If you don't have enough glycogen you can feel very tired, which will effect your athletic performance. With longer exercise or endurance exercise such as cycling or long distance running, your body uses the glycogen stores first and then uses fats stored in your body to fuel performance. Protein is necessary to build and repair muscles but gives you only a small amount of the energy. Minerals and vitamins such as calcium and iron are needed for strong bones and blood cells, but do not provide energy. Athletes should eat a balanced diet with a variety of foods, and make sure they are getting the right amount of vitamins and minerals, but high doses of vitamins do not improve performance and can be dangerous.
Different foods offer different nutrients.
For example, oranges provide vitamin C and carbohydrates but not iron or protein. Beef offers iron and protein but not vitamin C or carbohydrates. In order for your body to be in peak condition, you need to eat a variety of foods. Cutting calories and carbos keeps you from performing at your best. Your training demands energy and if you want to compete at full strength you need to meet your body's energy needs.