Exactly.
I read through the supplementary literature and it included oral glucose insulin assays and HOMA scores and there are 3 people who are lean’ish but certainly on the path to type 2 diabetes. Probably a decade away from a diagnosis.
Most of the rest have HOMA scores under 0.5 so twice as healthy as a reference 30 year old man with no metabolic syndrome.
I find this figure really perplexing. What is surprising is that the number is low enough that it seems like I should not be able to fast successfully. I weigh 157# and two years ago (when I also weighed 157#) I had a DEXA scan that said I have 13% bodyfat, so I should have about 20-21 pounds of fat tissue. (From looking in the mirror, I actually think I’m somewhat leaner now, but let’s skip that.) That suggests I can access only about 650 Kcal/day of energy from fat. So that, plus sometimes ~200 Kcal from coconut oil and HWC in my tea in the morning (I usually but not always have this when fasting, rarely have it when not fasting), would be only 850 Kcal per day of available energy. I would expect that to mean that I would feel terrible, but instead I feel fine and as energetic as on a feeding day. Before breakfast the next day (about 36 hours since my last full meal), I actually feel less hungry than I did the previous afternoon.
Last year, I did a 44 hour fast, with five mile runs at 18 and 40 hours of fasting, then a short weight workout before eating. (I’m not a big runner, but I dabbled in it last spring and summer.) I felt fine, and the 18 hour runs was one of my fastest, 8:15 per mile, while the 40 hour run was right on my average fed pace. (I’ll admit I probably pushed harder than usual to keep up my pace on those runs, particularly the 18 hour time-point.)
A couple of weeks ago, we did our first >48 hour fast, from dinner Wednesday to breakfast on Saturday. On Thursday afternoon I did a fairly fast 2-mile kayak paddle (36 minutes at 140 to 150 bpm heart rate; I’m 56 years old so that is probably Zone 4 for me) then a 4.5 mile walk. On Friday I did a moderately hard, by my standards, strength training workout: this https://connect.garmin.com/modern/activity/7258425788 plus some banded leg abduction and adduction that I forgot to log. Again, no problems with energy.
After three years on low-carb, I’d believe that I’m capable of partial glycogen repletion, at least, via gluconeogenesis (didn’t Jeff Volek demonstrate that in one of his studies?) but I’d still be surprised if I was using only 650 Kcal from my other stores…
The numbers don’t mean you will necessarily ‘feel terrible’ rather than not ‘as energetic as on a feeding day’, only the max amount of body fat you can utilize. Being fat adapted on keto for several years means that you are metabolically flexible and change fuel sources seamlessly. The rest of the energy comes from another source: mostly lean mass, but as you note also probably some glycogen. The glycogen is muscle mass dependent specifically, plus a little more in your liver.
Another example: myself. I weigh 145 pounds and have 14-15% body fat - so about 22 pounds. Close to your numbers. I’m a bit older, though, so aren’t into long fasts or your intensity of exercise. But I’m no couch potato. I do what I call an overnight IF 3 or 4 times per week. That’s simply timing the last meal of a day to be about 12 hours prior to the first meal of the next. I feel very little hunger on those mornings. If my mid-day meal on a work day (I work full-time at Walmart) happens not to come until early to mid-afternoon - up 6-7 hours after my morning meal - I still don’t feel very hungry. Just a passing and gentle reminder that it’s time to eat. My energy remains the same regardless.
This is what metabolic flexibility feels like. If you and I were to fast for several days or longer, then we would quickly see first the fat mass and the lean mass disappear and overall energy expenditure drop as our bodies attempted to compensate for the lack of incoming fuel. If you don’t believe me, try a 14 day water fast.
I would think that theory about how much energy you get from fat reserves is highly suspect for many people, particularly those with high insulin resistance.
I’ve always thought that when you transition to keto from SAD, it can be quite hard to access your fat due to high “insulin resistance” (let’s ignore that I hate this term, as it’s basically meaningless without a definition). Now, you also have a lot of fat mass, so maybe you can access enough of your fat to have a fast.
But, for me, after fasting for the last 5.5+ years and gaining muscle, I have a harder time fasting now than I did when I started. I remember fasting 4.5 days and exercising twice during that period. Now, I do not exercise at all if I do a 4.5 day fast.
If I fast 36 hours, I will exercise at about the 32 hour mark. That seems to be OK.
On the other hand, I know people like Peter Attia who is really “thin”, yet also can fast 7 days (while exercising what I consider to be an extreme amount).
