I gave a 30-minute presentation this morning for the cardiology department at my hospital on vegetarian diets and heart disease. I've given similar talks before, but to lay people, not to cardiologists.
An important point was brought up, and it's a source of frustration for me as well: There are hundreds of randomized controlled trials of various drugs and devices for heart disease, but there isn't a single large, randomized controlled trial on vegetarian diets and heart disease.
There's evidence that a plant-based diet is good for the heart, including a small randomized controlled trial by Dean Ornish (published in Lancet in 1990) and a small observational study by Caldwell Esselstyn (Journal of Family Practice, 1995) that showed that patients on a low-fat vegetarian diet have fewer repeat cardiac events. Epidemiologic studies demonstrate that cultures with plant-based diets have very low rates of coronary disease.
But, no one is going to do a large, randomized trial of vegetarian diet and heart disease. It isn't profitable -- what pharmaceutical company would sponsor it? And even though the data that is out there strongly supports a vegetarian diet for prevention of heart disease and cancers, there will still be skepticism for lack of a large randomized-controlled trial.