Hey everyone! I’m having a hard time reconciling 2 (seemingly) opposing perspectives on things. Drs Phinney & Volek mention in their awesome book “The Art & Science of Low Carbohydrate Performance” that even for lean endurance athletes, the body has plenty of fat to fuel them for a very long time (something like 30K+ calories). Lately, I’ve heard a few people in the Keto and/or I.F. communities say things like “if you are already at your target weight, there’s no need to do extended day fasts” and “if you’re a really thin person, you shouldn’t do extended day fasts”. However, fasting provides many health benefits; it’s not just a tool for weight loss. Furthermore, if you pulse or cycle a feast and famine approach as some doctors and experts are suggesting are good for you on the cellular level (not to mention peaking your metab before a fast), then you can actually gain muscle mass and/or body fat depending on how you feast while still reaping the benefits of extended day fasts. Is my thinking here illogical? TIA
Extended Day Fasting For People Already at Lean Target Bodyweight
Having a theoretical supply of fat “on board” and being able to utilize it in the moment are 2 different things. Lean people can derive benefit from extended fasts, they just might have a different experience:
@Kirkor That’s a very interesting blog and calculator - thanks! I am 178 at about 12% body fat; 156.6 lbs lean body weight. That is 21.4 lbs body fat. My BMR is about 1,746. My required metab deficit according to that calculator is 1,072 kCal (61.4%). Does this mean if I don’t want my metabolism to slow down by 61.4% while doing a water fast that I must consume 1,072 calories worth of fats (approx 119 grams of fats)? WOW! Good thing I won’t be doing extended day fasting for the sake of weight loss because this is a pretty big dip in metabolic rate.
Yeah the first (roughly) 18 hours or so you are drawing down your circulating lipids, chylomicrons carrying fat from your last meal, LDL carrying fat’s that your body fat has been releasing over the past few days.
Once you have drawn down circulating lipids, you are now running on the energy from body fat as it can be released.
That is why a lot of lean people can’t fast more than about 18 hours, and shouldn’t because your body doesn’t only cut back non-essential services, but it also starts hunting down alternate sources of energy. You don’t have enough fat, or glucose, then it will have to be amino acids that are drawn from your labile pool of circulating protein that can be easily retasked to be used for energy. Once that is gone you should start catabolisng protein structures not essential for immediate survival.
That 119g fat top up may be breaching the spirit of a fast, but if it stops you having to use protein for energy it’s worth it as it is protein sparing.
@AGG Thanks for posting this. Your numbers are almost the same as mine.
I, inarticulately, attempted to share this info on the main, recent, extended-fasting thread. Fast/Feast cycling
For me, I commented on that thread that about 1800 kcal is “fasting”. Using the calculator above, I am told that it is about 3600 kcal! I had already started, today, a weight loss plan with about 2650 calories per day - I will lose one or two pounds per day until 20 pounds are lost. That will bring me to my actual “lean body mass” (as tested in a variety of ways over decades). I have gone below my lean body mass many times with water cuts. I am looking forward to a dexa scan in August.
If I recall, Dr. Fung said (in a 2KD podcast) that he did not agree with the maximum rate that energy can be drawn from body fat as expressed in the calculator. However, there absolutely is something to it.
So, do we go and beef up 20 pounds then fast to get the benefits of fasting?
Thanks @richard
Sorry to belabor this. Are you saying that for a very lean person, beyond the 18 hour mark would probably not be advisable to fast much longer as after that mark your body hunts down circulating proteins to break down for energy and if not present, the muscles are the source? (catabolic). My concern is I am very interested in extended day fasts for other benefits - I have heard that doing a week long water fast 1 time yearly may help fight cancer/prevent cancerous growth.
Thanks again!
I don’t know that EVERYONE reaches an end of their circulating lipids capacity at precisely 18 hours, but the fact that so many people notice the switch over around that point indicates that that may be the cause. It would be different based on how much fat you had in your last meal (which will be still circulating up to 10 hours later).
I do know that EVERYONE will reach a point where their energy is coming completely from body fat, at whatever rate their body fat is willing to contribute. The maximum rate is roughly 315 kCal for every 10lbs of body fat. Lean people will necessarily have much less energy available, as will fat people who have high basal insulin.
