Why fasting is easier for some people

A friend told me that she has tried to fast, but can’t do it more than 24 hours.  And then she gets ravenously hungry afterwards.   As you may know I have done 7 day water fasts, and still had enough energy to ride 100km on my bike.  She asked me why it seemed so easy for me … so we did the math.

We know the maximum rate that body fat can transfer energy thanks to A limit on the energy transfer rate from the human fat store in hypophagia – Alpert (2005)   This paper used Ancel Keyes’ Minnesota Starvation experiment data and a little calculus to derive the value of (290±25) kJ/kg day  or  (31.5053±2.72) kCal/lb day as the maximum rate that energy can be drawn from body fat.

I’m 230 lbs, my lean body mass is 170 – so I have around 60 lbs of body fat. This means just from body fat alone, I can draw 1890 kCal/day of energy.

She’s 135 lbs, her lean body mass is 110 – so she has around 25 lbs of body fat. Which means from body fat she can only draw 878.5 kCal/day of energy.

My daily metabolic rate is around 2000 kCal, so if I fast I only have to find 110 kCal of energy savings – around 5.5%.  If I don’t want my metabolism to slow by 5.5% then I can eat an additional 12.2g of fat per day to remain fasted. But if I don’t eat a little fat I probably won’t even notice a 5.5% slowdown.

Her daily metabolic rate is around 1500 kCal, so if she fasts she has to find 713 kCal of energy savings – around 47.5%.  If she wants to maintain her metabolic rate then she would have to eat an additional 79.2g of fat per day to remain fasted. It’s no wonder she can’t fast for long.

And that is why fasting is easier for me, and harder for her.

You can calculate your own required metabolic deficit;

Total weight

Lean body mass
Body Fat (lb)
Metabolic Rate (kCal/day)1
Required Metabolic Deficit 2
Required Metabolic Deficit 3
Additional fat 4
1 average Metabolic rate of US Males = 1696, US females = 1410
2 How many calories you will be short
3 What % of energy savings you would have to find
4 How much dietary fat you would have to eat to prevent slowdown

60 Responses to “Why fasting is easier for some people

  • Great calculator – am just madly translating kg into lb so that I can use it! (Aargh, the dilemma of dealing with a predominantly US audience!) May use your explanation (cheerfully attributed to your page!) when explaining to my patients how this works. I get a lot of disgruntled wives watching their husbands doing intermittent fasting, which the girls can find a little tricky. Am trying to educate patients as to Ketogenic lifestyle, and your page is great, thanks. (Am a naturopath in Australia).

  • So, mine’s 0. I guess I’m good on having enough body fat…

    • richard
      8 years ago

      Yeah that’s the idea. If your bottom line is a 0g (or %) deficit that means that you are able to get more energy from your body fat than you need for your energy expenditure.

    • There clearly is more going on than this calculation. I weigh 145, Lean Body Mass of 121 (24 pounds of fat) and definitely do not have problems going 24 hours. Currently in Day 4 of a “pretty much” fast where I eat about 200 calories a day in bone broth, Kombucha and maybe the odd olive or two when I want to chew on something.

  • S. Haskmann
    8 years ago

    Can haz metric plz? You know, that silly decimal system that most of the world uses? 😉

  • Jay Morris
    8 years ago

    This calculator does not seem to work for weights above 230 pounds. Please let me know if and when it is corrected.

    • You can use the Slider below the preset buttons to adjust the Total Weight and the lean body mass. Total weight will go up to 500 lbs. LBM will go as low as 50 lbs.

  • I hate math. Please help me figure this out. I weigh 163 and according to my DEXA I have 114.3 lean mass and 26.3% body fat. I don’t know what to put in the metabolic rate box.

    • Total Weight: 163
      Lean Body Mass: 114
      Body Fat 49 lb (30%)

      btw I suspect the day you had your DEXA your total weight was 154 lbs as the math doesn’t work for a 26% body fat and a total weight of 163.

      Metabolic Rate: 1500 (this is how many calories you will use today – I’m guessing but the average for women in the US is 1410 and for men 1696)

      Required metabolic deficit: 0 kCal – so you are adequately able to provide all your 1500 kcal of energy from your stores of body fat.

      Let’s try a different Total weight. For example the 154 lbs you might have weight at the dexa for them to give you a 26% body fat.

