Metabolic adaptation associated with fasting

Measurement Phase I (0-3 days fasting) Phase II (3-30 days) fasting Phase III ( 30 days fasting) 

Plasma free fatty acids Peaking at day 2 at 0.8 Falling steadily to 0.2 0.2 

Plasma alanine (mM) 0.5 0.5 falling steadily to 0.4 Rising sharply to 0.7 

Plasma ) -hydroxy- Rising steeply 0 to 7 Rising to peak 22 at 15-20 days, Continuing to fall to 5-10 

Principal changes Adaptation to lipid utilisation Adaptation complete, reduced Increased use of protein as 

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EXAMPLE 13.21 Type II muscle fibers are subdivided into type Ila and type Ilb. Type Ila can use both aerobic and anaerobic metabolism to produce ATP whereas type lib fibers use only anaerobic metabolism. In contrast to type I muscle fibers, type lib fibers contract rapidly after stimulation by a nerve impulse. They have evolved for short-lived, powerful contractions by the possession of characteristically active myosin ATPases and a dense packing of contractile filaments. So much of the cytoplasmic space is taken up with filaments that little exists for mitochondria. Similarly, these fibers are associated with a relatively poor blood supply. Type lib fibers are also known as white and fast-twitch muscle fibers and are adapted for short-lived but powerful contractions. The relative paucity of mitochondria and the poor blood supply impose obvious constraints on the generation of ATP during exercise. 

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