Here is an excerpt from a book I just read about repairing a leaky gut:

When the gut barrier is compromised, it results in a condition called
‘intestinal hyperpermeability’, informally known as ‘leaky gut’. In a leaky
gut, these junctions become consistently bigger, allowing the passage
of harmful, larger elements such as toxins, pathogens, and even food
particles to leak into the bloodstream [2]. It’s like having the castle gates
continuously open. Additionally, every time an unwanted particle or
pathogen enters the bloodstream, an immune response is triggered. So,
if the gut continues to be ‘leaky’, the immune system has to be on constant
attack. Thus, when a leaky gut is not healed, it leads to continuous and
body-wide inflammation.
Unfortunately, this is taxing on the body, and can lead to a person
developing a secondary inflammation-related condition or more [3]. It
is not uncommon for people with chronic GI inflammation to develop
autoimmune ailments such as Crohn’s disease, rheumatoid arthritis,
multiple sclerosis, and ulcerative colitis.