There may be some medical benefits to periodic fasting, especially for people with excess adipose tissue. But in terms of "no harm" the Memel et al paper you linked doesn't seem to mention anything about loss of lean muscle tissue. This can cause serious harm for some patients — especially older patients with the "skinny fat" body type — by leading to sarcopenia (higher musculoskeletal injury risk) and endocrine dysfunction (muscle is a glucose sink). Loss of muscle can be limited to an extent by doing resistance training and maintaining high protein intake during non-fasting periods. But overall there are still a lot of unknowns in this field.
A lot of that would be covered under the “other issues” I mentioned. Obviously, consult with your doctor first.
In terms of muscle loss, from what I’ve read, muscle loss is more of an issue for low calorie carbohydrate based programs. When fasting there might be some muscle loss when at the very start, but then it tends to preserve muscle, and like you said, adequate protein intake and resistance training can mitigate that.
Those, like Peter Attia, who saw muscle loss from long term keto and fasting over several years, weren’t using fasting as a medical intervention. He was already metabolically healthy, but maintained a pretty extreme fasting protocol in an effort to gain longevity benefits, effectively experimenting on himself based on some results of early studies around the benefits of fasting for longevity.
The general theory now seems to be making the fasts much less frequent once healthy. Maybe only a couple times per year.
I’m sure this area of study will continue to evolve slowly.
