Base metabolic rate is down by 25% after 28 days (https://www.ncbi.nlm.nih.gov/pubmed/7275468). We don’t have a lot of data, but the biggest loser study shows a metabolic rate of 500 calories below baseline after 6 years in which patients had regained most of the weight (RMR was adjusted for weight loss). https://www.ncbi.nlm.nih.gov/pubmed/27136388
The lowering of RMR with prolonged fasting is real. The only thing we’re talking about is whether a 3-day fast will lower RMR in the long term or not, there we obviously have no data. Maybe it does, maybe it doesn’t.
Well, as before, Dr. Fung mentions that in the 4th day of fasting, the average metabolism was increased by 13%. The increased secretion of hormones that rev us up is real - it’s easy to test for increased noradrenaline/norepinephrin. There certainly is individual variation, and some will be above that 13% figure, and some will be below it, but the average increase in metabolism was there, as was the confirming increase in V02.
These facts totally fly in the face of “The metabolic rate does go down a lot when fasting after 3-4 days,” and that’s what caught my eye.
All right, cool, man - some good data there, apparently, and I don’t doubt it. (And I wondered, and it’s good to know that.) No question that eventually the metabolism will slow. Now of course - the body weight is declining during fasting, so after a substantial number of days of fasting, metabolic decline would be expected, all other things being equal. 28 days will usually have people down 14-20 lbs on fat (really big people can lose more per day) so even if maintaining the same metabolic rate per unit of body mass, an overall decline would eventually be expected.
I’ve got about 30 fasts of 3 days to 12 days duration in, and things vary - I’ve felt “normal,” or “sluggish” or “really jacked up…” I don’t doubt that after 28 days I’d have a slower metabolism. And I really wish there would be exhaustive studies on fasts of many lengths.
The ‘Biggest Loser’ contestants weren’t fasting, as far as I know - I think they were eating less (restricting calories) and working out, even working out like crazy.
This illustrates why this is a complex calculation. Over 28 days, you should lose a lot of weight. If you determined your metabolism slowed, would that solely be due to your lost weight? It’s like when people lose muscle mass when they lose a ton of weight. Is that only muscle they lost due to not needing it to move their previously much larger bulk around?
Say you fast 5 days and do DEXA scans before and after (and assume DEXA scans are perfect, which is never the case). You lose some lean body mass. What does this mean? Phinney thinks this is bad, while Fung thinks this is good. Who is correct? I personally think Fung is, because I’ve done tons of fasting and gained muscle mass while losing body fat according to DEXA scans. But I could be wrong, as I’ve never tested over one fast, only over many months. Maybe when I was eating, I compensated for the “muscle” mass I lost when fasting?
Unless we have large scale studies with many people, we can’t really answer these questions.
amwassil
(Michael - When reality fails to meet expectations, the problem is not reality.)
#27
Don’t want to be a smartass, but RMR is mainly based on lean mass so it doesn’t go down proportional to the weight. Curious. How does Fung measure RMR? It’s not exactly easy to determine. Maybe different measurement methods lead to different results.
The biggest loser study dscribes how RMR is measured: The RMR measurements were performed using indirect calorimetry following a 12-hour overnight fast. Participants rested supine in a quiet, darkened room for 30 minutes before making measurements of VO2 and VCO2 for 20 minutes with the last 15 minutes used to determine RMR according to: which assumes that protein oxidation contributes 15% to the energy expenditure.
Edit: It might make a difference whether you come from low carb or high carb when starting the fast. After all, the body decreases RMR with an undercaloric high carb diet but doesn’t with a low carb diet (unless it’s extremely undercaloric).
The muscle loss is almost negligible unless you fast for a long time, 12-15 grams per day and declining with time. https://www.ncbi.nlm.nih.gov/pubmed/6764569 We can measure this pretty well from some nitrogen compound which (as I understand) is excreted via urine exclusively.
Still, I recommend drinking a bit of bone broth when fasting. It will replenish electrolytes, reduce keto flu and give just a little bit of protein, for the price of a minor insulin response that shouldn’t impede fasting significantly.
