An man of average size will lose about 1500 calories of body weight each day. 1500 calories is equal to about 1 lb of body weight, biking for about 3 hours, or running 4 hours. I've noticed that fasting is alot like extreme training except you don't get winded. Fasting makes a person tired in a similar way as hard work. Fasted muscles may be a little slow and crampy for this reason. It is alot like the after effects of exercise. These observations probably explain why is fasting better than exercise from a longevity perspective, although you really need both exercise and fasting.
Fasting gives a similar effect as exercise, but without the high oxygen exposure. It should be possible to better isolate the beneficial effects by thinking about the comparison of fasting and exercise. In this round of fasting I lost weight faster than in previous rounds, which can only be due to improved conditioning over the past years. Endurance training improves the capacity to store energy.
After fasting two days, the next morning I got my best running time ever. It would actually be a good idea, to get an estimate of improved muscle conditioning from the higher rate of weight loss during fasting. I did a little calculation. The faster time is apparently directly related to the weigh loss, so this result is ambiguous, but there's also an incentive.
The lore says that fasting is easier in the cooler months. Why not get an early start before the holidays. You can fast for peace ;-). After fasting over 3 days I realized that fasting a well-conditioned body really does result in more cramps. I also woke up with minor stinging in the calves. It is easy to conclude that the difference may be a shift in metabolic rate. The cramps went away immediately, when I broke fast. The minor stinging was resolved with a few stretch/flexes. Later that day, I broke my fast. It may be possible to speed the release of energy stores by combining exercise and fasting. The effect of this combination was manageable, and I realized that I might have gone running instead of breaking my fast.
It is not yet clear whether the combined regimen lead to the increased cramping on the 4th day of the fast. Next experiment is whether additional bouts of combined fasting and exercise will eliminate the cramps. The model suggests a recovery period of 2-3 days should be sufficient, but there is also a barrier to initiating a fast. Fasting intervals could be delayed some additional days to reduce any resultant muscle loss. Of course, the muscle loss is easily reversible, and the benefits of fasting are now plainly evident. If you undertake this program, make sure to include supplementation to get the full benefit. Happy fasting!