Made a few short comments in response to a blog post I just saw on The Fitness Spotlight.

The post is here and below are my responses to some of the claims made.


Just a few short comments on some claims here. Sorry if this has been covered somewhere among the comments, I haven’t read through them all.

Liver glycogen levels are depleted within 8-10 hours. Muscle glycogen falls by 50% over 24-hours, even without exercise.

False on both accounts. Liver glycogen is completely depleted in approx 28 hrs (Cahill et al). And the second statement is also incorrect. In humans, muscle glycogen is barely affected at all after 24 hrs, assuming no strenous activity.

After depleting glycogen, amino acids are recycled to be broken down for glycogen through gluconeogenesis.

This is an ongoing process, but liver glycogen does not need to be depleted in order for amino acids to contribute to the maintenance of blood glucose. The longer the fast, the greater the contribution – for example, after 16 hrs, aminos will start contributing more than 50% to the amount of glucose in your blood stream. After 24-28 hrs, 100%.

We see increases in three of the four hormones driving lipolysis, indicating a propensity towards fat burning. Somewhere around 12-18 hours, lipolysis becomes a major energy pathway, producing energy from body fat.

Yes. In fact, after an overnight fast, 2/3 of the energy burnt are free fatty acids. Eat breakfast and you’ll be putting the brakes on this process, of course.

T3 levels fall slightly, indicating a slightly lower metabolic rate. Urinary nitrogen excretion falls, indicating less catabolism of muscle proteins.

No. Are you looking at rats now again? Because the downregulation of T3 takes more than 72 hrs to occur in humans. A short fast in the 16-24 hr range certainly doesn’t impact negatively.