A study found that partially substituting protein and monounsaturated fat for carbohydrates can improve cholesterol levels, further lower blood pressure and reduce estimated cardiovascular risk.

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, 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, 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, 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.

In an accompanying editorial, Myron H. Weinberger, of Indiana University Medical Center, Indianapolis, discusses whether the results from the OmniHeart trial can be applied to the general population.

" The participants in both of the DASH trials and in the OmniHeart Trial were highly selected from a motivated group of relatively young, well-educated, overweight individuals, a majority of whom were ethnic urban dwellers. Previous studies have shown that individuals with similar characteristics are likely to respond to sodium reduction and the DASH-type diets with a reduction in blood pressure and lipid levels. Thus the blood pressure responses of the participants in the present trial are not very surprising. Although the benefit of blood pressure reduction in those in the prehypertensive group is clear, the ability to control blood pressure adequately with diet alone among the stage 1 hypertensives studied in the OmniHeart Trial has not been presented. It is likely that more than diet will be required to reach goal blood pressures for the majority of these individuals, particularly with the lowering of blood pressure goals being advocated at present."

" The provision of all meals and snacks for a period of several months may have served as an inducement for study participants to remain in the trial. This may not be applicable to unselected patients or to the general population who have to purchase and prepare their own meals. Moreover, the practical application of these findings requires the demonstration that the lifestyle changes inherent in the tested diets can be maintained for periods longer than the few weeks studied in the OmniHeart trial. The composition of and the apparent lack of commercial availability of these very carefully designed diets also may be limitations to the broader application of these findings. Finally, because the OmniHeart Trial only used the surrogate outcomes of blood pressure and lipid levels, longer trials examining actual cardiovascular event outcomes will be needed to convince a skeptical public of the benefit of yet another unique and difficult-to-achieve dietary regimen," Weinberger concludes.

Source: American Medical Association, 2005


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