Every cell in the body relies on glutamine, yet some tissues use this molecule more heavily than others. Fast-growing cells such as those of the intestinal lining and specific immune cells (thymocytes, lymphocytes and macrophages) use glutamine for energy. If reserves are weakened, the body will begin to break down protein tissue (which contains most glutamine). A lack of glutamine will lead to muscle degradation, a "leaky" gut, and weakened immunity.