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Comment by lottin | Hacker News Reader

I plan to do a 3-day fast twice a year from now on, already did the first one in June. The main problem for me is that these long fasts take a significant toll on your muscles. After the first one, I noticed that I had lost strength and endurance. Basically you'll have to spend a few weeks training only to get to the level you were at before the fast.

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This is the main reason why I don't and won't fast. The author claims that food is not actually a daily need, whereas I know for a fact that I will injure myself cycling or climbing if I skip even a single large meal in a day. It's a noticeable difference.

I do maintain a workout schedule but not as intense as yourself. Sounds like you're running pretty close to the edge of all your limits if you can't skip even a single meal.

or alternatively, addicted to something in his regular intake; An opiate addict will also not function as well without their regular dose.

(This is the first account I hear from someone who will get injured from missing a meal. That said, if you have to be addicted to something while keeping in top shape, food sounds like a better deal than opiates).

I didn't mean to exaggerate that badly but climbing is a sport where your ups and downs are very noticeable. If I don't have a solid lunch and climb in the evening, chances are I will get injured trying to do limit moves. I try to ensure I stabilise my diet and not risk that aspect, just no point for me to skip any.

Agreed. If you do any intensive training for endurance sport, the math just does not add up. Even with a very efficient fat-burning metabolism (that I don’t have), there’s only so much fat on your body and muscle mass will be affected. Yes, I do do metabolism-focused sessions of long uniform low-intensity effort without prior eating to get my body used to burning fat. But it takes a lot of time for it to learn it

Were you exercising on these 3 days?

My experience (from longer fasts - never felt anything like that after 3 days) was that muscles that aren't in heavy use do indeed get metabolized for energy (which makes a lot of sense - maintaining muscles costs energy; if you're not using them and running on fat reserves, why keep them?)

However, if I exercise while fasting, I hardly lose any muscle at all.

One interesting thing is that unaerobic exercise, such as boxing or rope skipping becomes harder for me while fasted; but aerobic exercise, specifically, running - becomes easier and it feels like I can keep running forever (whereas unfasted I usually feel my ability degrading near the end of a 10km run)

Yes, when the body is in a caloric deficit in enters a catabolic state where it will consume existing tissue, i.e. fat and muscle, for energy. For me it was definitely noticeable after just 3 days of not eating. Exercising might reduce the loss of muscle mass to some extent, but most people will still lose some or a lot.

> Exercising might reduce the loss of muscle mass to some extent

That does indeed seem to be the case, at least for a 16:8 intermittent fasting regimen (an 8 hour eating window with 16 hours of fasting): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5064803/

"Our results suggest that an intermittent fasting program in which all calories are consumed in an 8-h window each day, in conjunction with resistance training, could improve some health-related biomarkers, decrease fat mass, and maintain muscle mass in resistance-trained males."

I think this has more to do with your body switching fuel sources than actual loss in strength or endurance.

No, there was an actual, measurable loss. I noticed, for instance, that I could do fewer push-ups. That was after I had resumed eating. During the fast I wasn't exercising at all.

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