Trading about 10 percent of carbohydrates in the diet for beans and healthful fats such as olive oil can help control high blood pressure and raise the level of "good cholesterol," according to a new study.
Experts said that the findings will help alter the standard dietary advice for people at high risk of heart disease. They also underscore the health benefits of popular foods including nuts, avocados and olive oil.
"We've got evidence to say that people should be more selective about what they eat," said Edward J. Roccella, coordinator of the National High Blood Pressure Education Program at the National Heart, Lung and Blood Institute. "If they follow a diet similar to that in the study, it may give them a chance to lower their risk of heart disease. And for some this might mean taking fewer drugs."
The results, presented yesterday at the American Heart Association's annual meeting in Dallas, caught even some of the study's researchers by surprise.
"I thought that we were going to see a protein effect" on cholesterol and high blood pressure, said Lawrence Appel, professor at Johns Hopkins School of Medicine and lead author of the trial. "But the addition of unsaturated fat in lowering blood pressure was a surprise."
Diet has long been an important tool in reducing the risk of heart disease. That is why experts urge limiting salt intake, unhealthful saturated fats, trans fats and cholesterol. But there has been great debate about what role other nutrients might play, including lean protein, whole-grain carbohydrates and healthful fat.
The 2001 Dietary Approaches to Stop Hypertension (DASH) trial conducted by the National Heart, Lung and Blood Institute proved that diet could be as effective in controlling blood pressure as some medications. The low-sodium DASH diet is "a hallmark to prevent and treat hypertension," said Frank M. Sacks, a co-investigator of the new study, in a statement released by Brigham and Women's Hospital, where he is on staff.
The trouble is that the DASH diet -- which includes plenty of fruits and vegetables, moderate amounts of foods with healthful fat such as nuts, and low-fat dairy products, along with high amounts of carbohydrates and lean meat, fish and poultry -- produces mixed results when it comes to cholesterol. The DASH regimen lowers low-density lipoprotein (LDL), one of the most damaging types of cholesterol, but has little effect on triglycerides -- another unhealthful type of blood fat. But the regimen also reduces levels of the protective high-density lipoprotein (HDL), the so-called "good cholesterol."
That is why "there are additional growing cardiovascular health risks that we need to address," Sacks said.
To do that, Sacks and Appel launched the Optimal Macronutrient Intake Trial to Prevent Heart Disease (OmniHeart) to compare the effects of three diets on blood pressure, blood cholesterol and heart disease risk in a group of 164 people, age 30 and older. The results are being published in this week's Journal of the American Medical Association.
Like more than half of Americans, all were considered "pre-hypertensive" or having stage one high blood pressure. Their systolic pressure -- measured when the heart contracts -- was in the range of 120 to 159 millimeters of mercury. Their diastolic readings -- the pressure when the heart rests between beats -- were 80 to 99.
About 75 percent of the participants were overweight or obese, and slightly more than half were African American, a group at high risk for hypertension.
The participants, from Baltimore and Boston, were randomly assigned to three healthful diets, all low in saturated fat, cholesterol, trans fatty acids and sodium -- much like the DASH diet. All diets were also rich in fruit, vegetables, fiber, potassium and other minerals. The researchers varied protein, carbohydrate and fat content, however, so that one diet was richer in carbohydrates, one richer in protein and one richer in healthful fat.
The volunteers followed the assigned diet for six weeks. All the food was provided by the researchers, who also monitored participants' body weight and adjusted calorie amounts to make sure weight stayed steady throughout the study.
All three diets produced large enough drops in blood pressure and LDL cholesterol to cut the 10-year risk of coronary heart disease by at least 16 percent. But the diets rich in protein and healthful fats outperformed the standard high-carbohydrate diet on both measures, cutting risk by 20 percent.
"We have a two-part take-home message here," Appel said. "All three diets were healthy and had favorable effects. But the current recommended DASH diet that is rich in carbohydrates can be further improved by partially replacing some of those carbohydrates with lean protein from plants and low-fat dairy products, or with monounsaturated fats" such as olive oil or nuts.