3 ms·

Doesn't the brain preferentially metabolise ketones and only in their absence fall back on glucose?

arghwhat 3y ago
No, it is the opposite. Ketone bodies are an emergency pathway of sorts. When in a high-glucagon and low-insulin state (that is, there is a demand for more fuel in the bloodstream than is present), after the liver has used up its glycogen stores for fast and easy glucose production, liver cells redirect most or all of its oxaloacetate supply to the process of gluconeogenesis to produce more glucose for the bloodstream from free fatty acids and glycerol released from fat tissue, as well as some other non-carbohydrate sources. In this state, the liver's own cells lack the key incredient to finish metabolism of free fatty acids, where acetyl-CoA is combined with oxaloacetate to form citric acid for the Kreebs/citric acid cycle. Instead, the cells start converting acetyl-CoA into ketone bodies and releasing these into the bloodstream to further increase fuel availability. Cells with mitochondria can metabolize various things including free fatty acids directly, but free fatty acids cannot pass the blood-brain barrier. Ketone bodies can, and while they have to be converted back into acetyl-CoA to be used, the liver does this to keep the central nervous system alive in a situation where glucose production might not cut it. You only have high levels of ketone in your body after long periods of fasting/starvation (e.g., 24 hours or more), prolonged exercise or if on a diet that forces the pathway with insufficient carbohydrate.
- sydbarrett74 3y ago
  Thank you for the correction and reasoned response. :)
  - arghwhat 3y ago
    Bonus fun fact: People in ketosis tend to have a breath with a hint of nail polish remover. This is because one of the ketone bodies is acetone, a result of spontaneous breakdown of one of the other ketone bodies. Only the liver can break down the acetone, and as it floats around the blood some of it ends up exhaled. (At very high levels of ketone bodies you end up in something called ketoacidosis. In this state your blood turns acidic, which can quickly develop into a medical emergency. This mainly happens to those with health issues like type 1 diabetes.)

No, it is the opposite. Ketone bodies are an emergency pathway of sorts. When in a high-glucagon and low-insulin state (that is, there is a demand for more fuel in the bloodstream than is present), after the liver has used up its glycogen stores for fast and easy glucose production, liver cells redirect most or all of its oxaloacetate supply to the process of gluconeogenesis to produce more glucose for the bloodstream from free fatty acids and glycerol released from fat tissue, as well as some other non-carbohydrate sources. In this state, the liver's own cells lack the key incredient to finish metabolism of free fatty acids, where acetyl-CoA is combined with oxaloacetate to form citric acid for the Kreebs/citric acid cycle. Instead, the cells start converting acetyl-CoA into ketone bodies and releasing these into the bloodstream to further increase fuel availability. Cells with mitochondria can metabolize various things including free fatty acids directly, but free fatty acids cannot pass the blood-brain barrier. Ketone bodies can, and while they have to be converted back into acetyl-CoA to be used, the liver does this to keep the central nervous system alive in a situation where glucose production might not cut it. You only have high levels of ketone in your body after long periods of fasting/starvation (e.g., 24 hours or more), prolonged exercise or if on a diet that forces the pathway with insufficient carbohydrate.

sydbarrett74 3y ago
Thank you for the correction and reasoned response. :)
- arghwhat 3y ago
  Bonus fun fact: People in ketosis tend to have a breath with a hint of nail polish remover. This is because one of the ketone bodies is acetone, a result of spontaneous breakdown of one of the other ketone bodies. Only the liver can break down the acetone, and as it floats around the blood some of it ends up exhaled. (At very high levels of ketone bodies you end up in something called ketoacidosis. In this state your blood turns acidic, which can quickly develop into a medical emergency. This mainly happens to those with health issues like type 1 diabetes.)