I’m a normal weight woman (BMI 22) trying to lose the last 10 lbs with an extended fast but I’m scared that not eating protein and not exercising will make me lose lots of lean body mass and end up skinny-fat. I noticeably lost lots of muscle on a previous extended fast (11 days) but seem to have gained it all back after 2 weeks of high protein refeeding. Am I driving my LBM and TDEE down with water fasting as a normal weight person?
How are you measuring this loss and gain? I believe it’s more likely muscle glycogen and its associated water stores that’s deflating and inflating your muscles on a fast. Any actual muscle breakdown is likely beneficial for cleaning out damaged proteins and the rebuild feast after the fast will make you better than ever.
Why are you abstaining from exercise during your fast? That isn’t a requirement as far as I am concerned. I wouldn’t do an extended fast without doing heavier resistance training if one of your key concerns is the preservation of lean muscle mass. Now you won’t be able to grow lean muscle mass during the fast, but it does help send the signal that your muscles are needed and should be given preference.
This Jason Fung article might reassure your worries about losing muscle while fasting:
In a nutshell your body is smart enough to burn fat when it is available. It will also burn useless proteins e.g. loose skin, cysts, scar tissue etc. If no fat is available then it enters starvation mode and starts to burn muscle.
So there is no reason that your LBM would change during fasting whether you exercise or not… your body is smarter than that.
Your muscles may have looked smaller but that was probably caused by decreased glycogen stores and water.
I know what Dr Fung says but here’s my experience:
Yes. You will lose lean mass when going on an extended fast.
Is it bad? No. Not necessarily. Your body will use anything you don’t show it it should keep. I exercised my upper body while fasting and neglected my legs… so I lost leg mass. I lost mass in the digestive organs because I wasn’t eating = no need to keep all that tissue.
Is it permanent? No. The most rejuvenating part of an extended fast comes during the refeed. The body is stimulated to make and use stem cells to make new tissue that is younger metabolically… this is strong healthy tissue that is superior to what you lost.
Will you also lose fat? Yes. That will be the majority of the energy spice, but the body isn’t going to keep old useless cells and continue to feed it all with previous fat. It’s going to throw out anything you don’t signal a need for. So stay active, keep your mind activated… what you don’t use, you will lose.
Fasting is overall healthy - especially compared to calorie restriction where much more of the lean mass will get used up. If you train your body to use fat before fasting, then it will adapt and predominately use fat during the fast also.