As part of a healthy diet, partially substituting protein and monounsaturated fat for carbohydrates can improve cholesterol levels, further lower blood pressure and reduce estimated cardiovascular risk, according to a study in the November 16 issue of JAMA. This study is being released early to coincide with its presentation at the American Heart Association’s annual meeting.

Despite widespread consensus that a reduced intake of saturated fat lowers cardiovascular disease (CVD) risk, the optimal type of macronutrient (protein, unsaturated fat, or carbohydrate) that should replace saturated fat is uncertain, according to background information in the article. Two major goals of dietary recommendations are to lower blood pressure and improve serum lipids, two of the primary determinants of CVD risk.

Lawrence J. Appel, M.D., M.P.H., of Johns Hopkins University, Baltimore, and colleagues with the Optimal Macronutrient Intake Trial to Prevent Heart Disease (OmniHeart) study compared the effects on blood pressure and serum lipids of three healthful diets: a carbohydrate-rich diet, similar to the DASH diet (Dietary Approaches to Stop Hypertension); a diet rich in protein, approximately half from plant sources; and a diet rich in unsaturated fat, predominantly monounsaturated fat.

Each diet was reduced in saturated fat, cholesterol, and sodium and rich in fruits, vegetables, fiber, potassium, and other minerals at recommended levels. The randomized feeding study involved 164 adults with prehypertension or stage 1 hypertension. Participants ate each diet for 6 weeks and body weight was kept constant. The study was conducted from April 2003 to June 2005.

The researchers found that blood pressure, low-density lipoprotein cholesterol, and estimated coronary heart disease risk were lower on each diet in comparison to baseline. “In OmniHeart, [compared to a carbohydrate-rich diet], a diet that partially replaced carbohydrates with protein, about half from plant sources, lowered blood pressure, LDL cholesterol levels, and triglyceride levels, as well as HDL cholesterol levels among adults with prehypertension or stage 1 hypertension. A diet that partially replaced carbohydrates with unsaturated fat, predominantly monounsaturated fat, lowered blood pressure and triglyceride levels and increased HDL cholesterol levels but had no significant effect on LDL cholesterol levels. Estimated coronary heart disease risk was similar on the protein and unsaturated fat diets and lower than that of the carbohydrate diet,” the authors write.

“Results from OmniHeart have important implications. First, our results provide strong evidence that, in addition to salt, potassium, weight, alcohol, and the DASH diet, macronutrients also affect blood pressure. Second, the DASH diet, as tested in this trial [under weight-stable conditions], can be improved; partial substitution of carbohydrates with protein, about half from plant sources, or with unsaturated fat, predominantly monounsaturated fat, has beneficial effects on blood pressure and serum lipid levels. Third, the magnitude of effects have both public health and clinical importance. The blood pressure reductions and improved lipid profiles should reduce CVD risk in the general population and mitigate the need for drug therapy in persons with risk factor levels above treatment thresholds.”

“In conclusion, in the setting of recommended levels of saturated fat, cholesterol, fiber, fruit, vegetables, and minerals, diets that partially replace carbohydrates with protein or monounsaturated fat can further lower blood pressure, improve lipid risk factors, and reduce CVD risk,” the researchers write.

(JAMA.2005; 294:2455-2464. Available pre-embargo to the media at http://www.jamamedia.org)