If you have never been in ketosis before, then running
out of glycogen on a run will obviously mess you up.
Unless I am misreading it seems like the pace — 15 seconds is roughly the pace it would take run
out of glycogen stores at 26.2 miles.
If you're partaking in a lower carb type of diet your liver will convert the stored glycogen into glucose and then release it into your bloodstream, then
when out of glycogen, it will convert fat and protein for energy.
When you restrict carbohydrate intake below 20 - 50 grams, your body runs
out of glycogen stores and starts producing ketone bodies.
The idea is that when you restrict carbohydrate intake below 20 - 50 grams, your body
runs out of glycogen stores and starts producing ketone bodies.
As hypothesized, the «low» group learned to burn more fat instead of carbohydrate — a physiological strategy that some experts think might allow your body to last longer before running
out of glycogen.
Once the liver is
out of glycogen, the body starts transforming your stored fat into usable energy.
So to break it down: whenever your body runs
out of glycogen (glucose sugar, for all intensive purposes), the body will release glucagon from the pancreas.
If you exercise intensely for more than 90 minutes, your muscles may run
out of glycogen.
Here is the big difference: when you are in a state of metabolic ketosis, any time that you are not getting enough fuel, your body will convert fats into fuel (and will eventually use your own fats, if you run
out of glycogen).