>>23003494the liver can't produce glucose out of thin air. if it doesn't have stored carbs (glycogen - the liver can store up to 100-150grams of carbs for later use), it has to use amino acids, glycerol or lactate to produce new glucose, this is called gluconeogenesis, to supply glucose to the blood so you don't drop down and die. Glycerol is very innefficiently produced from fat breakdown, think 1lb of fat can only produce about 45 grams of glycerol, but your body doesn't burn fat to supply glycerol to the liver directly, it's only used as a by-product for side help. Lactate is just the metabolic byproduct of muscles burning up carbs (since fast twitch muscle uses glucose/glycogen as its main fuel source), it's somewhat more effective than glycerol but since you'll be running out of carbs completely in a few days of fasting it won't contribute to anything, your muscles won't have any carbs to burn to produce lactate. So all that's left is amino acids (protein), and since you aren't eating anything, guess where your body will find those.
Also, once your body enters ketosis, it will also start breaking down muscle for ketogenic amino acids, ketosis cannot happen without aminos, it's not just fat that is magically turned to ketones in the liver. (Theyre called ketogenic cause different aminos are used for ketone production vs glucose production via gluconeogenesis)
>>23003507In the study the participants lost 2.3kg of fat on average and 3.5kg of lean tissue on average after 10 days of fasting, although only 42% of that lean tissue was actual muscle mass, the rest was stored glycoge n/water so it doesn't really count. But that's still 3lbs of muscle gone. Which can take months to build normally, or weeks to rebuild.
