... Interesting stuff. If I recall correctly glycogen is how the body stores glucose during ketosis. This bit from the Wiki article above suggests that as well: ...
... I wasn't able to find any info about what happens when glycogen is digested, though, which I'm curious about. Does it get broken down, or does it go straight into the liver and muscles to be stored?...
Ketosis is triggered after a period of carbohydrate restriction, such that liver glycogen is depleted and gluconeogenesis (production of glucose from protein, glycerol from fatty acids, or pyruvate/lactate) eventually becomes too stressful. In ketosis, it's unlikely you'll be storing glucose as glycogen, since as you continue gluconeogenesis the glucose produced will more likely be continually released into your bloodstream to fuel your (glucose dependent) red blood cells. Those endurance athletes likely weren't ketotic or well fat-adapted since I believe "hitting the wall" is what occurs when a sugar-burner runs out of their preferred fuel. Perpetually depleted glycogen stores is what will keep you in ketosis.
Glycogen, or "animal starch", can be digested by the same enzymes as plant starches(which makes sense, since both are glucose polymers with similar bonds); the amylase enzymes you make in your mouth and pancreas. Glycogen is more branched and compact than the analogous plant amylopectin, but in both cases the the alpha(1-4) bonds between the linear chains and the alpha(1-6) bonds at the branch points can be digested by alpha/beta amylase and gamma amylase, respectively. You'll break down the glycogen into various combinations of simple glucose, maltose (two-linked glucose units), and polymers of various lengths which can all be absorbed and then will most likey be reassembled into glycogen again.
I just came across this, which I found interesting. It's from a site with a mainstream worldview, but this part stood out regarding ketosis.
Protein and CarbohydratesHas this been discussed before?
Grass-fed beef liver is a good source of protein, meeting nearly 100 percent of your daily value. An 8 oz. serving contains 44 g of protein. As a complete source of protein, beef liver provides your body with all the essential amino acids necessary to build the proteins found in your body. Unlike other cuts of meat, such as the beef eye of round, beef liver does contain a small amount of carbs, 11 g in an 8 oz. portion.
The recipe below results in probably one of the most delicious meals I've had in a long time, but that also comes with a price. If you want tasty you might have a dopamine/reward circuitry problem to fend off when it comes to over-eating (and there are probably hidden carbs in these spices too, so watch out for those). Also, make sure you go for an onion that's relatively low in carb (I guess some can go from 7 to 22 carbs per cup). So, if you want a delicious liver (or maybe switch that out with other gamey meats) , then the recipe below really takes the cake IMO. Of course we all have different tastes, but this one blew me away!
These two points I connect together, as it plays into what I think of as sense vs. sensation. Essentially, IMO, cooking food is so appealing to us because of the flavors (or otherwise indigestible starch) we get as rewards which then reinforces the behavior. We'd likely be better off eating things raw, namely animals, but the flavor sensation of cooked food outweighs the sensible deductions that come from observing how all other living things eat their food raw.
The carb content of liver (or the presence of its best friend onions) is part of the reason why it's a bad idea to overcook it. What we perceive as flavor is generally the result of the Maillard reaction (http://en.wikipedia.org/wiki/Maillard_reaction
) where proteins react with sugars to create complex taste molecules. This reaction is the basis of the entire flavor industry. This reaction is part of the process in forming advanced glycation end-products, which can also occur in the body, but pre-forming them through cooking and then ingesting them will likely accelerate the processes associated with their accumulation in the body. "The total state of oxidative and peroxidative stress on the healthy body, and the accumulation of AGE-related damage is proportional to the dietary intake of exogenous (preformed) AGEs, the consumption of sugars with a propensity towards glycation such as fructose
 and galactose." (from wiki on Advanced glycation end-products).
That being said, I often (1-2x/week) fry up liver with onions and garlic in some semi-rendered suet. I think liver is the best food we can eat. Also, I personally find raw beef heart to be tasty, and eat kidneys raw(although I don't enjoy it), and used to be fond of eggs and brains cooked up in the gelatin from a bone broth. Now that's rich, and you can eschew your phosphatidyl serine supplements for sure.