The intestinal lining is a living interface between the contents of the gut and the rest of the body. Its cells, mucus, immune cells and microbial environment work together to allow useful exchange while helping limit unwanted entry.
Barrier function is not a simple on/off switch. It can be affected by infection, inflammation, medicines, nutrition and other health factors. A symptom by itself cannot show whether the barrier is impaired.
The immune system also interacts with microbes and food-derived compounds. This relationship is complex and individual; claims that a particular product “repairs” the gut barrier need evidence for that product and outcome.