i agree with lulu. i just completed a 29 day fast. as my normal cycle is 25 days, i knew it would come up. i searched the internet and found very little information about periods and fasting. well- they had a lot about ramadan, periods and fasting, but that wasn't what i was looking for!
i disagree with chris on this one. i do think it is natural. given that, it is well known that percentage of body fat affects menstruation. anorexic girls will stop menstruating when they get below a certain %age of fat. if you are of normal weight when you start, fast until you fall below, say 10% body fat, then i would predict you would stop menstruating. if you are overweight, and do not fast until you fall below that %age, you will most likely continue on with normal menstruation. BUT there are many other factors that could be at work in each individual.
let me give my own anecdotal evidence. during my fast, i started my period almost 4 days early, which threw me off. because i started so early, i was due to have it again the day after i stopped fasting. my cycle is short- 25 days. wondered what would happen as i continued fasting- would it stop? would it be early again? would it be later?
it was exactly on time 25 days after the previous. i was happy to note that, as i was beginning to worry about perimenopause. it had been getting irregular the last few years as i got less and less healthy.
good luck with it, and as a woman i am quite curious to see what happens for you.