Bob, it definitely is complex… For many people it’s no big trick to lose 5% or 6% of body weight during a 28 day fast, so some decline in metabolism would be expected, all other things being equal. It’s not going to be linear, though - we’re mainly losing fat, which is metabolically “cheap” to maintain (versus muscle, nervous tissue, etc.). Take it to a further extreme - the average person who weighs 150 will burn more than half the calories of the average person at 300.
There’s also the thermic effect of food. I always read that this is somewhere around 10% of our energy expenditure - when we’re eating, of course, not fasting. So, if there is a “real decline” in metabolism due to fasting, I’d say allowances have to be for weight loss and no food being processed, i.e. ‘all other things will definitely not be equal.’
I’m not saying this explains all metabolic slowdown when fasting. After some time, I do think the body starts trying to conserve energy by hormonally adjusting things. I’ve definitely felt sluggish and low-energy on some days of my longer fasts - and I am sure that things would be different if I was eating.
The accounts of long, really big weight-loss fasts I’ve read about are from the 1960s and 1970s, so there were no DEXA scans etc. It’s too bad they weren’t more closely studied - the most I can say is that the people generally looked okay when done fasting. Not sure if they lost more muscle or not, versus what would be expected due to having to move less mass around.
Sometimes it’s water loss from tissues that the DEXA scan reads as “lean” - this will be reversed once eating resumes and hydration/electrolyte balance returns to normal. There’s also the refeeding period after fasting, which does some impressively good things (while not directly related to weight loss or regain, it’s an area that makes me think that fasting at least once in a while is really advantageous).
To an extent, the refeeding period will make some lean mass “reappear,” usually, on a scan, but I don’t know if it accounts for everything.
Especially during the first few days of a fast, there is a significant and measurable nitrogen loss from the body - it does appear that some protein is being metabolized (and I don’t know how much of this is really muscle tissue or not). I’ve seen Phinney extrapolate from this, proclaiming that more muscle tissue is lost than there really is - the nitrogen excretion declines fast as the body goes into full recycling mode. As most of us usually note, too, Phinney has worked with more athletic, often younger people, than Fung. His subjects likely have substantially less fat to access, i.e. they will get into consuming lean mass earlier and more substantially than those whose fat stores can provide all the fuel needed.
I don’t give Fung a total pass - he sometimes over-simplifies things, or generalizes with nearly bumper-sticker style quotes, i.e. “The body is not so stupid as to burn muscle when fasting - this would not fit, evolutionarily, since we’d need to be strong to go out and hunt” (paraphrasing there).
There’s a lot that confirms what he says - the increase in growth hormone, for example, is profound as we get into fasts of several weeks in length, but it’s still not 100% effective at preserving all muscle that was there prior to fasting, and things are not as ‘black and white’ as I feel Fung sometimes portrays them.
amwassil
(Michael - When reality fails to meet expectations, the problem is not reality.)
#31
Something else to consider is the body’s requirement to maintain 98.6°F overall temp. That’s estimated to require 800-1000 kcal per day min. See my link above. When fasting, that must come from body fat and/or protein. If one is too lean to get it from fat, then it must come from protein. I’d guess.
Man, I love Phinney but I think he’s off there. I doubt that Keys fed those men significant amounts of protein during the 4-month recovery phase, at least during the first 8 weeks that were participants were fed set amounts and coudn’t eat as much as they liked. Thus, protein buildup will have been slow.
Refeeding syndrome is a thing but hasn’t been observed in any medically supervised fasting AFAIK.
I do agree with Phinney that there is a downside to fasting, but I also believe that we are built for feeding patterns consistent with hunter-and-gatherer populations, where fasting periods of a few days would be pretty normal. So it would be very surprising to see serious downsides to, say, regular 48h fasting. Going weeks without food are a different matter.
No worries, Thomas. It’s an interesting area, and no argument from me on the proportionality of RMR decline and weight loss. Muscle tissue is ~3.5 times as metabolically costly to run, versus fat tissue, and the individual will still have the very-costly-to-run brain and some other organs there, after the weight loss as well as before (the average person who weighs 150 will burn more than half as much energy as the one who weighs 300).
