Yes, but even lifestyle changes (like a diet low in glycemic load and building muscle) can help reduce many of the harmful effects of type 2 diabetes, even sending it into remission for some people in early stages.

Type 1 is a different story. It’s the lack of natural insulin production (due to a damaged pancreas, autoimmune or other causes), basically the opposite problem to type 2, and no amount of lifestyle changes will replace of need of insulin doses.