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The physiological purpose of ketogenesis is to spare the loss of lean mass that would otherwise occur during prolonged fasting. Meeting the glucose requirement of the brain entirely by gluconeogenesis from amino acids would cause us to lose 2.2 pounds of lean mass every day. This would cause us to die much more quickly from starvation than we actually do. Ketones can fuel 75% of the brain’s energy requirement. Glycerol from triglyceride hydrolysis and acetone derived from ketogenesis can together spare 55% of the amino acids that would otherwise be used to sustain the brain’s remaining glucose requirement. The acidity of ketones requires the kidney to hydrolyze glutamine to neutralize it with ammonia, however, and this attenuates the sparing of lean mass. In fact, it doubles the amount of protein breakdown over that used to synthesize glucose alone. When considering all this, ketogenesis cuts down the amount of lean mass lost during fasting 5-fold, allowing us to survive for a much longer time during fasting than we would be able to without ketones.
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