Can someone explain to me why we have glycogen in our liver - its obvious why we store it in our muscles, but why the liver?
And therefore how does one deplete this store?
Thanks
Glycogen stores in the liver are used as a “first responder” when blood glucose levels drop. When needed, the glycogen is converted to glucose. The body tries to keep blood sugar levels in a certain range, since the brain (and a couple of other things) can only use glucose for energy. Muscle glycogen is used to provide energy for anarobic (without oxygen) exercise; i.e., lifting weights, sprints, non-endurance (with oxygen) type exercises.
The glycogen in our muscle is “fixed” and is only useful for exercise or predators (if they eat you). You use that glycogen as a source of energy for your muscle cells.
Your body needs glucose for lots of other functions, however. For example, your brain needs ~125g of glucose (and only glucose works properly) a day - but it has very low glucose stores. As well, all your cells need a proper supply of glucose to function as designed. Your liver acts as the ‘storehouse’ for body glycogen. It will sop up extra glucose (if you get too much or are insulin resistant). It will release extra glucose into the blood if your body needs it.
It’s fairly easy to reduce the amount of glucose in your liver, just wake up after 8hrs and it will be a little drained. Emptying it completely would be impossible, since your liver can convert both fats and proteins into glucose - and the body will happily cannibalize itself to keep the liver in good form.
In advanced starvation (planned or accidental) your brain can start to function using chemicals called ketone bodies which are derived from fat and they’re made (you guessed it) in the liver. But I’m of the opinion that getting the brain to run on ketones is like putting leaded fuel into an unleaded engine.
Keeping the stores “low” is good for weight loss, because the liver has a bit of a ‘buffer’ to fill up before excess calories (from a meal) are turned into fat.
For example, when I’m trying to lose fat, I eat a P+F meal in the morning and keep on eating those types of meals until after I exercise. Because muscle can sop up glucose nicely after a workout, a post-workout P+C meal will preferentially fill muscle and liver stores before any excess being used as fat. And even then, the damage you do during a workout needs to be healed. This means that post-workout, you have a calorie glut that needs to be filled.