impaired. Total exhaustion during exercise occurs at 15-25 mmol/kg. Additionally, when glycogen
levels fall too low (about 40 mmol/kg), protein can be used as a fuel source during exercise to a
greater degree (4).
Following glycogen depletion, if an individual consumes enough carbohydrates over a
sufficient amount of time (generally 24-48 hours), muscle glycogen can reach 175 mmol/kg or
higher (1). The level of supercompensation which can be achieved depends on the amount of
glycogen depleted (5,6). That is, the lower that muscle glycogen levels are taken, the greater
compensation which is seen. If glycogen levels are depleted too far (below 25 mmol/kg), glycogen
supercompensation is impaired as the enzymes involved in glycogen synthesis are impaired (7).
A summary of glycogen levels under different conditions appears in table 1.
Table 1: Glycogen levels under different conditionsCondition Diet Glycogen level (mmol/kg)
Supercompensated High carb 175
Athlete Mixed diet 110-130
Normal individual Mixed diet 80-100
Normal individual, Ketogenic diet 70- aerobic exercise only
Fat burning increases 70
Exercise performance decreased 40
Exhaustion 15-25
Glycogen resynthesis without post exercise carb intake
Even without the consumption of carbohydrates there is some replenishment of muscle
glycogen stores following exercise. This raises the question of whether carbohydrates are
necessary while on a SKD. A few calculations will show that the small amount of glycogen
resynthesized during exercise is insufficient to maintain glycogen stores for more than a few
workouts.
When zero carbohydrates are consumed following training, there is a small amount of
glycogen resynthesized. This glycogen comes from the conversion of lactate, a by-product of
glycogen breakdown in the muscle, to glucose in the liver. This newly made glucose is released into
the bloodstream and stored again in the muscle as glycogen. Two mmol of lactate are required to
resynthesize 1 mmol of glycogen (8). Approximately 20% of the lactate generated during weight
training can be used to resynthesize glycogen after training.
Lactate levels in the muscle during resistance training may only reach 10-15 mmol with a
maximum of 21 mmol (seen only in highly trained bodybuilders). At 2 mmol of lactate/1 mmol
glycogen and an efficiency of 20%, this would have the potential to resynthesize only 2 mmol/kg
of glycogen, an insignificant amount.
Two studies have examined the phenomenon of post-workout glycogen resynthesis. One
study using weight training with no carbohydrate given found a resynthesis rate of 1.9
mmol/kg/hour following resistance training with a total of 4 mmol/kg being resynthesized (8). As