      Total Weight: 154
      Lean Body Mass: 114
      Body Fat 38 lb (26%)
      Metabolic Rate: 1500

      Your daily deficit of energy you are short is now 240 kCal/day

      in other words to not see your metabolism slow down you woul dhave to consume 26.7g of fat.

      Let’s say we go back to the 163lbs total weigh and increase your metabolic rate, maybe you go fo a jog for 340 mins and burn an additional 100 kCal and now your Metabolic Rate is 1600 kCal a day.

      Total Weight: 163
      Lean Body Mass: 114
      Body Fat 49 lb (30%)
      Metabolic Rate: 1600

      Now your deficit is just 57 kCal/day. You would need to eat 6.3g of additional fat to prevent your metabolism slowing.

      • So does this mean if I fast I should not consume any fat to get the best results? As for what I weighed the day of the DEXA scan I suppose their scale could suck that bad, but it was only 1.5 pounds more than my scale at home.

        • Yeah. It means you have enough body fat. Just. You can likely comfortably fast without noticing a speed bump in energy.

          If you go for a 30 min jog as well as fast then you might want to take a small amount of fat (like a teaspoon of butter blended into a coffee).

          And if you lose another 9 lbs and then try the same fast you won’t have enough energy and may need 1 1/2 tablespoons of fat per day 🙂

          • Missy
            7 years ago

            Okay – thank you. That makes much more sense to me.

  • This is brilliant! I have been feeling so frustrated with my super low energy while doing a 3 or 5 day fast. Now I understand that I am not broken, just don’t have enough body fat (128 lbs, 25% body fat.)

    Thank you so much for the science and the calculator!

  • I’ve done calculations for my figures (estimated BF). It seems I’ll never need extra fat while fasting- if the numbers come out as follows

    weight[kg] 90 85 80 75 70
    body fat[%] 40 37 34 31 28
    body fat[kg] 36 31.45 27.2 23.25 19.6
    body fat[lb] 79.36 69.34 59.97 51.26 43.21
    BMR [kcal] 1625 1575 1525 1475 1425 – acc to cron-o-meter
    max fat
    drawn from BF 2499 2184 1889 1614 1361

    I’m 49-female; I fast prolonged 4-5 days/month -lost 34 kg since Apr-and now switched to fasting on weekends.
    i’m not sure about BF% I will get when losing weight but it seems I have a good margin of fat to avoid feeding while fasting. Correct me if I’m wrong.

  • gpisabela
    7 years ago

    This calculator does not take into consideration the degree of insulin resistance that can reduce the amount of fat mobilized per lb per day…

    • That is very true – This calculation is based on the Maximum rate that we can draw energy from a pound of body fat in a day, when all your “metabolic ducks” are lined up. That presupposes you have extremely low fasted insulin, or your body fat is insulin resistant (where it will release free fatty acids even when insulin is high).


      I’m afraid I don’t have a good way to factor fasting insulin into this formula … but if I can find actual data quantifying the inhibition of lipolysis due to insulin, then I could further finesse this model.

  • I have been keto for about a year or so, lost 30lbs and blood pressure stable also my blood panel is improved trigs down HDL up, i have been trying fasting the last few months with IF but recently tried 2-3 day fasts, i noticed after this last 3 day fast i got quite light headed and my flight or fight reaction seems to be way up the last couple days, just wondering if this could be due to lack of sodium as Dr Phinney suggested.? any tips would be great, i really enjoy fasting but do not like the anxiety after the fast. I am a 50yr old male 185lbs with about 143lbs lean mass. Thanks

    • It could be sodium. It could also be that your body fat can generate 42lbs x 31.5 kCal/lb = 1323 kCal, which may be just not enough to run your daily energy expenditure. The first 2 days you probably supplemented body fat energy with energy that was circulating in lipoproteins.

      Or it could be a panic attack – it’s a lot easier to notice the counter regulatory hormones kicking in when we are running on a low base level of energy. We don’t even need something tangible to cause a spike in those.

  • How did you get so friggin smart, Richard? A PhD in math perhaps? Or a zillion ketones? ?

    • curiosity mainly and paying attention to really smart people

      I did do pure math at Uni, but quit after 18 months to switch to programming. Ain’t no career in math if you aren’t in the top 0.1% except teaching math.

  • 60 year old female How can I figure out my lean mass so I can accurately use your calculator? I am 5’7′ and weigh 178 lbs. I have lost 27 lb.

    I have been doing 18/6 and 24/2 with added 72 hour fast and feasting on weekend. I do not want to lower BMR but I also want autophagy….how can I have both and can you supplement that I still stay in autophagy?

