This. The Wahl's protocol https://www.amazon.com/Wahls-Protocol-Autoimmune-Conditions-...

It's very simple. Avoid foods that cause inflammation and this helps reduce MS flare-ups

Diets have been (anecdotally at least) shown to be effective in the treatment of autoimmune diseases like Type 1 diabetes, multiple sclerosis, Chron's disease, and many others. A popular book on the subject is The Wahl's Protocol: https://www.amazon.com/Wahls-Protocol-Autoimmune-Conditions-...

Now, that's mostly anecdotal. There are very few studies on diet in general. As far as type 2 diabetes is concerned, though, low-carb diets have empirically been shown to reduce the need for insulin injections, slow progression of disease, and prevent the disease in the first place.