Here's one of the placebo-controlled studies that suggests lifestyle can reverse coronary artherosclerosis (hardening of the arteries): http://www.ncbi.nlm.nih.gov/pubmed/1973470

And for something more digestible, here's Dr. Dean Ornish's TED talk discussing his research on the same:

http://www.ted.com/talks/dean_ornish_on_the_world_s_killer_d...