[eupharis]

Something I didn't know:

"A very small part of your energy comes from breaking down your muscles — but you can avoid this by doing some resistance training, otherwise known as pumping iron. The majority of your energy comes from breaking down fat."

I thought fasting would always cause your muscles to weaken. But this makes more sense for evolution. If your muscles are being used, the body takes energy from all other sources first. The preserved muscle could make all the difference between eating and not eating.

[vidarh]

You will definitively feel weaker and be unable to work out as hard. Even a days worth of poorly timed meals and/or too little protein can mess my weight lifting up for the following day, and fasting quickly forces my lifting volume down substantially (though amino acids from your protein intake can stay in your blood stream for a few days, so if you've kept protein intake high, you can keep things normal through a couple of days of fasting).

But the main predictor of how quickly you'll recover your strength afterwards will be whether or not you keep up resistance exercise. After a weak of lazing about during vacation, it can take me 2-3 weeks of hard exercise to be back at my peak. After eating too little or fasting for a short while, it's mostly just a matter of getting well fed enough again. I don't have experience with longer fasts, so I can't say how much they'd affect me, but it does take time to trigger substantial muscle breakdown, and even if you do lose muscle, getting back up to the same strength is much quicker.