    • Brenda how often do you do 72 hr and in that time what do you take in?

  • Ok I figured out my lean body mass is 133 weight 178 with I believe 26% body fat. Do I put my BMR into the calculator are my kcal required for maintenance? I used a keto calculator and it reads BMR 1414 and calories to consume for maintenance 1867…however I did another lean mass calculator and it says my BMR is 1462. Can you please help me figure this out I don’t want to Lower my BMR because I’m not supplementing with fat.

    • Total weight: 178 lbs
      LBM: 133 lbs
      Body fat: 45lbs (25% BF)
      Metabolic rate: 1414 (that’s probably close too as 1410 is the average for US women)

      required metabolic deficit = 0 kCal

      Which means you have exactly the right amount of body fat to supply your body with that energy evry day.

      Let’s say however you lost 10 lbs.

      Total weight: 168 lbs
      LBM: 133 lbs
      Body fat: 35lbs (21% BF)
      Metabolic rate: 1414 (that’s probably close too as 1410 is the average for US women)

      required metabolic deficit = 312 kCal

      Which means you now don’t have enough body fat to supply that energy, and if you don’t supplement your body will lower your metabolism and start recruiting other source of energy (protein).

      You could avoid that by supplementing with 34.6g of fat a day

  • Richard does this in fact mean calorie deficits work for losing fat as long as you can mobilize your body fat? I realize this equation is based on maximum amount of fat that can be burned.

    • I believe that is true. Insulin inhibits healthy adipose tissue from releasing stored energy, some people say this is it’s most important role. If you can get your basal insulin down then when you caloricly restrict, your adipose tissue supplements and replaces dietary sources of energy.

      If your basal insulin is high enough to inhibit lipolysis even without any stimulation, then that side of the metabolic ledger has only a shrinking dietary intake, and the other side has all the things we want to do when we have energy PLUS all the things we HAVE to do to survive. What ends up happening if you can’t get insulin low enough is your metabolic rate slows as you homeostasis puts off all the things it wants to do when energy is abundant.

      This was seen by Kevin Hall in his 6 year follow up of the Biggest loser competitors, they maintained their exercise, they also maintained their high protein low calorie diet (though they presumably relaxed a little once the cameras were off) and they all gained most of their weight back and their HOMA:IR (indicating insulin resistance) scores at the end of 6 years were WORSE than when they entered the competition.

  • Madeleine
    7 years ago

    Richard, I just finished a 5-day water (and electrolyte) fast. I’m 5’4″ and about 130 lbs, and according to the calculator I would have needed 110g of fat to supplement but I decided to just see how I felt as I went along. It was a terrific 5 days except for one night of leg cramps, and I added in 3 Bikram classes, which are pretty intense, with lots of walking and milder exercise on the other days. No cold hands or feet, steady clear energy the whole time (I’m still hoping for the fasting euphoria to kick in when I do more fasts 🙂 but basically I felt pretty good). I’m trying to square my experience with your post here. I know that there are lots of individual variations (I’m probably much more fat adapted than those poor Minnesota Starvation fellows, I was feasting beforehand, I’m fairly insulin sensitive, maybe my particular genes… I don’t know) but I wonder also if the results of the experiment only make sense – or primarily make sense – in light of calorie restriction rather than all-out fasting, which seems to be a very different physiological/hormonal state.

    I know that you probably have your hands full with KetoFest prep (so great to see that happening! I can’t go but I’m following the updates happily) and I don’t even know if you check these comments anymore – but if there’s a way to pick this up in the future I’d be eager to know your thoughts.

    Maybe a corollary question is: if fasting is easy, can we assume that we’re not slowing down our metabolism? I’d love to continue 3-5 day fasts every month but I don’t want to slow down my metabolism or do any harm….

    • Yeah we’re “all hands on deck” for Ketofest right now … but briefly it occurs to me that the flaw in this Alpert study is that the population was made up of all young white men, also with very similar cultural norms (mostly from christian sects that are anti-war) and they all were fed a high carb diet (cos it was Ancel Keyes) … and then extrapolated that to assert that was true for all humans. Unfortunately Seymore Alpert died in 2008 from surgical complications so I can’t ask him about extrapolating his math to the rest of the population.

