Originally Posted by Randy Gurley
Point taken on what to do to avoid muscle loss. I usually don't IF most of the days on weekends anyways so I'll start to eat something before morning cardio. I do have a question about what Darryl wrote about liver glycogen. I haven't done a search here for this, but say someone has eaten and has full liver and muscle glycogen stores, and goes running at say, 70% VO2 Max. At what percentage does liver glycogen delete, my understanding is that a lot of muscle glycogen is used due to the exercising muscles, but unable to find the rate of depletion for the liver.
From reading other sites, it seems the liver holds around 70-100 grams, so 280-400 calories worth. Running at an intensity burning 50% carbs/50% fat, if muscle and liver depleted at the same rate, you would have to run 1.5 to 2 hours to deplete a full liver. I found a very large study today that went into detail of exercise induced hypoglycemia and about all the hormones involved, and lots of other things, I still haven't finished reading it yet.
Again, this is just for my knowledge, I find the study of the body very interesting, and can't seem to get enough. If I need to do a search as this subject has already been discussed, tell me.
At rest liver glycogen is used at a rate of ~10g/hour and during exercise ~60g/hour. If you've done some serious carb loading you might have ~130g of liver glycogen available so at best you can run at a good pace for a little over two hours before hypoglycemia develops forcing you to slow down and eventually stop. In practice that means that if they don't take on board CHO during a marathon an average runner might make it to somewhere around the 20 mile mark before they're forced to quit.
Edit: I think this might interest you -