I haven’t quite figured out why some people fast so easily and others don’t. “Insulin resistance” provides part of the answer, but am I still “insulin resistant” after almost 8 years low carb/keto with many, many 36 hour, 3.5, and 4.5 day fasts under my belt?
Should say I have to look at the original post in more detail. Will have to do that later.
Studies on Starvation begins at the bottom of page 5.
I confess, the original post was longer than I have time for right now … but that won’t stop me from commenting 
…
As a rather lean guy (5’9" hovering just below 140lbs) who typically eats TMAD - quite comfortably in terms of lack of hunger - I don’t fare so well when I reach around 24 hrs without eating anything.
My hunger is not great until about the 1 full day mark … then apparently I’ve got no Krispy Kreme storage left from 10 yrs ago (in truth, I was never much of a donut guy even pre-keto anyhow).
In short, being 2+ yrs fat-adapted on low-carb and feeling wonderfully energetic every morning/noon/night, I need to get my fuel from dietary fat within a day or so else I start to peter out. I can function, but I feel hungry 
Science?
Based on my experience, after fat adaptation I’d guess that serious hunger starts when your energy requirement exceeds your ability to access stored fat fast enough. Your metabolism does not want to eat lean mass since that leads rather quickly to bad outcomes. So the signal to eat fuel increases. But even so, it goes away eventually if you don’t. Read Scott’s Diary. Folks don’t starve to death feeling hungry, but very tired, weak and often befuddled.
I have given this a lot of thought, and I (no surprise) do not have a definitive answer, but I “feel” that you do not get nearly the benefit of lowered insulin resistance from fasting until your insulin drops hard. This can vary greatly on the person, as you saw from my last fast, that took around 4 full days for me, whereas I have seen other insulin/time graphs of fasters who had it fall precipitously at 3 days or 2 days if they were metabolically healthy and of course, longer than 3 days if they are not. If you were very resistant, it would not surprise me if many of your fasts “just” got to the stage of actually dropping your insulin significantly before you broke your fast. Is that optimal? /shrug, not sure.
This is part of the reason I feel it is important to really note that time and then go for at least a few hours/days within that lowered range. I expect my next fast will be +2/3 days longer than it takes for my insulin to drop substantially (as I plan on an insulin lowering fast before another autophagy fast in 6 months or something). That might be 5-8 days total. Not sure when I will pull that trigger, but it will probably be not for a while, and totally spontaneous when it happens. Right now, I am just loving 3 steaks a day, so I am going to really enjoy eating for a few weeks fist. Only problem is I am eating those 3 steaks and NOT putting fat on my body, I need more fat, to fast, faster and harder!
Yes the data that formed that hypothesis was from young fit mostly Mennonite men, conscious objectors of WWII being starved on 1400 kCal/day for 6 months.
Who knows what it would be in females? in the obese? in hyperinsulinaemics? Insulin doesn’t only blockade fatty acids in fat cells it also inhibits their transfer into mitochondria to be oxidized - so it surely must reduce the factor.
The problem is you couldn’t redo the study today. So it is all we have for now.
“Oates’ last thoughts were of his Mother, but immediately before he took pride in thinking that his regiment would be pleased with the bold way in which he met his death. It was blowing a blizzard. He said, ‘I am just going outside and may be some time.’ He went out into the blizzard and we have not seen him since.”
Yikes. I once thought it might be a fine thing indeed to do a tour in the Antarctic.
“We want more food yet and especially more fat.”
Indeed.
Just doing some back of the envelope math, your lean mass is 1-13%*71.3kg = 62kgs. You labile pool of amino acids available to use for making new protein or oxidizing to make energy is around 620g. Multiplying by the Atwater approximation gives you a buffer of roughly 2480 kCal that you can dip into that your ancestors could use to through a bad day of hunting.
Your athletic results are fascinating because you know your regular fed results and can compare like for like changing just one factor - length of fast.
Whoa. A bit disturbing to think that I might be burning hundreds of grams of protein to get through a short fast!
Sadly, I don’t really have much data. I’m too lazy to take careful notes (although I might be able to extract the data from my Garmin GPS watch’s files) and my running career was very short: started some time in March 2020 due to pandemic boredom and quit in August 2020 when I realized that there was no way the Seattle Marathon would be held in November 2020. Then I tore up my knee cartilage doing yoga in December (Half Moon Pose bad!), so may need a partial knee replacement before I do any more running.
