Leucine

Leucine breaks a fast. Not only does it carry calories, but it also warrants more caution than other nutrients and amino acids in a fasting context because it is the single most powerful dietary activator of mTOR, the growth pathway that signals the body to build rather than recycle.

When mTOR is active, it directly suppresses autophagy, the cell's repair and recycling process. In human studies, even a few grams of leucine raise the markers of mTOR activity and lower the markers of autophagy, which makes leucine one of the most effective ways to switch off the autophagy that a fast is often undertaken to promote.

How much this matters depends on the goal and the amount. Amino acids are not equal here: leucine acts strongly on this pathway, while other amino acids taken in small amounts have little such effect, so not every amino acid affects a fast the way leucine does. For a fast aimed at autophagy, leucine is the clearest thing to avoid; for a fast aimed only at fat-burning the picture is softer, since leucine can be converted directly into ketones and in isolation has little effect on ketosis. For practical purposes, leucine belongs to the eating window, and its effect on autophagy is the main reason it matters when deciding what breaks a fast.

Source: Glynn et al., leucine, mTOR and autophagy signaling in human muscle, J Nutr (2010)