[rsync]

Everybody is different, and my anecdote should have very little weight attached to it, but ...

I do an 18 hour fast once per week, which is a very modest fast. On the other hand, I have a very, very comprehensive and intense exercise regimen, so my resting metabolic rate is quite high, and 18 hours with zero calories clears my body out very thoroughly.

And I notice it greatly - it feels fantastic. I feel very quick and light on my feet. It's noticeable.

Am I improving my immune system ? Who knows. However, it is definitely less than a 3 day fast, and it definitely improves my health.