I haven’t seen Dr. Fung do his own studies on RMR - he quotes others, so I’m not sure of the exact method. (I think he does have a solid point about increased secretion of hormones that rev us up in the first few days of a fast, however.) To me, the ‘Biggest Loser’ contestants had the worst of both worlds - feeling deprived and suffering an average metabolic decline of 789 calories per day. Some of that was due to losing weight, but as above, not enough to make it even remotely worthwhile, especially in the context of losing/maintaining body weight. We can debate the merits of fasts of different lengths, but I think we’re on the same page as far as “calorie restriction” diets being a very poor way to go for most people.
No question about it. If we’re already burning fat for energy (and have sufficient stored fat to access) then it makes sense that the body could much more smoothly and evenly go to the fat stores.
I’ve seen this before, and I question whether Dr. Phinney or Dr. Volek actually wrote the text, there, themselves.
In brief, any regimen involving fasting beyond 24-hours has not been proven effective in sustaining weight loss long term.
I guess this one would be arguable, but at the present there are so many individual accounts - as well as famous examples from the past - that give evidence in the other direction that at the least I’d say it’s an incorrect generalization.
And while it can temporarily speed weight loss, this comes with a long-term price.After just one day of fasting, you begin to lose body protein from lean tissue – from places like muscle, heart, liver, and kidneys.
This is the one that blew me away. It’s so incredibly alarmist. Even people who involuntarily starve all the way to death don’t lose much mass at all from their heart. One day of fasting, and are we really supposed to then be in fear for all those important organs? C’mon, man…
These organs and their functions are things that we want to preserve, and definitely should not be given up lightly.
Geez… All the 3 to 12 day fasts I’ve done, and here I was giving up my muscles, heart, liver and kidneys… Damn it!
To me, that sounds like it was written for a clickbait site, by somebody not too familiar with English.
The body has the option to lower its temperature. Some people experience being cold very soon, it doesn’t take several days or even fasting, low energy intake is enough. It’s a effective way to conserve pretty much energy and we easily survive some temperature drop, it’s better than dying from hunger, our body “thinks”. Mine is a wasteful one, it seems, it never “thinks” about famine if I don’t eat for a few days, it stays warm and demands food, that’s super effective every time. It probably would change later but I never do really long fasts.
But if we do need energy and nothing else is available, it comes from protein, of course. I never could understand how someone is able to think we just don’t lose muscles when fasting (or just eating way below our need). We obviously do, it’s usually little but in some cases, it isn’t… Our body is skilled but it has its limits, it can’t create energy or amino acids from nothing. If we reach the limit of getting enough energy from fat, it’s a serious case and people usually feels unwell and gets cold then, it’s a sign our metabolism slowed down, the body does its best to spare energy. It’s better than burning through precious muscles too fast though using up costly muscles when there’s plenty, that’s a good idea too. But those aren’t nearly as needed for survival as the modest amount of muscles of a not muscular one.
Fung did say that, but I don’t think this can be generalized to subjects fasting for a second time (or 50th!). All published data on RMR and extended fasting has been done on subjects fasting for the first time. To my knowledge, there is not a single journal article that has ever been published investigating the RMR effects of subjects fasting for the second time, let alone every single week! It’s crazy to think that the human metabolism adapts to everything else, but not a stimulus as massive as extended fasting on a serial basis.
The best data we have on serial fasting and RMR is from this forum’s very own @primal.peanut . He fasted 5 days a week for months and tracked his RMR daily with an indirect calorimeter. Take a look at this chart of his daily RMR after adapting to fasting. Look at how his RMR drops by 500kcal in the first 24hrs of fasting every single time (those are the red lines to highlight the matching size of the drop). His RMR data does not even remotely resemble the published data of people fasting for the first time. His body adapted to extended fasting because he does it often.
Like you mentioned, there is probably a lot of individual variation in the adaptation response to frequent extended fasting, but I don’t think we a clue what those adaptations are because they have never been studied.
Quite true, Nick, but it wouldn’t necessarily be slowing the metabolism or increasing it less. It’s very common for one’s first fast to be harder to get through, versus later fasts - if anything I’d think this indicates beneficial adaptations and more energy available later on. Most of us really do “get better at fasting” the more we do it.
