BCAAs

BCAAs break a fast. Branched-chain amino acids, leucine, isoleucine, and valine, are protein building blocks, which means they carry calories, so taking them as a supplement ends a strict fast.

BCAAs are often assumed not to break a fast because they contain no carbohydrate and are widely used as a supplement taken during workouts. The more relevant point for fasting is that branched-chain amino acids are among the strongest dietary triggers of insulin release, leucine especially. That rise in insulin moves the body out of the fasted state, even though no carbohydrate is involved.

Leucine is a direct activator of mTOR, the growth pathway that holds autophagy in check. Autophagy is the body's process for clearing and recycling damaged cell components, and modest amounts of BCAAs work against reaching it. For this reason, they should be kept out of a clean or autophagy-focused fast. People doing a looser fast for appetite control or calorie reduction sometimes take them around training and accept that the fast is broken during that window. BCAAs are best kept to the eating window rather than taken during a true fast.

Source: Branched-chain amino acids, insulin secretion, and mTORC1 (Frontiers in Nutrition, 2021)