For an omnivore, eating plant material is the most efficient way to gather the sun's energy. The plant has directly processed that energy and will then pass it on to the omnivore with little energy loss. From an energetics's standpoint, the omnivore could obtain the same amount of energy by eating more animal material. The animal material is a less efficient way of gathering the sun's energy. The eaten animal has already taken the directly processed energy from the plants it ate. Now the omnivore is introducing a type of 'middle man' between the original plant and himself. The omnivore could obtain the same amount of energy as consuming plant materials by eating more animal materials but it would be far less energy efficient. Most omnivores strike a balance between the energy efficiency of plant materials and the nutrient value of animal materials.