A several-thousand year-old diet is not a fad. This is the ancient expression of fasting, not a modern brand like intermittent fasting.

These same ancient diet experts did things like bloodletting. The ancients aren’t always rational.

Bloodletting is a valid medical procedure for hemochromatosis which presently affects around 1 in 200 of those with European ancestry. It's very possible the incidence rate was greater in the past when populations were more locally homogeneous.

It's exceptionally lazy and unimaginative to dismiss out of hand a procedure practiced globally for thousands of years.

What about divination? Water wishing? Human sacrifice? All practice for a long time to no effect. People are highly superstitious and rarely logical.

calling it "ancient" and putting it in italics doesn't change the reality

Ancient is what it is and fad is what it is not. That's the reality.