According to Jennifer Hutchinson, a dietician, nutritionist, and USAT Level 2 coach, "There is an endless debate on how much and what kind of electrolytes an endurance athlete needs, and whether or not electrolyte supplements are necessary."
Hammer Nutrition, maker of Endurolytes says its surveys indicate that successful athletes took in 3-6 Endurolytes, with 4 capsules being the most often reported dose. Endurolytes contain 40 mg of sodium, a moderate dose. Hammer also says, "Salt: less is best!" and recommends consuming less than 2,500 to 3,000 mg of sodium daily.
Let's review the important electrolytes in the body.
Two electrolytes play key roles in fluid balance. That is because these are abundant and exist as hydrated, or "surrounded by water" species. These two, sodium (Na+ and K+) can flow through specific pores in membranes or are pumped across those membranes by specific (energy consuming) pumps. These pumps make up a large part of what is called basal metabolism. The fluid content of a "compartment" such as the intracellular space (i.e. all the cells), or the intercellular fluid space, is effectively controlled by these electrolytes.
Sodium is the extracellular electrolyte. It is an alkali metal that is a component of table salt (or sodium chloride). Sodium is also the key component of any fluid secreted from the extracellular space, such as sweat. Blood makes up 1/4 of the extracellular fluid.
According to Hutchinson, Chris Leigh, a pro triathlete lost 2.2L of fluid per hour in a standard test at 70F or 21C. The fluid contained 1.5g Na/L and so his total loss was 3.3g per hour (3,300 mg). I have personally measured losses as high as 2.5-2.6L per hour. I never measured the Na content of my sweat but I have reason to believe it is high. I sweat a lot and my sweat is very salty. Amounts of fluid lost and Na content in fluid varies from one individual to the next.
Potassium is the "other" electrolyte. It maintains intracellular fluid levels. For that same reason K+ is plentiful in foods with high numbers of cells, such as fruits, vegetables, etc. A normal diet supplies potassium far in excess of what is needed. The amount of K+ lost in sweat is also very minimal (100-200 mg/L). Supplying extra K+ during exercise is of limited value. Excess blood K+ is toxic and the kidney has an enormous excess capacity to remove K+ lest it becomes dangerous. Unless you inject K+, which can be deadly, you need not worry about either too much or too little.
Two other electrolytes play key roles in the body. These are often added in supplements and as part of multi-vitamins. They are calcium (Ca++) and magnesium (Mg++). Ca++ is the mineral component of bone. Largely because of osteoporosis, calcium is added to anything and everything people can think of from juice to water, to tablets. That is ironic since Ca++ plays no role in osteoporosis and this fact has been well known in the medical community for decades. Yet everyone recommends women ingest a lot of calcium.
Calcium is also necessary for muscle contraction and nerve conduction. Yet excess intake before, during, or after exercise is a waste of time (and money). The amounts of calcium needed for these important functions are extremely minuscule. Furthermore, more than 1 kg (yes 1,000,000 mg) of calcium is freely available and in equilibrium with the blood. Excess calcium only leads to kidney calcification, kidney stones, and other unwanted and nasty side-effects.
Magnesium too is important for nerve function and muscle contraction. It is the fourth most common mineral in the body, with about 50% in the skeleton. Magnesium deficits are extremely rare and only occur in patients with genetic disorders, advanced kidney disease, and those taking diuretics.
Mg has been implicated in cramping and it is often added to supplements and drinks. There is no evidence to support this and it is extremely unlikely that anyone would suffer Mg deficits during a race. It is totally unlikely that oral Mg supplements could correct such deficits if they existed. Both calcium and magnesium for contraction and nerve conduction are highly sequestered in separate compartments with tightly controlled levels. Furthermore, there is evidence that fatigued muscle cells actually dump Ca and Mg in the extracellular fluid. Such active mechanisms cannot be counterbalanced by oral ingestion of minerals. In these cases oral intake just adds to the already increased plasma load and puts stress on the kidney.
The "negative" electrolytes such as Cl- and HCO3 are no less important than the positive ones However these are packaged along with the positive ones as NaCl or NaHCO3, etc. to maintain electrical neutrality. Bicarbonate is also in equilibrium with CO2, and respiration can quickly restore any imbalance.
In short, all that is needed during a long race is to replace some of the Na lost. It is probably best to replace the bare minimum. Contrary to what you hear all the time, electrolytes do not prevent or cure cramps. To prevent cramping make sure you train properly. Proper training will also mitigate any negative effects from fluid or electrolyte losses. Some amount of dehydration is normal during a long event. The better trained you are the less it will affect your performance. Replenishing during the race will only work if you are trained well. It will not make up for lack of training.