During the early phase of fasting, which tissues use ketone bodies as fuel to spare glucose for the brain?

• A. Brain
• B. Liver
• C. Adipose tissue
• D. Heart and skeletal muscles

Answer: (D)

During fasting, the liver ramps up ketone production from fatty acids, and these ketone bodies become an important fuel for tissues with high oxidative capacity. The heart and skeletal muscles readily oxidize ketone bodies, which reduces their need for glucose. By using ketones, these peripheral tissues spare glucose for the brain, which continues to rely primarily on glucose (and can use ketones later as fasting progresses). The liver itself cannot use ketone bodies because it lacks the enzyme needed to oxidize them, so it remains the source rather than a consumer. Adipose tissue mainly supplies fatty acids and glycerol rather than using ketones for energy. Therefore, the heart and skeletal muscles are the tissues that use ketone bodies to spare glucose for the brain.