Maria C. Linder is on the faculty at California State University
Fullerton, California in the Department of Chemistry and Biochemistry.
All quotes are from her textbook; "Nutritional Biochemistry and
Metabolism: with clinical applications", Maria C. Linder. pages
87-109. Chapter Eight: Nutrition and Metabolism of Protein.

There is a difference between starvation, prolonged fasting and
controlled carbohydrate eating. There are similar metabolic mechanisms
at work, but the differences are key to understanding the safety
and efficacy of controlled carbohydrate diets.

"The whole process of muscle protein catabolism and liver
gluconeogenesis is regulated principally by glucocorticosteroids and
glucagon and a relative lack of insulin. Early in fasting glycogen
reserves are depleted, and protein (mainly from muscle) becomes the major
source of carbon for glucose production. Glucose is required in
substantial amounts by blood cells and the central nervous system on a
daily basis. There is also an initiation of ketone body production by
the liver to provide a more water soluble form of fat-derived fuel."

THIS IS WHERE THE CONFUSION BEGINS

"A very similar adaption of protein and energy metabolism
occurs in persons consuming diets very low in carbohydrates, where there
is little or no glycogen reserve. However, in this instance, DIETARY
PROTEIN largely or fully SUBSTITUTES for muscle protein in gluconeogenesis.

NOTE: "DIETARY PROTEIN LARGELY OR FULLY SUBSTITUTES FOR MUSCLE PROTEIN IN GLUCONEOGENESIS."

Even when discussing starvation many critics of lowcarb eating miss the proverbial
boat. "The body adapts to starvation and reduces the need for
protein-dependent gluconeogenesis by boosting its production of ketones,
a fuel ALTERNATIVE to glucose for MOST CELLS. Circulating ketones reach
maximum levels after about ten days and now substitute for
much of the glucose requirement of the central nervous system.
This drastically reduces the need for catabolism of muscle protein."

"With reduced protein catabolism, urinary nitrogen excretion also
declines. And there is a shift from the excretion of urea to a
predominance of ammonia loss. This shift toward ammonia versus urea
parallels the increased production and excretion of keto acids, and
serves to MAINTAIN ACID/BASE BALANCE."

"The overall point is that muscle is a valuable reserve of carbons
that can be used for glucose production when needed. However the body
prevents excessive losses of muscle protein over long periods of fasting
by adapting the central nervous system to utilization of ketone bodies
for fuel."

Adaptation avoids keto-acidosis in lowcarb dieters

"A parallel adaptation in the production and excretion of
ammonium ions by the kidney neutralizes the increased ketone bodies
(principally beta-hydroxybutyric and acetoacetic acids). Without the
latter adaptation, such large productions of keto acids would cause a
severe ketoacidosis, as well as a loss of large quantities of sodium and
potassium ions (accompanying ketones spilled into the urine)."