Keeping well hydrated involves more than just replacing fluids. When we sweat during exercise the body loses sodium and other minerals as well as water. Sodium and other ‘‘electrolytes’ are important in helping maintain fluid balance in the body and also play a role in cognitive function, absorption of nutrients, nerve transmission and muscle contractions. Those salts cannot be generated or stored by the body and typically get replaced in sufficient quantities through the food that we eat.
Generally, most people exercising for an hour or so at a time will not need to worry about this and although some sources suggest calculating precise amounts (based on weight lost during a session) drinking to thirst is probably sufficient to replace what you need. You may find you feel better for taking on an electrolyte mix but you are unlikely to suffer problems without it. When the duration of exercise increases, to two hours or more, it is worth paying closer attention to how that affects you.
Replacing only water and not replacing enough salts may initially manifest as fatigue, impaired performance, and muscle cramps but at very extreme levels can lead to a condition called hyponatremia which in some cases has been known to be fatal.
There is a lot of individual variance in the amount of salt in sweat. Some people sweat more salt than others and therefore, especially when exercising for extended periods or in hotter conditions, could benefit from including electrolyte supplements or simple table salt in the drink you take in while exercising. Some indications of whether you’re a more salty sweater are that you may be able to taste it on your skin or it shows as white marks on your clothes or kit.
As always, it also helps to make sure you’re recovering well between sessions, eating a balanced diet, getting plenty of sleep at night (I know this is harder in the heat) and listening your body.
If this sounds like something you might be affected by I would recommend that you take the Precision Hydration Sweat Test.