Collagen

Collagen breaks a fast. Collagen powder and collagen peptides are protein, which means they contain calories and provide amino acids that trigger a metabolic response, so a true fast ends the moment you take them.

Collagen is often assumed to be fast-safe because it has little effect on blood sugar and is marketed for skin, joints, and hair rather than as a food. But the relevant question for fasting is not blood sugar alone, it is whether the body registers incoming calories and nutrients, and with collagen it does. The protein supplies amino acids that raise insulin modestly and signal mTOR, the growth pathway fasting works to lower, which is the same reason any protein source interrupts a strict fast. Collagen is low in carbohydrate, so like bone broth it will not necessarily push you out of ketosis, but ketosis and the strict fasted state are different questions: the calories and amino acids still register, so the fast still ends.

Whether that matters depends on your goal. On a clean fast, or when the aim is autophagy, collagen is off the table. On a dirty fast focused on appetite control or simple calorie reduction, some people take it and accept the tradeoff. It is a conscious choice rather than a free pass, because even a small amount of protein moves you out of the strict fasted state.