What duration of fasts are you referring to? I had seen research on Ramadan fasts that indicated that short term fasting did not increase the total amount of cortisol, though the timing shifted.

I was under the impression that short term fasting was metabolically different from term fasting.

My source had been Martin Berkhan's blog. He cites the Ramadan literature in point 8. I'm not well placed to evaluate the papers as I lack expertise.

http://www.leangains.com/2010/10/top-ten-fasting-myths-debun...