These kinds of claims when made about fasting are IME usually talking about autophagy. https://www.niddk.nih.gov/news/archive/2013/working-out-heal... is the least shady synopsis I could find. That mainly talks about exercise - for fasting, people seem to be most interested in the mTOR pathway. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3266408/figure/...

I'm not qualified to judge the literature as a whole, and it appears to be an open question whether this can be translated to actionable advice, but we're not in complete bullshit territory here.