Just worried about bmr lowering on extended fasts. Only doing 3 days but let’s say I could do 30 with black coffee. BMR get shot to hell?
Theoretically...does a 30 day fast lower bmr?
late_to_the_party
(Susan)
#2
I’ve been listening to Jason Fung on Youtube. He’s the doc that has researched fasting the most. He says if you truly fast - meaning only water and minerals - you have the huge plus of autophagy. This means you break down your old, unneeded cells and protein and use that to keep you going. You lose loose skin, old buildups of protein here and there that you don’t need any more (including your brain, which is why some speculate fasting might delay Alzheimer’s). The problem is that if you eat anything, you don’t start the autophagy.
His take home message was that normal calorie restriction (most diets) lower BMR since you can’t get at your fat stores or old cells if any insulin is being released. Thus, you have to surivive somehow with lower calories so you lower your BMR. Fasting, however, opens up other sources of calories within your body and doesn’t lower BMR.
I didn’t pay attention to anything he might have said about coffee as I don’t like the taste. Search for Dr. Jason Fung on Youtube and you’ll find something like ‘Fasting 101’ and other videos he’s done. He’s the one I trust most for fasting. (Phinney for keto.)
This is what I got from the various videos. I may not have gotten it completely right. If I didn’t, please chime in and correct me everyone else. Thanks.
Don_Q
(Windmill Tilter)
#3
Personally, I do a lot of extended fasting because I find it convenient for weight loss. I did a few dozen 3 day fasts last year, and I’m doing 2 day fasts followed by 2 day feasts on a serial basis until May. I’m not anti-fasting, but nobody on planet earth understands wtf it actually does to the human body when done on a frequent basis. I’m ok with that.
No one has even studied individuals doing extended fasting for a second time as far as I’m aware. Since you’ve already been doing some extended fasting, nobody can really say what a fast of this duration would do to you. Extended fasting is poorly understood, and frequent extended fasting is basically a black box. Ever seen Fung’s data on BMR from all his clients that do weekly 3 day fasts? Me neither.
It seems probable that metabolic adaptations occur when you fast frequently given that water fasting is the most powerful metabolic stress possible, but as far as I’m aware, the adaptations are completely unknown and unexplored. Maybe they’re good, maybe they’re not so good. Probably there’s a bit of both.
If we assume that your metabolism responds in the same way that it does for someone fasting for the very first time, there is no reason to think that your BMR would be negatively affected.
Shinita
#4
Beware, I just have hypotheses and educated guesses.
Available fat (and probably muscle mass too) is surely a very important factor. Lowering BMR is a smarter reaction of the body than burning through the little reserves and killing itself pretty soon.
But there are so many factors, I can say this much with my tiny knowledge about fasting. It’s different with different genes, different fasting experiences (our first long fast is probably not as our 20th long fast)…
Very, very rarely can be something very specific said in general if the wonderfully complex human body is involved.
I personally would expect a lower BMR in general, very soon with little reserves but I can imagine BMR barely gets affected in some cases when the body gleefully burns through the plentiful excess fat reserves (but I don’t expect it in every cases when huge amount of excess fat is present, it’s probably genetics).
ctviggen
(Bob M)
#5
I would think any time body mass goes down, BMR goes down. So, BMR should go down after 30 days. The trick is to determine whether BMR decreases MORE than what should happen based on the person’s final body mass and activity level. In other words, your body has a BMR less than what it should have (which is what happened to The Biggest Loser contestants).
amwassil
(Michael - When reality fails to meet expectations, the problem is not reality.)
#6
I think this might be relevant, since in a way it represents (forced) EF:
Starvation. This is an extreme situation. It does not happen in a few days or a few weeks. We have evolved to deal with this in a very elegant manner. When the hunt was successful everyone ate and stored the excess energy as fat. When the hunt failed, our ancestors still had to continue hunting even when that meant days or weeks of not eating and still burning energy.
Read Scott’s diary. It’s truly extraordinary what these men endured before finally succumbing. Of course, they were not only dealing with a huge energy deficit, but an environment that necessitated equally huge energy expenditure to survive. Ultimately, they could not produce it.
Don_Q
(Windmill Tilter)
#7
Yup. The Mifflin St Jeor equation is a helpful baseline reference. Get your RMR tested prior to the fast. Ask them the duration you’ll be on the respirator. If it’s less than 25 minutes, ask for two back to back tests. Most indirect calorimeters that are intended for weight loss clinics and doctors offices have an abbreviated RMR test as a “feature”. Mine has the feature, and it recently came back from the factory calibration with this set to default. This allows clinics to increase patient throughput at the expense of accuracy.
Once you have a valid RMR, compare it to your predicted value given the Mifflin St Jeor equation using an online calculator. This will give you a reference for how fast your metabolism runs relative to the population mean. There is wide variation among st individuals. If you are 10% above normal, you should hope to remain 10% above normal at the end of your 30 day fast.
As @ctviggen mentioned, you’re RMR will change if your mass changes, but what’s important is how your RMR changed relative to the predicted amount given your height/weight/age.
Don_Q
(Windmill Tilter)
#9
Your best bet is to google Dexafit to see if they have a location near you. An RMR test will run you about $100. Google for a coupon and you’ll probably find one that will get you an RMR for $75. If there aren’t any near you, go to the KORR website and look for the “find a provider” link. Enter your zip code and you’ll find a list of facilitys with KORR indirect calorimeters. These could be weight loss clinics, high end personal trainers, bariatricians etc. I’m not affiliated with KORR or anything, they’re just the largest manufacturer of clinical indirect calorimeter devices, and it’s the kind that I own for personal use.
If you’re doing it to see what effect fasting is having on your metabolism, I strongly encourage you to do the RMR test 48hrs fasted. There are two of us here who do extended fasting on a serial fasting, and who own indirect calorimeters, and both of us have seen the same pattern. Our RMRs drop precipitously a short way into the fast, in the other guys case it’s a consistent drop of 500kcals! Mine is half that. Both of our RMRs rebound upon refeeding. It appears to be an adaptation. If on the other hand your RMR is in line with MSJ, you’re probably good to go.
I’m not a fan of fasts lasting longer than a week, but if you’re going to do it anyway, it wouldn’t hurt to know whether your metabolism (for which your RMR is the best proxy) is running normally to begin with.
I’m no doctor though, and certainly not an expert. Just my 2 cents. 
Bobt
#10
Fung also talks about an increase in hgh at a certain point of the fast. The body is definitely doing something different with bmr than what happens with calorie deficits. It would have a natural slow down due to just a change in body mass index. It will be interesting to see where this goes considering that fasting is being practiced by so many people.