it depends on how much I'm sweating. I usally try to drink some sport drink before I start riding on really hot days to boost electrolytes, sodium,potassium. if you start drinking all water this can further dilute your exsisting electrolytes only a problem if your drink a lot of water. usally best to alternate water and sport drinks. Most sport drinks use dextrose, maltodextrose or fructose and these don't spike your blood sugar. you probably could get by on water alone used in moderation for most rides. but any ride over 2 hours I use energy gels, sport drinks and water alternating between the three.