It is not quackery. It has been proven over and over and over that fasting is good for your health. Even longer fasts (10, 20, 40 days or longer).

Check these out for starters

https://www.richroll.com/podcast/alan-goldhamer-541/

https://www.amazon.com/Rational-Fasting-Arnold-Ehret/dp/1884...

https://www.diabetes.co.uk/blog/2018/02/story-angus-barbieri....

Even religions agree on the benefits of fasting. Muslims, Hindus, Jains... all do fasting.

Fasting gets bad rep because people do it wrong, especially breaking the fast.