      There has to be a limit because of how body fat releases it’s energy, I’ve also seen enough people experience the energy shortfall, and get relief with a small amount of energy incoming (dietary fat) – so I know for some people the numbers that Alpert gives are in the ballpark … but I am always looking for data from other “starvation” studies to do similar math to, to see if the theory holds up.

      Your experience might indicate that there may be more human variation (which would not surprise me).

      • Madeleine
        7 years ago

        Thank you, Richard!

        Yes, lots of considerations: the study was restricted-calorie and probably LFHC, drew from a narrow demographic slice, and probably triggered a very different psychological and therefore hormonal state for the participants. I would think that fasting as an enthusiastic choice rather than as a mandatory constraint must put us into a profoundly different hormonal state – cortisol et al…

        Anyway, interesting stuff. I’ll continue with 3-5 day fasts and see how they go.

        Happy KetoFest prep!

  • Kathryn
    7 years ago

    Well this calculator sure explains why I just finished a 48 hr fast, in July, in South Florida, with temps in the 80-90 range; ready to inflict bodily harm if anyone tried to take my sweat shirt or pants away from me. I was freezing the entire time!
    According to the calculator I should have added 53g of fat/day to the fast. You can be sure my coffee will have fat added next time. Thanks for the info.

  • Deb Griffith
    7 years ago

    I’ve been using this formula for weeks now, and I think it is a vital tool that many people need to use but don’t know about. I have passed it on as much as possible.

    One thing I would change, however, is that your friend’s BMR, at 135 lbs, with 110 lb LBM, would not be 1500 kcals, most likely. I am 55, am now down to 136, am 5’6″ and my BMR is 1228. Even if she is 35 yrs old, it would only be 1326.

    And to make up the metabolic deficit, if one learns how to be more creative than just guzzling fat, and adds good carbs from veggies, moderate proteins, then the deficit is easily made up when not fasting, and when fasting, we learn to breeze right through it.

    But so glad I found your article. This is such an important issue.

  • I’m 80kg with around 15% body fat, but I’m doing long fasts with no energy problems. The calculator says I should be 1150 kCal short (dying) every day but I’m doing just fine. I think that the energy limit must be much higher than that and many other variables must be the problem with people that can’t handle the fast well.

    I’m not saying it’s completely wrong, and I understand that they are based only in one study, but I think that those numbers must be way off.

    • You could be right, but if the Alpert number is accurate there may be another explanation.

      I know some people who are able to lower their metabolic rate to 800 kCal, and of course you body can always make up the arrears by burning lean tissue for energy – which is like the last thing you want to do with amino acids.

  • Melanie
    6 years ago

    I’m 123lbs, lean body mass 50, fat 73lbs (59%) and metabolic rate is 600 caps??? I’m guessing that means my metabolism is very slow?? Can you please explain it to me??

    • Ouch. I would guess that you have been calorie restricting for several decades to get your metabolic rate so low. It should be up around 1600 for a woman, 2000 for a man. Mine is 2700 kCal/day. I can still lose weight at that rate, by simply eating food that doesn’t result in a large amount of insulin secretion (carbs).

      It’s really fighting a losing battle to calorie restrict. Eventually you HAVE to eat more than your metabolic rate and you gain weight. Plus to get your rate that low you would have been using lean tissue as a supplement for energy. So when you put the weight back on, you will end up with more body fat and less lean tissue than when you started.

      The only way to not have this NOT happen is to NOT restrict calories.

      I would suggest going as ketogenic as you can; bare minimum of carbohydrates, certainly under 20. Protein you should keep to the minimum you need to support your lean mass. For me that is around 1g/kg of lean mass, but everyone is different. If you have access to a scale with impedance measurement you can use that to determine the minimum protein that you can still maintain your lean tissue. And in your particular case I would eat quite a lot of fat. You have been good at calorie restricting so you are good at tracking … for a couple of weeks aim to eat 1600 kCal/day. You may gain a little weight, because on the first day you only need 600 and you will be eating 1600. But that will cause your body to relax it’s constraint on your MBR and on the following day your body should be able to use a little more.

      I don’t know how long it will take to get back to a normal metabolic rate, it could be weeks. Once you do you can decrease trhe fat and increase the protein if you want to eat a more normal diet. But keep the lid on carbs, they take calories off the table and put them on your thighs and your body can’t use them when it is calculating how much energy it can budget for.

    • Can the calculator really tell if your metabolic rate has slowed like this, just based on your lean body mass vs fat mass? I don’t see how it can do that unless the metabolic rate is solely based on amount of lean mass. But we know it is based on more than that.

