do collagen peptides break a fast？

Scientifically, it depends on the goal of the fast.
If a consumer is fasting for strict autophagy (cellular cleanup) and gut rest, then yes, collagen peptides absolutely break the fast. Collagen is pure protein, and amino acids stimulate the mTOR pathway, which immediately halts autophagy. However, if they are fasting strictly for weight loss and blood sugar management, pure peptides cause almost zero insulin response, making them highly acceptable for a “dirty fast.”