Liza:

> My understanding is that the body uses glucose available in the blood
> during the first day, until it is depleted (very quickly). Then it goes
> to glycogen stores to extract glucose there (stored in muscle tissue -
> thus resulting in loss of protein and muscle mass). The body does not
> "want" to waste muscle tissue this way, so when still no glucose is
> coming in from food, it then resorts to another mechanism
> (gluconeogenisis) where it "cumbersomely" synthesizes glucose from
> compounds in the liver. But by the 3rd-5th day the body has converted
> to another ATP production system, ketosis, where it is able to
> synthesize and use ketones from the fat stores, instead of using glucose.

What I learned about fasting is similar, but a bit different:
All glycogen stores (from the liver and muscle tissue) are depleted after
24 hours (on average). Note that the brain can't use the glycogen present
in the muscles, but only the one which is stored in the liver. On the 2nd
and the 3rd day, since the nervous system needs glucose, the body uses
gluconeogenesis, which means that muscle protein is transformed into
glucose. Then, by the 4th day, the body becomes to use its own fat stores
for energy. However, the brain then uses about 50% glucose and 50% ketones.
By the end of a long fast, the body spends approx. 1000 calories/day (more
if you are a young and fat man than if you are an old and thin woman),
90% of which from fat.

Best wishes,

Jean-Louis
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