When carbohydrate is removed from the diet, the body undergoes at least three major adaptations to conserve what little glucose and protein it does have (5). The primary adaptation is an overall shift in fuel utilization from glucose to FFA in most tissues, as discussed in the previous section (5,6). This shift spares what little glucose is available to fuel the brain.
The second adaptation occurs in the leukocytes, erythrocytes and bone marrow which continue to use glucose (6). To prevent a depletion of available glucose stores, these tissues break down glucose partially to lactate and pyruvate which go to the liver and are recycled back to glucose again (5,6). Thus there is no net loss of glucose in the body from these tissues and they can be ignored in terms of the body's carbohydrate requirements.
The third, and probably the most important, adaptation, occurs in the brain, which shifts from using solely carbohydrate for fuel to deriving up to 75% of its energy requirements from ketones by the third week of sustained ketosis. (5,6,26) As the brain is the only tissue that continues to deplete glucose in the body, it is all we need concern ourselves with in terms of daily carbohydrate requirements.
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