My own experience as someone with about 50lbs of body fat but enough insulin to lock it away from access is, at 18 hours I feel “UGH” and around 48 hours I feel “leap over small buildings” awesome.
My hypothesis is that around 48 hours is when my pancreas says “enough of the BS we’re cutting off the insulin” and then I get access to my body fat.
Yes I heard the same thing. From Thomas Seyfried
I think in your situation I would either try to supplement your energy resources during the fast with fat, or have a high protein meal before the fast to make sure your labile reserves were topped up, then fast for a week accepting the hit on your lean stores, and then have a good high protein meal for several days after the fast.
Believe it or not, fat gives us humans a lot more options.
Thanks @richard ! Yes, I believe I heard the suggested 1 week fast from Thomas Seyfried.
I really enjoy the 2 Keto Dudes podcast by you and Carl. I just mentioned it to my friends on our Facebook group “Life In The Fasted Lane”.
@pinnacle Yeah, I’m not sure of the solution. I have been hearing more about “pulsing” the feast and famine cycle to attempt to increase your metabolic rate right before going on a fast (thus being in a fast with a higher metab rate than if you had gone into it with a previous days of chronic caloric deficit). I think 2KD may have talked about it on one of their podcasts. I also heard Dr Joe Mercola mention it as a guest on a another podcast. He said it was good for MTOR or something else. Can’t remember - so I don’t want to misquote 
I am really mostly interested in the autophagy aspect of an extended fast which is why I intend to try a 7-day fast although I am already lean by some standards. I suppose I should just try to be more accepting of myself if I can’t sustain it for the entire week. I will be pleased if I get to 5.
Anecdotal evidence here:
I’m 5’11"/160 pounds, 9-15% body fat, and I fast every month for at least five days. For me, the autophagy, self-control, cost savings (yes… even that) and recovery impact (I feel SOOOOO GOOOOD after a long fast) are worth it.
I did a 15 day fast on nothing but water and black coffee, it’s doable.
Now, I might be an anomaly with a high threshold for pain/suffering/B.S. but I don’t really believe I’m that much different from the average person, just that I “laddered up” into this process, I started in FEB of 2016, waiting until 1300 to eat anything, then started doing 24 hours and so on, that made it much easier when I did the 15 days.
I did 5 days last week, started out at 165 (beer weight!) and ended up at 161, water, coffee and some heavy cream only, the heavy cream made it much easier but it was not impossible without.
Bottom line (IMHO) even those of us at our ideal/desire BW and those of us who are lean will still benefit from fasting. Going for periods without having an “insulin hit” HAS to be good for us, giving the body time to “rest” from digestion and the side effect/impacts of those biological process HAS to have a positive net effect. Clearing out old cells and forcing the body to use old proteins and such, rather than staying in a fed state, shocking the system and stressing the processes HAS to create a stronger organism.
I’m working on my own “unified theory of fasting/diet/exercise etc” and am working on providing evidence for these kind of thought processes, note I said evidence, not proof. IMHO we can provide evidence that XXX works for/against us as an individual but we are all so unique, N=1 rules…
Thank-you for this. I was searching for info for my husband who is a cancer-survivor and is doing his first 3-day fast with me this week. His goal is autophagy, not weight loss.
So far he’s lost 4 pounds and we have one more day to go. He has never been insulin resistant so has access to his fat stores. Will he gain it back after he breaks his fast and feasts? Should he eat more than normal after his fast? He mostly follows a keto diet because it helps his joint pain and allergies… but not as strict as me (more carbs, but not near SAD carbs) because he can get away with it.
You are welcome!
I would expect that he’ll gain most of the lost weight back. Fasting weight loss is mostly water/hydration based.
I don’t think it’s necessary to eat more before or after a fast, just follow the bodies signals and, if hungry, eat! I would not worry about how many calories etc but I would stick to keto, don’t use the fast as a reason to eat lots of carbs/sugar/junk before or after a fast (this is common, folks think “I’m fasting next week so I’m eating junk this week!”).
Weight loss during a fast isn’t a problem per se, but if the focus is on autophagy and not weight loss, ensuring lots of hydration with sodium might help. Additionally, having a single avocado a day might help, and shouldn’t be that big of an impact on autophagy (Dr. Longo does a lot of work on this).
Best of luck!