Does Fasting Cause Muscle Loss?


Hard to believe I am asking the same questions as seven years ago! Loss of Lean Muscle During Fasting, Old Study My old post came up when I started typing.

I would like to do some extended fasts
Goals at 55+

  1. Lose more weight without losing muscle
  2. Build muscle in general (I find heavy groceries, dog food bags are more of a struggle)
  3. Improve my speed on long bike rides, have enough muscle to start skiing again (20 - 30 years or more have barely skied but find biking helps those muscles)
  4. Avoid loose skin if possible

Have been Low Carb on and off since March 2017! Anniversary is next week! I have lost about 30 lbs (lost about 25 before I started Keto from my highest weigh). I could still lose another 50 to get to what I consider a good weight. Probably will never happen but I trudge on. Currently have a very short eating window, 11-5 or 6 depending on the day.

When I first started Keto I mainly lost weight through extended fasts, each time I would lose 2-5 lbs (I measure from the day after I refeed or even two days after). I would maintain with Keto in between.
In 2017 I did a lot of five day fasting. I also got a lot of mild colds, almost monthly starting from September 2017 or so until I started supplementing with various things and eventually I stopped fasting and my weight creeped up a little and then more during Covid.

I rarely fast now although I do not find it difficult.

I have lost about 22 lbs since July when I started reducing my eating window and that seems to be working although I have been on a stall since I hit month 7. I know Fung believes Autophagy will help with everything but others and the old study in my initial post make me ask


Hi Saphire, When I’ve fasted either IF or EF consistently, I lose MM and gain both Strength and endurance. I’ve done it for climbing if that’s a comparison. Isn’t Haaland carnivore?

(Chuck) #3

The key if you don’t want to lose muscle is to eat enough that you don’t go into starvation mode. It is also important to eat enough protein and to stay active and do some resistance training. Your body will use your muscles as fuel if you eat to few calories for and extended time. I personally believe in intermittent fasting but not extended fasting. I don’t fast for longer than 18 hours anymore.

(Joey) #4

@Saphire Short answer: as @cvkemp notes above, yes.

Fasting beyond intermittent spacing of meals (to 1 or 2 a day) can be unhelpful in maintaining (no less building) muscle mass. Getting adequate protein and working out will maintain/build muscle mass and strength. (Cardio is important too, but that’s more about endurance.)

The other leading cause of muscle loss is a cable subscription. :vulcan_salute:

(Bob M) #5

And heart disease. Malcolm Kendrick has a nice chart in one of his books comparing the number of TVs (in Britain?) with heart disease. There’s a near perfect match. Of course, giving someone a TV does not CAUSE heart disease, which is what his point was.

(Robin) #6

Best last line!

(Bob M) #7

Here’s an intermittent fasting study (8 hour eating window versus normal eating, healthy, young males):

No differences in strength or muscle mass.

Another study, this one a review of other studies:

At least for intermittent fasting with lifting, it seems you can do well.

A 10 day fast doesn’t appear that detrimental:


YES! Me and no shortage of others have DEXA’s that show it, Fung has no business talking about muscle, as he has none. His stupid book is what got me so into Fasting, my RMR was trashed as well from it. Keep in mind, that’s as somebody that works out 5-6 days a week and has a physical job and eats high protein, didn’t save me!


Thank you. It is my RMR that I am most worried about. Up until a few years ago I tend to put on muscle easily. I am back to my stall weight (which is both good and bad, this is the weight I get to on most diets, WOEs, whatever you want to call it.) Without a major reason I have never been able to get below this weight since my second child was born (they are a working adult.) So its good in the sense that this has been as low as I get in 20+ years but frustrating too. One thing, I do not look as thin as I did in 2017 when I was at this weight and I have been wondering why. I am actually exercising more than I did back then. Mostly tennis and cardio but a small amount of circuit training. Hate anything medical so would prefer not to get a Dexa scan but may consider it if I can get one without a prescription


It can. Extended-day fasts may be good for the morbidly obese on a short-term basis, but not long-term. At 55 and older, it becomes harder to build muscle. Dr. Fung only recommends extended-day fasts for the worst of his patients. Fung does not fast himself. Newer research is showing that when doing IF it is best to shift you timing. Have you first meal within 30 minutes of waking up. Fung’s normal recommendation is IF 2-3 times per week. Dr. Peter Attia (50 years old) stopped all of his extended-day fasting because he discovered that he had lost approximately 10 pounds of muscle mass over a 3-year period. Keep in mind he also works out most days. His decision was based on the knowledge that as we get older, it becomes harder to gain muscle. He believes the risk is too great.