If the supposition is that repetitive fasting results in less increase or more decrease in metabolism, it would be bolstered by Primal Peanut’s having had a relatively higher RMR in his first fast, versus later fasts - but I’m guessing it was not recorded/we don’t know…? We also don’t know to what extent, if any, he’s an outlier.
Down 370 - 440 calories in RMR is substantial, no question about it, but measurements after digestion is completed would be expected to feature some built-in decrease, no? In PP’s case I’m guessing it would be 220 - 280 calories…? When measurements were taken before fasting hours were 12+ I’d think some increased metabolism due to digestion is probable.
I looked around on the internet today, and yeah… It’s an empty wasteland out there, as far as repetitive fasts among human subjects.
We’re really talking about different metabolic substrates, and hormonal changes and alterations in what they regulate. There are some things that we know generally happen on a recurring basis (with fasts of a few days): small to moderate decreases in blood sugar and insulin, big increase in fatty acid mobilization, huge increase in ketones.
As far as RMR, the elephant in the room is norepinephrine. In the study Fung references, it more than doubled by Day 4 - this is what we really need to know for serial fasters. The study subjects were pretty consistent on resting energy expenditure - there was an initial range of ± 23%, and Day 4 had a range of ±20%.
Individual variation - even among individuals, one fast may differ substantially from another. On my last fast, days 5 and 6 were really rough, while days 8 and 9 were great, easy. No way to predict that in advance.
I agree 100%. This is what drives lipolysis. I did some research into adrenal fatigue and adrenal resistance last spring when I was fasting 2Feast:3Fast on a serial basis for 4 months. This was my big concern. It was my hypothesis that adrenal resistance was responsible for people who were fasting on a serial basis getting “colder” during faster the more often they did it. It happened to me too. It happened to everyone I talked to.
Also, people who start fasting often generally reported that they would get too wired to even sleep for the first month of serial extended fasting. It happened to me, and it felt like I had just had one cup of coffee too many. Sounds a lot like nor-epinephrine. After a few weeks that stops, and you can sleep just fine. Sounds like adrenal resistance.
That got me researching adrenal fatigue and adrenal resistance similar to what happened back in the 90’s when you could buy epinephrine diet pills right over the counter. People lost fat like crazy, and it also F’d them up. Enough to make it illegal.
Anyhow, that’s the stuff I was looking into, but I never came to any conclusions.
I think serial extended fasting is pretty safe, and I did it for 4 months with no apparent ill effects. That said, I don’t know what the heck it did to my metabolism either. I believe adaptations occurred that make it easier for me to fast, but I don’t know that they are all positive.
For what it’s worth, I’m on 2 day fast right now, so it’s not like I’m anti-fasting by any stretch! I just think that we have no idea what happens to the human metabolism when we do extended fasting on a serial basis.
Nowhere in the literature has an RMR drop of that magnitude in that period of time ever been recorded as far as I’m aware. That’s a positively staggering drop. His RMR should have dropped 50-100 at most or even risen a bit in the first 24hrs. It dropped 5x that every single fast for him like clockwork.
Again, I’m not saying it’s necessarily bad, just that it’s off in uncharted territory.
If you eat that much you WILL gain weight Unless you poop a lot of it out before it gets absorbed, ie you poop a lot. I’m in a weight gaining mode right now (or was up til’ last week) on keto because I eat more than I burn because I dont get enough exercise in the winter. Once I hit 200 I start eating less. It’s not hard except when I get bored.
Curious. How does Fung measure RMR? It’s not exactly easy to determine. Maybe different measurement methods lead to different results.
For many people it’s no big trick to lose 5% or 6% of body weight during a 28 day fast, so some decline in metabolism would be expected, all other things being equal. It’s not going to be linear, though - we’re mainly losing fat, which is metabolically “cheap” to maintain (versus muscle, nervous tissue, etc.). Take it to a further extreme - the average person who weighs 150 will burn more than half the calories of the average person at 300.
It’s so incredibly alarmist. Even people who involuntarily starve all the way to death don’t lose much mass at all from their heart. One day of fasting, and are we really supposed to then be in fear for all those important organs? 
C’mon, man… 
It’s an empty wasteland out there, as far as repetitive fasts among human subjects.