The term "Isotonic" means "the same concentration", so an isotonic drink will contain the same concentration of mineral ions (e.g.- pottasium, magnesium, iron etc.) as your own body. This is to make sure that there is no osmotic gain or loss in the cells of the body.
In answer to your question, a long distance runner should choose an isotonic drink rather than cola because the drink will provide and replace the lost mineral ions through sweating and breathing. The isotonic drink has the same concentration of minerals in body as it does in the drink whereas the cola will not replace any of the ions lost from the body (it'll just replace a bit of glucose).
Hope this helped you out a bit :) x
Answered Sat 24th August, 2013 @ 12:16 by Aishwarya
Yes it is the mineral ions that makes the sports drink better. They make sure that the concentration of body fluids remain the same even if lots of whater is lost from cells.