basically drinking sea water and your body will react accordingly.