I ramped up pretty steadily from 3 to 4 to 5 mile runs along the same route (then extended to 7-9 mile runs along varying routes), so I had a set of about a half-dozen 5 mile runs to compare. My 5-mile pace was usually 8:30 to 8:45 per mile, worst was 9:00, which was fed but I think I’d eaten a bunch of peanuts before, so I am now suspicious of peanuts. The 40 hour fasted run was close to 8:45 pace, IIRC.
As I said, because I was afraid I’d be slow I deliberately pushed the pace on the uphill blocks for the fastest fasted run, rather than trying to keep a more nearly constant effort as was my usual habit, so it’s a bit screwy.
Thanks for the reply!
Knees are why I cycle.
Not precisely protein. If I am right about how much energy you could get from fat, and assuming glycogen isn’t sufficient to make up the arrears, then you used amino acids from your labile pool. The labile pool is a buffer for amino acids which by weight comprises roughly 1% of your total lean mass. Those amino acids are potentially going to be used to make proteins only if you don’t need them for energy.
That buffer is filled by your cells recycling proteins excess to requirements, or digestion of dietary protein, and emptied by your cells making new proteins (and other molecules based on amino acids), or using them for energy.
Sure if you use a lot for energy persistently and don’t refill them you may inhibit the production of new proteins and because we’re continually tearing down and rebuilding proteins you could be losing muscles for a fast at that point.
Same - knees are why I cycle.
best answer!!!
in general to all--------- life. natural life of what it was meant to be.
eat, thrive, live and live another day to eat and live and if no kill, we ‘can make’ it like all predators do cause we don’t eat weeds or grass for our survival and predators like lions and ALL of them go longer cause THEY must which is why I say all humans are ketogenic burn vs. glucose burn…glucose burn is for 'secondary back up for survival…to survive life…fat burn ketone life is what life is all about YET humans went glucose nasty fake food sugar burn in full and we all are losing health point blank now.
and remember our lives ARE NOT and WILL never natural.
We work in cubicles. We fret over money for survival to pay bills and survive on a dime. We have noise pollution as an assault to us, we ‘sleep’ on demand thru work issues to pay for it all, family-past experinces that hit and LIFE of today for humans is not and won’t be natural again for each of us…in that big term of ‘real life natural flow of life’ and what it takes.
to SG–If you are hungry SomeGuy then you ain’t eating enough, simple as that if one is chatting carnivore plan 
but I also do not no your life in any way that could be effecting why you feel hungry and why your eating on carnivore is not what it should be to make ya not say that…if your fat content means more to you on vitality then darn eat the fat to help you at all times 
no need to peter out truly if you know you, ya know LOL
but you know you, you see it and correct it as you need, I did the same, I found me on what works with my time on plan.
“Fasting doesn’t work”
Comfortable, safe dry fasting is what motivates me to extended fast. Keto allows me to extended fast. OMAD makes exercise easier.
Fasting works. Look at Terry Crews. 53 years old with baby skin. He’s been doing it before science had much to say about fasting.
Eating is an inconvenience. Fasting always works when it allows you to avoid public restrooms.
and dry fasting at that HA speak for some but never for the entire population out there that will never ‘be just you’ and a few others that follow suit. 8 billion on the planet, give me some truths. Blanket statements like this without big support mean nothing to be a select few truly thru the 8 bil on the earth
not a darn thing ever saying…what worked for me personally but to put it thru as real truth is oh so tough for many to read and wondering why what worked for U will never be them
and @BuckRimfire I used to run marathons. Knees are so frustrating.
I am currently trying to help my wife get better use out of her knees. She wanted to start with Prolo therapy which seems like witchcraft and ineffective voodoo. I would like to try BPC-157 an amino acid peptide which I see has some positive traction along with Platelet Rich Plasma. She has degenerative knee cartilage as well as a possible Ehlers Danlos diagnosis on top of fibromyaglia and more.I even had a protein tested by a Professor at Urbana Champaign who is working on a vaccine to treat fibromyalgia and her score was 88 out of 100 and is awaiting the second round of trials.
I hope that a keto carnivore diet might reduce some of her inflammation and have made plans on how we might address this.
@richard @marklifestyle @BuckRimfire
I agree about cycling. Take care of your knees, you’re going to miss them when they’re gone. If you get to the point where even cycling hurts, slow down and/or add motor assist.
I’m buying a back inversion table to hopefully eliminate clicking in my left knee. I can’t imagine a life without decent knees. Give it another decade we could be powering our houses with stationary bikes. Don’t put your money in Peloton.