The human intestines have two functions that are partly contrary. The lining needs to be permeable to absorb nutrients from digested foods, but it also needs to protect us by being impermeable to viruses and bad bacteria.
Bad diets, environmental stress and Coeliac disease (to name a few) can cause the protective membrane in the intestine to change or breakdown resulting in leaky gut syndrome.
As the membrane/barrier changes, it can cause an increased absorption of bad bacteria. This can then cause food allergies which, in turn, can cause damage of the mucous lining once again. It’s a vicious circle!