      I do agree with Richard’s response though. My metabolism dropped on calorie restriction and increased on periodic fasting. (Same number of calories but more on eat days and less/none on fast days.)

  • Sorry to be late to the party, Richard.

    Ok, so I ran my numbers through your calculator: weight 168lbs; lean body weight 144lbs; MBR ~1980 C/day.

    So, by fasting I am running a daily deficit of about 1224 C and would therefore require 136g fat/day.

    Ach du lieber Gott! What is that? 5 or 6 BPCs? 1/3 pound of mascarpone or cream cheese? Oy gevalt … how can that be fasting?

    Maybe I shouldn’t be fasting anymore?

    This is food for thought, though. Thank you. 🙂

    • Yikes, you have 24lbs of body fat * 31.5 kCal/lb = 756 kCal.

      Remember that’s not how much you’re eating, with your body fat supplementing that. That is all you are getting.

      Can your metabolic rate drop by that much? Probably not easily. In which case the arrears will come from burning lean mass for energy … which is like the last thing you want to do with amino acids.

      I wouldn’t fast in that circumstances.

  • I’m confused about how this fits with someone trying to lose weight, where you need a calorie reduction below your burn rate.

    • If you are fasting then you are drawing energy from storage as fast as your body fat can supply. That’s how you lose body fat, which is the ideal way to lose weight.

      If you need more energy than your body fat can deliver, then you will still lose body fat at the fastest rate you can, but you will additionally slow down your metabolic rate and you will lose a little lean mas to make up the arrears.

  • LeOnnie Brathwaite
    6 years ago

    Richard, I am so confused. I weight 150lb, lean body mass is 108, 28% body fat. Attempted a 3 day water only fast but had a green smoothie before work on day 2 as I noticed I felt like I may faint after work on day one of only water. Day 3 of water only and limited activities for over 24 hrs I experienced Headaches, lightheadedness, cold hands and feet was so tired even after sleeping 9 hours so broke my fast with beet juice, green probiotic, apple. What could I have done differently? Or is this kind of fast just not going to work for me?

    • You have 42 lbs of bodyfat (150-108), so the maximum rate that you can generate energy from body fat is (31.5 kCal/lb * 42 lbs) 1323 kCal. That’s not a lot. It really depends how many calories you burn. If you are weight stable then the number of calories you eat to remain so would be a fair estimate. Let’s say you normally eat 1800 kCal/day and are weight stable – you will have a deficit of 477 kCal. That explains the cold extremities. Heating is an optional use of energy.

      You could supplement 477 kCal with 53g of fat/day, and then you might notice fewer symptoms.

  • marty kendall
    6 years ago

    Can you explain the basis of “How much dietary fat you would have to eat to prevent slowdown.” Couldn’t this energy be the amount of protein you need to maintain LBM based on your level of activity or even the protein/fat/carbs you need to get your micros?

    • The point of fasting is not calorie restriction. It may be a side effect, but depending on the rate your body fat can deliver energy even that is not necessarily the case. The point of fasting is nutrient signalling.

      Jason Fung explained during one of the obesity codes the difference using a reductio ad absudum.

      Imagine 2 identical twins, let’s call them Adam and Barry both with 40 lbs of body fat. Each is measured and found to burn 2400 kCal/day. Each eats 1200 kCal, both appear to be at a 1200 kCal deficit – let’s assume the eat the same macro nutrient ratio 10 fat:25 protein: 65 carbs.

      Adam eats all 1200 kCal of his daily meal at midnight. His body responds by switching him to a fed state, his insulin goes up, as it does his fat cells switch from releasing energy to storing energy, his energy consuming cells switch from transporting fat into his mitochondria to be burned via beta oxidation, to oxidizing glucose through aerobic glycolysis. Eventually after 2 hours his insulin has returned to it’s nadir, and his fat cells now release free fatty acids into circulation, and his energic consumers switch back to shuttling fatty acids
      into the mitochondria to be burned.

      Barry eats his allotted meal over the entire day, so every hour on the hour he eats 50 kCal. His body responds by switching him to a fed state, his insulin goes up switching him from fasted to fed state with the same result as Adams … but the difference here is that before Barry’s insulin has had a chance to return to it’s fasted state … his next meal is already on him and he is back to a fed state again … and he stays there all day.

