There is a growing trend for endurance athletes to replenish electrolytes and calories separately. Traditional sports drinks provide calories in the form of carbohydrates as well as essential electrolytes (sodium, potassium, calcium, magnesium). Often this combination is adequate, but as event distances get longer, intensity increases. In extreme weather conditions, this single source approach can be inefficient.
More: 15 Hydration Facts for Athletes
Environmental Considerations
When temperature or humidity rise, our bodies must sweat more to keep us cool. This increases our need to rehydrate and replace lost electrolytes.
Regardless of how hot or humid it is, our calorie requirements per hour remain pretty much the same. In fact, our body's ability to digest carbohydrates actually decreases in hotter conditions because blood is diverted to the skin's surface to cool us off, leaving fewer resources available for digestion.
More: Measure Sweat Loss for Optimal Hydration
This is actually a good thing. Our body's survival instincts prioritize keeping us cool over getting additional calories. Heat stroke is much worse than bonking.
Training and racing in cold conditions creates the opposite challenge. Since we don't sweat as much, our hydration needs are not very high but our calorie requirements remain the same. When conditions are cold, we don't drink very much. The danger here if we are relying on our beverage for calories is that we may not be consuming enough calories to fuel our activity.
Intensity Considerations
As intensity increases, our bodies use more resources to fuel muscles. The higher the intensity, the more heat we generate and thus additional resources are required to cool ourselves off. To cool, we sweat more, increasing our need to rehydrate. During higher intensity exercise we consume more calories per hour, but our ability to digest calories decreases.