Which of the following statements regarding glucose is correct?

• A. Most cells will function normally without glucose.
• B. The brain requires insulin to allow glucose to enter the cells.
• C. Blood glucose levels decrease in the absence of insulin.
• D. The brain requires glucose as much as it requires oxygen.

Answer: (D)

Energy supply to the brain hinges on two essentials: glucose and oxygen. The brain relies on glucose as its primary fuel, and it uses a lot of it to meet its high energy demands. Importantly, glucose can enter brain cells without insulin because the brain uses insulin-independent transporters at the blood–brain barrier and in neurons. At the same time, oxygen is needed to fully extract that energy from glucose through cellular respiration. Because both glucose availability and oxygen supply are crucial for brain function, saying the brain requires glucose as much as it requires oxygen reflects the reality that both inputs are indispensable for energy production in brain tissue. The other statements misrepresent brain metabolism: insulin is not needed for glucose entry into brain cells, absence of insulin raises blood glucose, and while glucose is essential, saying most cells function normally without it isn’t accurate.