The role of dietary fat, protein, and carbohydrates in lowering lipids and blood pressure has not been satisfactorily clarified. The Dietary Approaches to Stop Hypertension trial demonstrated that a high-carbohydrate diet emphasizing fruits, vegetables, and low-fat dairy can lower blood pressure and low-density lipoprotein (LDL) cholesterol, but it also lowers high-density lipoprotein (HDL) cholesterol while having a neutral effect on triglycerides. Appel and colleagues conducted a randomized trial to compare the effects of three healthful diets: one emphasizing carbohydrates; the second, protein; and the third, unsaturated fats.

Study participants were 30 years or older with systolic blood pressures of 120 to 159 mm Hg and diastolic pressures of 80 to 99 mm Hg. Patients with diabetes, coronary vascular disease, and unhealthy habits were excluded. An effort was made to recruit a high percentage of black patients. Each diet, regardless of macronutrient emphasis, was low in saturated fats, salt, and cholesterol, and rich in fruits and vegetables. Five caloric levels of each diet were provided with the idea that weight would be maintained at stable levels throughout the study. Participants ate their main meal on-site and kept records of their off-site meals. Primary outcomes were systolic blood pressure and LDL cholesterol, with diastolic pressure, HDL cholesterol, and triglycerides serving as secondary outcomes. Each feeding period lasted six weeks in the three-period crossover study.

Compared with baseline, HDL cholesterol levels decreased in the carbohydrate and protein diets and were unchanged on the unsaturated fat diet. Triglycerides remained the same on the carbohydrate diet but were lower on the unsaturated fat and protein diets. Blood pressure, LDL, and total cholesterol levels were reduced from baseline on all diets. The unsaturated fat and protein diets lowered blood pressure better than the carbohydrate diet. Fewer hypertensive patients remained hypertensive on the protein and unsaturated fat diets than those on the carbohydrate diet. There was a more significant drop in LDL and HDL levels for patients on the protein diet than those on the other two diets. The unsaturated fat diet increased HDL levels more than the carbohydrate diet. The protein and unsaturated fat diets reduced triglycerides, total cholesterol, and non-HDL cholesterol levels more than the carbohydrate diet. Applying 10-year risk reduction estimates, all diets lowered the risk of coronary heart disease, with the greatest reductions associated with the protein and unsaturated fat diets.

The authors conclude that substituting some carbohydrates with protein or monounsaturated fat in healthful diets can improve lipid factors, lower blood pressure, and reduce the risk of cardiovascular disease.