How much of size is inflammation?

I found I had a bacteria in my small intestine and went on a diet for this and lost 9 pounds in 30 days. Went down 2 sizes too. It seems when I go back to eating like the everyone else, I gained 4 pounds in 4 days back. It seems my 59 year old body is very unhappy with what they have done to the food. I will be on meat, fruits, vegs, and nuts for ever.