Even if you eat the same thing before and during every race, you can still suffer from GI distress because even the most seemingly insignificant alteration, such as the brand of coffee you sipped or the fact that you ate two bananas instead of one, can impact your digestive system when you're going all-out. The evil double-edged sword: Instinctively, you might reach for an extra sports drink or gel when you feel a bonk coming on, but if your body can't metabolize the extra carbs quickly enough, you could be in for a rough, stomach-sloshing stagger to the finish line. You'll be lucky if you can make it across the line without dropping to all fours and barfing your guts out.