(Robin) #11

I can relate to hitting what seems to be the weight your body prefers. Mine was 180. Having been 250 at my highest, 180 was great. I felt and looked good.

So when I began keto, I had my sights set at 180. I got there and happily “stalled” for a few months.

3 years into this journey (I quite naturally became carnivore after a year) I have been holding steady at 135-138…. With zero effort.

If someone had said it would take 3 years to get here, it might have sounded like a long time. But slow and steady wins the race…
to cruse control and maintenance.


Don’t need a prescription, only need a doctor involved if you’re going to attempt to not pay for it, and unless your doc wants to go on the books that he’s worried about Osteoporosis, it’s not happening. You can try to see if anybody in your area is registered with them, but alternative health places, high end gyms, personal training and performance places typically have them. Some Universities will scan you as well. Literally takes like 10mins and you’re out the door with WAY more info than you’ll ever get otherwise, and a REAL BF% on yourself. They’re cheaper in packages which are pretty popular, but average $65-$99 most of the time if you go with a single scan.

More muscle = higher RMR, always a good thing! I’d definitly get one at least yearly, and with your activities listed of Tennis, Cadrio and Circuit training, I read Cardio, Cardio, and Cardio. Cardio is great in it’s own way, and burns a ton of calories, but that all comes to a screeching halt the second you stop doing it, and cardio is anti muscle, and that is anti metabolic rate.

If you’ve never seen what you get out of them, here’s my (almost) most current one, can’t find that one but this still has two of them on there, so you can see how awesome it is to compare to previous ones whether it be muscle gain, fat loss, or hopefully not, bone density loss (or gain!).

dexa_public_052723_2.pdf (855.3 KB)


Thank you but after 7 years I doubt I am suddenly going to drop 50 lbs! It would be nice

Thank you Bob for the studies. I finally was able to really read them. It sounds like eating windows have benefits and don’t impact muscles in a negative way. The last study was interesting although I thought it was bizarre that they gave them 250 calories of carbs daily. Curious how it would have gone without that

(Bacon is a many-splendoured thing) #14
  1. A ketogenic diet should be able to do this. In fact, over the years, study participants often add lean mass while shedding excess fat. The key is a truly low-carb diet (carb intake under 20 g/day) and ad libitum eating, since restricting calories tends to inhibit fat loss (for a number of reasons).

  2. Building muscle requires specific types of excercise. Weight lifting to failure is what is needed, because muscle growth and strength is a response to stressing the muscle. The actual weight being used is not as important. Bikman says that 25-30 reps at a lower weight are just as effective as 8-10 reps at a higher weight, provided that you continue to lift until you can’t do another rep. Sean O’Mara highly recommends sprints as a way to eliminate visceral fat and improve overall fitness.

  3. Can’t help here, but surely just getting out and doing it will help.

  4. Loose skin is the result of losing fat too fast, so a slower rate of loss will help keep your skin tight. Jason Fung used to have before and after pictures of a guy who tightened up a great deal of loose skin, simply by fasting properly.

Phinney’s big concern with fasting is that the data show a certain irreducible loss of lean mass each day. However, he feels that two or three days of fasting should be fine. Fung, who is often assumed to be Phinney’s opponent on the issue of fasting, doesn’t recommend fasting beyond three days at a time. So they agree on that much.

(Bacon is a many-splendoured thing) #15

You can get them without a prescription at walk-in imaging centres in many U.S. states. If your state doesn’t allow it, perhaps there is a centre not too far away in a neighbouring state.