Racing MX i usually only drink water or gatorade in the camelback (i suggest you get one of those) and more gatorade simply because i dont get stomach cramps with it like i do water. When i race XC (XC/GNCC/Enduro) i tend to take vitamins in the months leading up to the season, as well as all though the season. Eat well consistently during the season, and exercise. In the average season i ride 3-5 times a week for at least 2-4 hours each time, even on race-weeks. This works well for me, and i think atv racing takes more out then what your talking. You figure the average track is 1/4 mile to a mile in length with jumps, berms, whoops, ruts, etc. At 10 laps per heat 3 heats per series at x amount of series per day with a 350 pound machine and probably close to 30 pounds of gear on it adds up to alot.
And thats MX. Enduro's are often close to 50 miles through all terrain with obstacles and very difficult courses in all weather conditions.
Some guys swear by Cardio Stack but i tend to think things like that are all in the head.