      The difference between the two is that in the 22 hours AFTER his one meal, Adam has access to as much energy as his adipose tissue can release in that time (31.5 kCal/lb/day * 40lbs = 1260 kCal). So over the 24 hours Adam has been able to budget for 1200+1260=2460 kCal of energic consumption – That’s a normal day for Adam and he’s just lost roughly 1/3 lb of body fat.

      Over the same period Barry has had 1200 kCal from food to spend on his budgeted activities, and zero contribution from body fat – because his insulin was kept aloft like a beach ball at the cricket. So Barry has to make up a shortfall of 1200 kCal from budgetary cuts (reduction in MBR) and from alternate sources of energy (glucogenic Amino acids). So Barry has not only not lost any body fat, he may have lost lean tissue sacrificed to supply the short fall AND his metabolic rate has been kneecapped.

      Adam fasted. Barry caloricly restricted.

      If Adam’s meal was just 300g of protein, then his insulin response would have been less but the remaining 22 hours would be identical. Barry’s would have still been a disaster, he would have still lost lean mass, and had his metabolic rate kneecapped. If the meals had just been fat for energy then both of them would have comfortably fasted.

      Obviously the intended nutrient signalling for fasting (lower insulin, less MTOR, more glucagon mediated autophagy) is affected if you eat protein, or a combination of protein and carbs in order to supply essential nutrition from food. A fast for a few days likely won’t deplete stores of essential nutrients, or the labile store of protein. Besides for the first few days we can scavenge urea and ammonia and manufacture non-essential amino acids. For a longer term fast, that may be an instance where like Angus Barbieri, essential micro nutrition from a multivitamin might be a reasonable strategy.

  • I’m wondering if the assumed fat conversion numbers are accurate. According to these numbers I should be eating 1000 cals a day of fat. Yet, here I am on day 3 of my second 5 day fast this month. Just as happy as a tick on a dog. True I’m not moving or exercising much but I am moving around. I love this concept though and it explains why my wife at 110 lbs goes bonkers if she isn’t fed. I always thought it was the insulin. I think also that with IF fasting for the last couple of months that my system has gotten really, really good at converting fat to energy.

    And a couple of other things — First there is this disclaimer from the college student author. “This value disagrees with the results of other observers who have measured metabolic rates of diverse groups. The disagreement is explained in terms of individual metabolic properties as opposed to those of the larger population.”

    Second — Keys was a charlatan and his study was not of fasting men but under fed men. The hormonal processes change significantly for those of us on keto who fast. If I was in deficit (not converting enough of my fat to energy) I imagine that like the skinny girls I would be starving. Fact is I am not. I am on cruising altitude with no wind and an expected arrival time of Friday..

    • Seymore Alpert wasn’t a college student, he was a professor, received his PhD in experimental physics. Sadly he died in 2014 from complications of surgery .


      I’m with you on Keys. He was a weasel. But he didn’t know the uses his data would be put to so he couldn’t influence this study from beyond the grave.

      What Alpert did was assume that a human body would rather use body fat for energy, than to cannibalise lean tissue. What he needed was not starving humans to study, but humans under various degrees of caloric restriction where their total weight both dry and immersed was measure regularly. He was looking for the contribution to their energy from adipose and from lean tissue. It was just maths, but clever use of Keys data.

      It could be wrong for many reasons, Keys may have fiddled with his data in ways we can’t subsequently rule out, the human body may be stupid and always dip into lean tissue as an energy source even when it has energy stored in adipose, the hormonal milleau of the subjects may conspire to change energy release from adipose and lean tissue (although the results were not as diverse as you might expect if that were the case). And I could have got my math wrong too 🙂

  • I’m new here and wanted to say thanks for this explanation of why fasting can be so hard for some of us. The widget is amazing! I’m referring people to it.

    I’m vaguely aware that the metabolism slows when a calorie deficiency exists but not in the complete absence of calories.

    Could you please elaborate on this for me? I know the podcast has mentioned it and I’d like to read the relevant study/studies.

    • Is there ever a complete absence of calories, in a body with energy stored in body fat? Not eating at all can be a restriction of calories, not an absence. The Alpert study proposes a maximal rate that body fat can contribute energy. If you have sufficient body fat, not eating may not be a caloric restriction at all, because you can completely replace the calories from food with calories from food that you ate earlier and stored.

      There are a lot of studies showing how we change our RMR based on our energy balance. Here is one


      I disagree with their assumption that this is the body defending a set weight. Studies that maintained caloric restriction for years and saw inevitable weight regain did not see a regain in metabolic rate.

