Fasting spikes stress hormones and lean tissue catabolism. It's clearly a very stressful thing for the body. Small doses of stress have hormetic benefits but it's easy to go too far.

Vigourous exercise is stressful, too. A bit is good for you, but a lot is clearly bad. High energy expenditure athletes prematurely age and have elevated death rates. I'm sure it's the same with fasting. I would be careful about doing "a lot" of fasts.