A lot of things are connected. Lactose intolerant is just a certain enzyme missing, but i think a lot of diseases are caused by stress and bad nutrition which can create inflammation in several areas of the body. Eg the immune system.
For example I am reading a bit up on histamine levels. It can be influenced bad diet and it will influence certain hormonal levels in the brain. High levels of histamine can cause anxiety disorders and depression, but also other more "physical" symptoms like allergies.