  • Primal123
    6 years ago

    Richard interested in your statement that average woman’s bmr is in the 1400’s. I’m using chronometer at it lists my bmr as 1890. I’m 52,180 lbs, I calculate my lbm at around 125. So if my bmr is 1890 I need to add a little fat daily to my fasts. If as you say my bmr should be 1400ish, I don’t need the added fat. Is chronometer completely off base?

    • I don’t use Chronometer myself, but I suspect it is probably calculating how much energy you are eating minus how much exercise you are doing, applying a factor based on whether you are gaining or losing or maintaining weight. I think that’s going to be a pretty reliable way to calculate total daily energy expenditure which is what the calculation works on. If those were my numbers and if I was starting to feel a metabolic slowdown (cold extremities, sluggishness, lethargy) I would try and get an additional 17.5g of isolated fat (like olive oil, or coconut oil) and see if that symptom went away. That will also prevent your body seeking out alternate sources of energy – such as recruiting lean tissue to waste for energy.

      I am frustrated with blogs that don’t reference where they got their data from … and when I went looking for where I got those average US BMR rates from .. I realized I didn’t cite and now I can’t find it. I’ll try to find better data. I should have used average daily metabolic rate not basal or resting rates too. Sorry for the confusion.

  • I wasn’t sure about this because I was doing fine with longer fasts even when the calculator told me I shouldn’t be. I have been growing muscle, have not been tired. But now that I am down to around 26# of fat, I am finding myself hungrier on fast days than I was before. Is hunger also a symptom one would notice if they have hit the limit of how much energy they can get from their fat? And is this something one should worry about if only fasting 24 or 36 hours?

  • Is Metabolic Rate the same as Resting Metabolic Rate? My RMR is 1258, but based on my activity level, it’s actually 1950/day. Not sure which I should be inputting for the calculation. Did a five day water fast, and really struggled, had no energy, and really felt like I lost muscle mass. My lean body mass is 89.6lbs, bone mass 5lbs, and fat mass 35.3lbs. Thanks for any guidance you can provide!

  • Ok here is a twist… I’m a middle of the pack 100 mile(in one day) trail runner and am trying to use keto to maximize my performance. I don’t want to win or anything. I would just like to run the whole thing feeling good and not having to each much (because it is difficult to do so).

    Dr. Noakes implies on his askprofnoakes podcasts that the non-elite athletes should be able to do these races on their body fat. But based on this calculator with my 185 lb weight, I’m guessing 150 lean mass, 3000 calorie per day usage, that I would not reasonably be able to get enough energy from my body fat. I would need to eat like 200g of fat for that day. Are we saying that it is easier to use fat ingested as food for exercise energy than body fat?

    I’m severely insulin resistant. Produce too much. My blood sugar is always below 100. Even after big meals. Even if I ate carbs. But I feel weak from the insulin blocking my energy usage.

    Right now, I’m easily able to run for a couple of hours in the morning while fasted and feel fine. But then energy wanes… and once I start eating, nothing seems to give me energy. Carbs, fat, or protien.

    My ketones are routinely 3.5 when I stop my runs. I’ve been low carb for over 3 months.

    Just wondering what a fueling strategy during a race might be. These things last about 30 hours for me. And trust me, you can get an insulin response during one of these races. I’ve eaten a lot of carbs 30 miles in and then was laying on the side of the trail until the insulin wore off.

    This last weekend I tried eating about 900 calories of creme after 2 hours. That was a mistake.

    I can eat biltong, pemmican, or just about anything. I can feel not hungry, satisfied, but I just can’t keep feeling like I have energy to burn. Is the only solution to find a way to eat more fat during the run?

  • tealhill
    6 years ago

    Ancel Keys’ subjects weren’t fasted. Fasting increases norepinephrine levels. And, when there’s sufficient epinephrine in the blood, the 31 kCal*lb per day limit no longer applies.

    For my sources, please see:


  • Tina Emmons
    6 years ago

    To clarify, does “additional fat” mean grams to ingest on fasting days?

  • This is based on Ancel Keyes’ Minnesota Starvation experiment data. Keyes is a notorious hack scientist. You cannot trust anything from him.

Trackbacks & Pings

  • Carmela Diomede :

    Why fasting is easier for some people

    […]But the smartest people are quietly making preparations making no appear occurs, they are going to be okay.[…]

    6 years ago

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