Final answer:
The macromolecule that the body uses third for energy is protein. Carbohydrates are used first, followed by lipids, and proteins are utilized when the other two are insufficient. Relying on proteins for energy can lead to muscle breakdown and is not ideal.
Step-by-step explanation:
Macromolecules Used for Energy by the Body:
Within the body, carbohydrates are the primary source of energy. When carbohydrate sources are depleted, the body then turns to lipids for energy. However, the macromolecule used third by the body for energy, typically in conditions of carbohydrate and fat insufficiency, is protein.
Proteins are crucial for building and repairing tissues, but they can also be broken down into amino acids and used for energy if necessary. This is not ideal, as proteins are not primarily meant for energy storage, and utilizing them for energy can result in tissue breakdown and body wasting.
The sequence in which the body utilizes macromolecules for energy is important for understanding how the body maintains its energy balance, especially during periods of fasting or starvation. Initially, the body will use stored glycogen from the liver and muscles, followed by fats stored in adipose tissue. Only when these stores are significantly depleted will the body resort to using proteins for energy.