Which nutrient do the brain and red blood cells primarily rely on for function?

The brain and red blood cells primarily depend on glucose for energy and function. Glucose is a simple sugar that serves as a critical source of energy, particularly for the brain, which is highly sensitive to changes in glucose levels. It accounts for a significant portion of the body's energy consumption, particularly in the central nervous system.

Red blood cells rely on anaerobic metabolism, breaking down glucose to generate ATP (adenosine triphosphate) for their energy needs. They do not have mitochondria, which means they cannot utilize oxygen-based metabolism to generate energy; thus, they depend solely on glucose.

In periods of fasting or prolonged activity, while other energy sources like fatty acids and ketones become more relevant for many tissues, the brain has a strong preference for glucose under normal circumstances. It can adapt to utilize ketones when glucose is scarce, but this is not the primary pathway for energy under typical conditions. Therefore, glucose is the nutrient that defines the primary role in sustaining the function of both the brain and red blood cells.