The nutrient that can be used to produce energy in the absence of oxygen is glucose. This process is called anaerobic respiration, and it's less efficient than aerobic respiration, which requires oxygen.

Here's a breakdown:

* Aerobic respiration uses oxygen to break down glucose and produce a significant amount of ATP (adenosine triphosphate), the main energy currency of cells.

* Anaerobic respiration occurs when oxygen is limited or absent. It uses glucose to produce a smaller amount of ATP and lactic acid or ethanol as byproducts.

While anaerobic respiration is less efficient, it allows cells to survive and function for a limited time in oxygen-deprived conditions.