Quick comment on 72 hour fasts and why it's a bad idea.
DNG = de novo glucogenesis.
Liver glycogen plays an ever increasing role for maintaining blood glucose during the fast.
Once liver glycogen is completely depleted, which happens after about 24-28 hrs, glucose from stored amino acids (aka muscle) will be the only way the body can maintain blood glucose in the right range (via DNG).
Hours 24-72 during a fast: often referred to as the glucogenic phase due to massive DNG.
Hour 40: Gene-expression for proteolysis kicks in. This is bad and it essentially means that your body is doing whatever it can to effectively shuttle amino acids out from your muscles.
After about 72 hours, ketogenic adaptation is maximized* and protein catabolism is reduced as a consequence of
1) ketone bodies contributing to brain metabolism (in place of glucose).
2) thyroid hormone gets down-regulated, which further reduces protein catabolism. This is true metabolic slowdown, the one some people foolishly believes happens if you miss a meal.
* Just a quick note here. Blood ketones starts contributing to brain metabolism gradually & in a dose-response fashion during the first 72 hours. The increase then plateaus a bit at about 30% efficiency after 72 hours (i.e. after 72 hours the brain's daily need of 120 g glucose is reduced to 80 g). However, they are not "maximized" in the true sense of the word. There is a gradual increase beyond 72 hours but this occurs slowly (i.e. not +30% in 72 hours). After a few weeks** in ketosis, it reaches 70% efficiency. At this point, usually full-blown starvation, protein metabolism is minimal.
** A note about this too: the reason for the mental numbness some people feel during the first few weeks on low carb/keto diets is the result of the adaptation process. That's why - in studies - people on ketogenic diets score lower on cognitive tests DEPENDING on when the test is taken. Take the test after a few days and you'll see a decrease in reaction time/various cognitive metrics but after a month or so there's no difference vs normal/non-keto diet.
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Re: insulin resistance
Forgot to comment on the comment about insulin resistance. You will burn a ****load of FFA during a 72 hour fast but a large part of that will be intramuscular triglycerides. I wrote about this briefly here
http://www.leangains.com/2010/06/int...born-body.html
"4. My research has indicated that the ideal state of fat burning is reached after 12-18 hours of fasting. Coupled with high levels of catecholamines, increased blood flow to stubborn regions, and low insulin for a2-receptor inhibition, this time interval is the "golden age" of stubborn fat mobilization.
Let me just explain real quick what I mean by the ideal state of fat burning. Studies have examined free fatty acid (FFA) oxidation from anywhere between the overnight fasted state to three days of fasting. While FFA oxidation increases the longer time you spend in the fasted state, the contribution of fatty acids to whole body fat oxidation changes.
In short-term fasting there's a significant increase in subcutaneous FFA oxidation. That's just a fancy way of saying that you're mainly burning body fat and nothing else. For up to 14-20 hours* after a 600-calorie meal in normal-weight subjects, fat is only mobilized from body fat stores in resting individuals.
* 14-20 hours in a completely sedentary state should easily equal 12-18 hours in real life.
Past this time point, fat burning increases further. That goes without saying. But it's not necessarily the type of fat you're after that you'll be burning. Somewhere in between the 10- and 30-hour time point, the oxidation of intramuscular fat increases greatly, but no increase is seen in subcutaneous fat. Subcutaneous fat simply can't keep up with demand, so you're playing a game of diminishing returns if you push the fast too long. Coupled with the escalating rate of de novo gluconeogenesis, and subsequent risk of muscle catabolism, fasting for too long may not be very conducive for a lean individual seeking optimal lean mass retention while targeting stubborn body fat."
Somewhere around the 30-hour mark, you will also start to develop significant peripheral insulin resistance (i.e. it occurs in concert with IMTG mobilization and your bloodstream being chock full of FFA).
This is temporary, of course, but consider the consequences of breaking the fast with a large meal during such conditions. It would not bode well from a nutrient partitioning point of view. Nor would it be particularly good from a health point of view to walk around with blood glucose sky high until insulin resistance is reversed.