But do we?
A study published way back in 1991 tried to answer that question.
The researchers took 8 cyclists and ran them through 6 gruelling hours of intermittent cycling in an 86°F room of 50% humidity, and kept their intensities at 55% of their VO2 Max. Each cyclist repeated this exercise 3 times on 3 separate occasions and 3 separate conditions. Once with no water or sports drink, the next time with just water, and lastly with just a sports drink where in both the water and sports drink treatments both were provided in quantities sufficient to replenish their volume of losses to sweat. The cyclists would cycle for 13 minutes, and then take a 2 minute break during which their heart rate, rectal temperature, and their perceived exertion were measured. Blood was drawn at the 15 minute mark and at hours 2, 3, 4, 5, and 6 to measure plasma sodium and plasma aldosterone.
The results were striking and easy to describe.
Simply put, if the study is to be believed:
1. You don't need to hydrate yourself at all during non-dramatic bouts of exercise. Certainly if you're thirsty, that's your sign to drink, but if you're not thirsty, probably no need. In this experiment's case, when drinking nothing, these cyclists DID have to stop the experiment short, but only after a long 4.5 hours of cycling (where the experiment was stopped short due to a marked rise in core temperature, heart rate and perceived exertion).
2. Unless you're exercising vigorously for longer than 6 hours straight, you don't need a sport drink to replenish your "electrolytes", because in this study, there was no difference in plasma sodium levels between the water drinkers and the electrolyte drinkers even after 6 hours of heavy-duty exercise!
I asked my friend Alex Hutchinson, author of the fantastic Which Came First Cardio or Weights his thoughts on the matter and he put it rather succinctly,
"electrolytes are irrelevant in the vast majority of situations".So definitely don't ignore your body's cues and do drink to satisfy any thirst you might have when you're exercising, but if your exercise isn't extreme and it's less than 6 hours in duration, you probably need never "replenish" your electrolytes.
[And just as a reminder of what's in most sport drinks, below is my homemade Powerade video from a few years ago]