This might all be a scam pushed by David Sinclair and his desire to spin up resveratrol companies. Apparently reducing insulin by reducing meal frequency or fasting does help reduce obesity, see "The Obesity Code". As far as the other magical benefits of fasting and cold showers and other "stressors" - specifically autophagy and neurogenesis - none of it is proven and can be considered a trend.

I do water-fast 3 days in a row once a month, and it does basically nothing for me (adversely or positively). This is very much a trend in my life - trying all these things to achieve pretty much nothing. I guess it's good to know with certainty though.