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

32 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).

  • Lauren
    1 year ago

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

    • richard
      1 year 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
    1 year ago

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

  • Jay Morris
    1 year 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
            1 year 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
    11 months 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?

  • 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
    6 months 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
        5 months 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!

  • 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.

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