Fasting reduces sebum production, a compound vital to keep skin healthy but which also has the drawback of producing acne for a lot of us.
Edit: Sources“Some studies found a strong relationship between a fasting type diet and acne in human adults and young subjects. In fact, during the period of caloric restriction, sebum level was found to be reduced by 40%, which influenced the degree of acne severity [64,65]”
Fasting and Its Impact on Skin Anatomy, Physiology, and Physiopathology: A Comprehensive Review of the Literaturehttps://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC6413166/
I moved to a new state and my skin freaked out and I then had bad acne for about two years—nothing helped. Then I did a four-day fast and my skin completely cleared and stayed cleared for six months!! Obvious ymmv but it was the best unexpected side effect!