Every religion since forever has claimed their religious practices are beneficial. Such claims are not science regardless of the religion and, importantly they aren't science even if the claim turns out to be true.

If you have a properly executed study gathering the data correctly and doing a dispassionate analysis on the effects of fasting as practised by Jainism or any other religion that's worth looking at. The easiest person to fool is always yourself and never is it easier than when looking at something about which you feel particularly strongly such as your religious beliefs and practices. Claims from religious devotees (or former devotees) about fasting or magic underwear, or genital mutilation benefits or miseries should be completely disregarded. Evidence properly gathered, analyzed and capable of being falsified about those same things should be taken with the seriousness it deserves. Religious claims might give one an idea about where to look but should but not be mistaken for any kind of rigor in themselves. Or you end up with truly barbaric, unnecessary medical "treatment" that needs to be consigned to horrific sections of history texts where mercifully at least some of it has now been.

Properly done science is how you know if any given claim is true, false or its truth is still yet unknown and possibly still unknowable.

Fasting is a very, very well preserved meme - across broad geography and many different ethnic and religious groups.

That means something. Regardless of whether or not we understand the science of it, or how we feel about "the messenger" (in this case, religious teachings that we may or may not harbor animosity towards), a very well preserved meme is worthy of our attention.

Is it true ? I don't know. It's interesting, though, and is worthy of our attention.