Metabolism of starvation/fasting =/= low carb diet
 
The Biology of Starvation (intro)

The Biology of Starvation: Renal Gluconeogenesis


One of the major [obvious] differences in metabolism between fasting and low carb dieting is nitrogen metabolism... because you're eating protein on a low carb diet, not so much while fasting. During fasting/starvation your body tries to downregulate the urea cycle because you can't really afford to be 'wasting' amino acids/protein. I bet you never thought about the muscle-sparing effect of fat-derived fuels like that!

"Glucose production in starvation" is important because you need to make all of it and reduce the use of it.


Sources of gluconeogenic precursors during starvation (in terms of grams of glucose):

Substrate     day 1     days 2-4     day 40

lactate               40              40                40

glycerol              18             18                  18

amino acids      60             106               16

TOTAL              118            164               74

For Brain          78              124               34

You make more glucose days 2-4 because your brain needs it, you're not eating it, and ketones haven't ramped up yet. A couple weeks into it and total glucose production is downregulated, thankfully, so your muscles don't waste away (this includes essential organ proteins). 


I imagine metabolism on a low carb diet is similar to days 2-4: your making a bit more glucose because you're not eating it; ketones aren't likely to get up to 3-4 mM, to where they can substantially spare glucose usage. Plus, you're eating more protein (or at least you should be), so some of the amino acid carbon skeletons are going to get funneled through hepatic gluconeogenesis with concomitant disposal of the nitrogens via urea cycle -- this is another difference between starvation & LC dieting -- during starvation nitrogen exits via ammonia salts, eg, with ketones.


That's all for now!

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