We don’t really know how much of the health protection of a vegetarian diet comes from avoiding meat or dairy and how much is due to the nutrients, fiber and protective compounds in plant foods such as vegetables, fruits, whole grains, beans and nuts. Most vegetarians do tend to eat more of these healthy foods than non-vegetarians. But replacing meat-oriented meals with these whole plant foods will almost surely have an entirely different health impact than meatless meals based mainly on processed refined grains, with a limited variety of vegetables and plenty of sweets and soft drinks.

Furthermore, studies showing better health among vegetarians don’t reflect dietary differences alone. Studies show consistently that vegetarians as a group are more likely to be non-smokers and more physically active than non-vegetarians. Vegetarians are also less likely to be overweight. Excess body fat is strongly linked to heart disease, diabetes and cancer risk. But vegetarian eating won’t automatically lead to a healthy weight if it still includes excessive portions and foods concentrated in calories from oils and sugars.

Although a vegetarian diet has a positive influence in supporting weight control, specific food choices may be the overriding influence of plant-based diets on health. Traditional Mediterranean-style eating patterns, which are plant-based diets that include fish regularly but also may include modest amounts of meat occasionally, were identified as the principal eating style tied to lower incidence of heart disease in one recent review of available data. Diets highest in fruits, vegetables, whole grains, legumes, poultry and fish and lowest in refined grains, French fries, and red and processed meats were linked with 36 percent lower incidence of heart disease among women in the Nurses’ Health Study. And for lower cancer risk, it’s low body fat, regular physical activity and a plant-based diet with a wide range of vegetables, fruits and other fiber-containing foods that can lower risk of cancer by about one-third.