Final answer:
Carbohydrates can be metabolized for energy without oxygen through anaerobic glycolysis, while fats and proteins require oxygen for aerobic metabolism.
Step-by-step explanation:
The question 'Which of the three main macronutrients can be metabolized for energy without oxygen?' relates to how the body breaks down nutrients to produce energy. Of the macronutrients mentioned, carbohydrates can be metabolized through a process known as anaerobic glycolysis, which does not require oxygen. During this process, glucose, a simple sugar, is broken down to produce ATP, the energy currency of the cell, along with lactic acid. This is in contrast to fats and proteins, which are primarily metabolized aerobically requiring oxygen to yield energy.