The Hopkins team found that shifting about 10 percent of calories from carbohydrate to either protein-rich foods, mostly from plant sources, or to monounsaturated fats, contained in olive and canola oil, provided a major benefit to the heart.

“Our study provides strong evidence that replacing some carbohydrate with either protein or monounsaturated fat has important health benefits,” says internist Lawrence Appel, M.D., M.P.H., a professor of medicine at the Johns Hopkins School of Medicine and lead author of the study. “There is already agreement that reducing saturated fat lowers risk for heart disease, but the question of which macronutrient to emphasize has been controversial.”

Appel makes clear that his study does not support extremely high-saturated-fat, low-carbohydrate diets such as the Atkins diet, which he says is not a healthy diet plan.

The study, called the Optimal Macronutrient Intake Trial to Prevent Heart Disease (OmniHeart), evaluated three healthy diets that differed mainly in the amount of macronutrients - protein, fat and carbohydrate - that provide calories used for energy in the body. All three diets were low in saturated fat, cholesterol and sodium, and rich in fruits, vegetables, fiber, potassium and other minerals. However, one diet was a traditional healthy diet, rich in carbohydrate, while in the other two diets approximately 10 percent of its calories from carbohydrate were replaced with either monounsaturated fat or protein. In the protein-rich diet, about half came from plants.

“All three diets reduced overall heart disease risk, lowering blood pressure and improving cholesterol levels,” says Appel. “But the protein and monounsaturated fat diets had an edge over the carbohydrate-rich diet.”

The Hopkins findings from OmniHeart, to be presented Nov. 15 at the American Heart Association’s Scientific Sessions 2005 and published simultaneously in the Journal of the American Medical Association, underscore the significant benefits from making dietary changes, the researchers say.

Overall, the protein-rich diet, derived from plant and animal sources, decreased cardiovascular disease risk by 21 percent. “Many people equate protein with meat, but it is not the only source of protein,” says study co-author Phyllis McCarron, M.S., R.D., a dietitian at Hopkins. “Excellent plant sources of protein are beans, nuts, seeds and certain grains.”

The monounsaturated fat diet, enriched with olive and canola oils, as well as various nuts and seeds, decreased risk by almost 20 percent.

The carbohydrate-rich diet used in the study decreased risk by roughly 16 percent. The carbohydrate-rich diet is similar to the Dietary Approaches to Stop Hypertension, or DASH diet, which Appel helped develop in 1997.

For the current study, which lasted about three years, researchers enlisted 164 generally healthy adults, both men and women ages 30 and over. “Because of the huge risk of stroke and heart attack in African Americans, the results are particularly applicable to this group, who made up roughly 55 percent of study participants,” says study co-author Jeanne Charleston, R.N., a research associate at Hopkins’ Bloomberg School of Public Health. Charleston adds that all participants either had high blood pressure (almost 20 percent) or were on the verge of having high blood pressure.

For six-week intervals, participants ate all of their food - including breakfast, lunch, dinner and snacks - from one of the three diets. After a two-to-four-week break, participants started the six-week feeding period over again, this time with a different diet. The process was repeated until all participants ate all three.

Researchers monitored each participant’s levels of blood pressure, cholesterol and triglycerides on each diet. These measurements were then factored into a standard mathematical model, called the Framingham risk equation, for estimating heart disease risk.