If you want to know the scientific basis for these guidelines and learn more about the physiology and biology, I encourage you to read Waterlogged: The Serious Problem of Overhydration in Endurance Sports. Otherwise, I shall summarize here his guidelines:
1. Your body will tell you what it needs, if you listen. (Keep in mind the human body has robust biological controls. Nothing in nature is wasted; everything has a purpose. To deny this would be to deny evolution or, if you prefer, the perfection of creation.)
2. So...drink ad libitum, according to the dictates of your thirst.
3. And...there is no need to increase your daily sodium intake above that dictated by your appetite, nor to ingest additional sodium during exercise.
4. Dehydration is not a disease, nor does it contribute in any way to any illnesses associated with prolonged exercise.
5. Much of what you believe about your personal well-being is the result of targeted manipulations by industries whose principal focus is their commercial fitness and not necessarily your health or safety. (Marketing!)
This advice is contrary to what many athletes are doing. It is also contrary to the advice I have given over the years. I am trying to remedy that now.
I am sure many athletes and coaches will question these guidelines, thinking that drinking less and being more dehydrated or not supplementing electrolytes for example will put athletes at risk. This is what the scientific evidence says:
Exercise-associated Muscle Cramps or "heat cramps" result from altered nervous control of muscles. Cramps are not improved or prevented by fluid or electrolyte intake. Recover from cramps with rest, maybe stretching.
Heat Exhaustion is just the condition (heat) under which an athlete choses to stop exercising. Basically, it is just exhaustion. It is a condition that the brain uses to ensure the athlete ceases activity before the body temperature reaches heatstroke range. Fluids and electrolytes will not prevent this. Acclimatization to the heat will. Treat this with rest and cooling the body.
Heatstroke occurs when the core body temperature reaches 42 degrees Celsius. Body temperature increases as a function of body size and exercise intensity, e.g. average running speed (think 5k pace versus marathon pace). Generally, with sustained exercise, the core temperature rises, but it seems that the body is able to maintain homeostasis, just at a higher (and still safe) core body temperature. Again, hydration has little effect and electrolytes have no effect. In those that get heatstroke, it seems to be a function of heat production, because even after they are cooled, there is frequently a secondary spike in core temperature. Heatstroke can be prevented with rest, allowing the core temperature to decrease periodically.
Most athletes who collapse after a sporting event are suffering from Exercise-associated Postural Hypotension. The blood-pumping action of the calf muscles, the second heart, stops and the body does not respond fast enough. The brain doesn't get enough blood and the athlete passes out. This can be treated by elevating their legs and hips above their heart and head for a few minutes. Fluids, electrolytes, or sugar will not help. Athletes should continue moving or lie down after an event to prevent this.
Exercise-associated Hyponatremia/Hyponatremic Encephalopathy is brought on by drinking too much fluids too fast, diluting blood sodium so quickly that the body cannot maintain a safe range. This seems to occur in predisposed individuals, possibly with hormonal differences: usually slow runners in long events--back-of-the-packers. It is not related to sodium deficiency. It was rare if not completely absent prior to hydration guidelines which instructed athletes to drink as much as possible, staying ahead of their thirst. This was the impetus for Noakes' research and book, leading to his hydration guidelines shared above.
To summarize, athletes do not need extra electrolytes nor do they need to drink any more than their thirst dictates. Doing otherwise will not improve performance or prevent illness, and in some cases, it can actually be fatal.