Short fasting has been shown to have positive health effects when done right. Even short fasting can be dangerous and should absolutely not be done without prior consultation of a medical professional.
That being said 'clesnding' has been disproven scientifically so many times, I think there must be a Necromancer for bad nutritional myths out there. I just can't understand why this myth (together with others) always comes back from the grave.
Regarding vegans. Yes a primarily plant-based diet has been shown in studies to be beneficial to health. Depending on the environment of the test subjects. For practical purposes in western/northern societies that holds true.
The important word is plant based. Because without either taking extreme care of the (especially) micro nutrients as a vegan you either need to substitute or be extra cautious. And of omnivores with a primarily plant based diet also look at their nutritional balance, caloric intake and such, the difference between diets in the studied groups vanished.
So one could say if eating primarily plant based and being mindful of our diet and nutritional needs, there is no difference scientifically.
The only remaining difference is one of personal values and believes. The question of how (or if) do we want to have animals treated to firm our diet. Personally I would add to look at how do we want the humans in the food production chain treated as well and how do we want our diets to have an ecological impact.
But as said - these are existential because deeply personal value questions that can't be scientifically discussed.