ketosis, muscles derive less only 4-6% of their energy from ketone bodies (22). This becomes
important when considering the time course for nitrogen sparing during ketosis.
Infusion studies
Several studies have examined the effects on protein breakdown during the infusion of
ketone bodies at levels that would be seen in fasting or a ketogenic diet. Of these studies, three
have shown a decrease in protein breakdown (33-35) while two others have not (36,37). One
study suggested that ketones were directly anabolic (38). One oddity of these studies is that the
infusion of ketones (usually as a ketone salt such as sodium-acetoacetate) causes an increase in
blood pH (36,38), contrary to the slight drop in blood pH which normally occurs during a ketogenic
diet.
At least one study suggests that the rise in pH is responsible for the decrease in protein
breakdown rather than the ketones themselves (36); and sodium bicarbonate ingestion can
reduce protein breakdown during a ketogenic diet (39). However, since blood pH is normalized
within a few days of initiating ketosis, while maximal protein sparing does not occur until the third
week, it seems unlikely that changes in blood pH can explain the protein sparing effects of
ketosis.
It should be noted that these studies are different than the normal physiological state of
ketosis for several reasons. First and foremost, the mixture of ketone salts used is not
chemically identical to the ketones that appear in the bloodstream. Additionally, the increase in
pH seen with ketone salt infusion is in direct contrast to the drop in pH seen on a ketogenic diet
suggesting a difference in effect. Therefore, ketones produced during metabolic ketosis may still
have a direct anti-catabolic effect.
Possibly the biggest argument against the idea that ketones are directly anti-catabolic is
the time course for changes in nitrogen balance. Most of the infusion studies were done on
individuals who had been fasting for short periods of time, overnight or a few days. The major
decrease in nitrogen sparing does not occur until approximately the third week of ketosis, at
which time muscles are no longer using ketones to any significant degree (22,40). All of the
above data makes it difficult to postulate a mechanism by which ketones directly affect muscle
protein breakdown. In all likelihood, contrary to popular belief, ketones are not directly anti-
catabolic.
Affecting thyroid levels
A fourth possible mechanism by which ketosis may reduce protein breakdown involves
the thyroid hormones, primarily triiodothyronine (T3). T3 is arguably one of the most active
hormones in the human body (42-44). While most think of T3 simply as a controller of metabolic
rate, it affects just about every tissue of the body including protein synthesis. A decrease in T3
will slow protein synthesis and vice versa. As a side note, this is one reason why low
carbohydrate diets are not ideal for individuals wishing to gain muscle tissue: the decrease in T3
will negatively affect protein synthesis.