At the same time that FFA and ketone use is increasing, the body's use of glucose and protein are going down. This is a critical adaptation for two reasons. First and foremost, there are tissues in the body which can not use FFA for fuel, requiring glucose. By decreasing their use of glucose, those tissues which do not require glucose for energy spare what little is available for the tissue which do require it. Thus, there is always a small requirement for glucose under any condition. As we shall see, this small glucose requirement can easily be met without the consumption of carbohydrates.
The second reason is that a reduction in protein losses is critical to survival during total starvation. The loss of too much muscle tissue will eventually cause death (6). From a fat loss standpoint, the 'protein sparing' effect of ketosis is also important to prevent lean body mass losses.
To examine the adaptations to ketosis in terms of glucose and protein, we first need to discuss which tissues do and do not require glucose. Then the adaptations which occur during starvation, in terms of the conservation of glucose, can be examined.
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