If they already eat a healthy diet and get enough sleep then about the only thing most people who exercise need to supplement is water.
I'll wholeheartedly agree there, but Imo the problem is most of us don't have the time or the knowledge to always stay within a properly balanced nutritional plan.
I know that BCAAs such as valine, leucine and isoleucine have been proven to reduce post exercise muscle damage, what better and easier way to insure I'm getting at least enough through supplements or vitamins.
Whey protein with some fruit added is an easy low fat and low calorie way for me to have pre and post workout snacks, and get many of my much needed amino acids.
Eating real food with some supplementation I can assure myself I'm getting most of what my body needs, and that makes planning healthy meals and watching caloric intake an easier task.
