I believe athletes who eat a balanced diet don't need extra protein or vitamins so only occasionally take some vitamin C, Echinacea and zinc when I feel a cold coming on. It's a matter of priority and not discipline. Any activity that keeps you from dealing with life's issues can be seen as addictive. Addictions become harmful when attention to family, friends and work suffer as a result. Amenorhea or missing monthly periods in women is a indication of over training.
Runny stools are also an indication of over training.
Feeling down and out of sorts could also mean you might be better taking the day off.