The first summer I was running long distances down here, I was only drinking plain water. One long run, I started getting tunnel vision, stopped sweating, felt nauseous and dizzy, and got chills. Classic heat illness heading toward heat stroke, and quite possibly on the verge of hyponatremia. I happened to have a physical scheduled a few days later and mentioned it to my doc. She advised that I was likely very low in electrolytes and told me to 1. start adding salt to my meals during the hot months (I normally never add salt when cooking- not for health reasons, but because I don't like the taste of salt), and 2. to start using Gatorade or any other electrolyte replacement drink. Doing that made a HUGE improvement in my ability to handle summer running! I use Nuun mostly now because I don't need the glucose in Gatorade (I get that form gels), and the Nuun tabs are so handy to carry with me for refills.