However, the Ornish Diet has a major drawback: It may be difficult for some people to follow, especially over the long-term. The popular diet is essentially a vegan diet, Weiner says, and people may find it hard to avoid all meats, chicken, fish, and egg yolks. Also, she says, the diet is extremely low in fats of all types, and it’s often fat that adds flavor to foods and makes people feel satiated