Along with potassium depletion, low-carb diets can cause fatigue to set in due to used up glycogen stores. A low-carb diet, for example, might cause a high-mileage runner to "hit the wall" (1). Generally endurance athletes practice carb loading simply to fill up those glycogen stores to ensure lasting energy (1).
Reference
1. http://www.faqs.org/sports-science/Je-Mo/Low-Carbohydrate-Diets-and-Athletic-Performance.html
