Fat is the secondary substrate that powers mainly aerobic exercise at low and moderate intensities. Stored fat in the muscle is highly limited as a fuel source compared to that of carbohydrates and is less sensitive to changes in training volume and intensity. The healthiest sources of fats include monounsaturated and polyunsaturated fats (e.g. olive oil; oily fish; nuts and seeds; avocados). Other sources of fat come from saturated (e.g. animal fat) and trans fats (e.g. margarine, heated oil, deep fried foods, baked goods) that should be limited.
High fat low carbohydrate diets can increase adaptations to improve the use of fat as a fuel source but in turn hinder high intensity exercise. The daily total fat intake for an endurance athlete does not need to be adjusted with training like carbohydrates and protein. Generally, fat intake should be around 20-35% of daily caloric intake.