Article below from the latest Trail Running Mag. We (humans) get used to what we're exposed to. For example, in one generation, low nutritionally dense foods that are falsely flavored to make them palatable has become the norm. Two diets that make an attempt to diverge from this norm are vegetarian and paleo. Since we do not have an extra ruminant stomach to ferment veggies and grains, paleorunners tend to lean towards paleo. Besides having one stomach, there also is the issue of carb-dependent or fat-dependent for fueling during a run. One convenient thing about being fat-dependent - we already carry fat with us.
another compelling point about deciding whether to eat high fat of low fat, simply look at micronutrients. There are two kinds water-soluable and fat-soluable, wait... no carb-soluable? How do carbs help me absorbs vitamins and minerals? They don't.
Good quote form the article below:
"As you become more aerobically fit, your body becomes more efficient at using its existing fat stores as fuel. The ability to burn fat stores efficiently not only improves endurance capacity by sparing glycogen, but also reduces the need to rely on eating carbohydrates or fat during a run, which decreases the risk of intestinal distress and a consequent decrease in performance."