Electrolytes and water
Osmosis is the movement of water from an area with lower solute concentration to an area with higher solute concentration; body fluids are hypotonic, isotonic or hypertonic if they have concentrations that are respectively less, the same as or higher than the solutes in the cells.
Electrolytes largely determine concentration: these are electrically charged cations such as sodium, potassium magnesium and calcium and anions such as chloride, phosphate, bicarbonate and protein.
Water in the body is either in the intracellular fluid (ICF) which accounts for about 65% of the total, or in the extracellular fluid (ECF). All cells are freely permeable to water but require either fluid (hydrostatic) pressure or osmotic pressure for movement to take place. The cardiovascular system provides the hydrostatic pressure but osmosis is only possible if solutes are present in the water.
The composition of plasma and interstitial fluid (found between the cells) is primarily made up of sodium cations and chloride and bicarbonate anions. The intracellular fluid (in the skeletal muscle) however, is made up of mostly potassium cations and phosphate and protein anions. Due to these differences in concentration, there is constant pressure for sodium to leak into the cells an for potassium to leak out; the role of the "sodium potassium pumps" is the cell membranes is to maintain these concentrations. If the concentration of sodium increases in the extracellular fluid (by heavy sweating, for example) water would move by osmosis from the cells and into the ECF in order to dilute the ECF fluid.
Increased water vapour in the air (humidity) reduces the ability of sweating to lose heat by evaporation, even if it is cold. In dry, cold environments, more fluid is lost by ventilation. Trained athletes start sweating sooner and have a higher sweat rate than sedentary individuals.
The immediate effect of sweating is an increase in the concentration of sodium; it is the consumption of hypotonic drinks such as water that dilutes the sodium concentration leading, in extreme cases, to the potentially fatal condition of hyponatremia. An increase in sodium will result in an increase in ECF (by osmosis).
In endurance events of 2 hours or longer, it is recommended to take in sodium. The sodium concentration of sweat is between 230 and 1310mg/l. The recommended amount to ingest is 1g/h for heavy or "salty" sweaters, in a concentration of between 0.5 and 0.7g/l. The Tolerable Upper Intake Level (TUIL) of sodium is 2,300mg/d in an effort to reduce blood pressure but these recommendations do not apply to athletes; some athletes consume more than 5,000mg of sodium a day and excess sodium is excreted in the urine.
Potassium supplementation is not recommended because o the risk of hyperkalemia (occasionally seen in bodybuilders); rather one should obtain potassium from fruit and vegetables.
The maximal rate for gastric emptying is approximately 1-1.5l/h which may be less than the sweat rate and is even slower at exercise intensities of 70% VO2 max or more.
There is no hard evidence to suggest cramps (other than all-body heat cramps) are influenced by dehydration or electrolyte imbalance, as is commonly believed. The current thinking is simply that they are caused by muscle fatigue and corresponding "malfunctioning" of the firing mechanisms.
It is possible to hyper- (over) hydrate by taking glycerol, for example, but no corresponding performance improvement has been scientifically established and it's use is not recommended, except under strict guidance of a qualified professional. A typical strategy would be to consume 1-1.2g/kg of glycerol with 25-35ml/kg of water. This could lead to fluid retention of approximately 500ml more than fluid overload alone. There is a risk of gastrointestinal distress (a polite way of saying "the runs"), however. (I tried this about 4 years ago and didn't notice any effect, positive or negative. The fact that I could only source glycerol by asking for glaucoma medicine made me a bit reluctant to repeat the experiment.)