It is well known that cutting down on saturated fat in the diet lowers the risk of heart disease but medical experts are less certain about what this fat should be replaced with.
The new trial suggests that those advising a traditional approach - including more carbohydrates to replace energy gained from saturated fat - may need to recommend more protein or vegetable oils instead.
The researchers at Johns Hopkins University found that substituting about 10 per cent of calories from carbohydrate to either protein-rich foods, mostly from plant sources, or to monounsaturated fats, contained in olive and canola oil, had a greater benefit on the heart than a carbohydrate rich diet, similar to that known in the US as DASH (Dietary Approaches to Stop Hypertension).
The study, called the Optimal Macronutrient Intake Trial to Prevent Heart Disease (OmniHeart), compared the effects of three different diets, each consumed over a six-week period, on blood pressure and fat in the bloodstream of 164 adults with pre-hypertension.
The first of these diets was rich in carbohydrates, but in the other two diets, approximately 10 per cent of the calories from carbohydrate were replaced with either monounsaturated fat or protein. In the protein-rich diet, about half came from plants.
Each diet was reduced in saturated fat, cholesterol, and sodium and rich in fruits, vegetables, fibre, potassium, and other minerals at recommended levels. Subjects were randomly assigned to follow each diet, with an two-week interval between each one.
Levels of blood pressure, cholesterol and triglycerides assessing during each diet was factored into a standard mathematical model, called the Framingham risk equation, for estimating heart disease risk.
Each diet was found to lower blood pressure, low-density lipoprotein cholesterol, and estimated coronary heart disease risk. But when people reduced their carbohydrate intake, the benefits were greater.
Overall, the protein-rich diet decreased cardiovascular disease risk by 21 per cent, and the monounsaturated fat diet decreased risk by almost 20 per cent.
The carbohydrate-rich diet only decreased risk by roughly 16 per cent.
The findings were presented at the American Heart Association's annual meeting in Dallas yesterday and published in today's issue of JAMA (vol 294, pp2455-2464).
Lawrence Appel, lead author of the study, said "the protein and monounsaturated fat diets had an edge over the carbohydrate-rich diet", although he noted that his study does not support extremely high-saturated-fat, low-carbohydrate diets such as the Atkins diet.
But the results provide strong evidence that, in addition to salt, potassium, weight, alcohol, and the DASH diet, macronutrients also affect blood pressure.
The authors note that previous studies demonstrate an inverse association between protein intake and blood pressure, although results have been inconsistent. At the same time, some studies have suggested that increased sugar consumption raises blood pressure. Given that both the protein and unsaturated fat diets lowered blood pressure to the same extent, it could be that lowering carbohydrates is the key to this benefit.
But protein did appear to have a direct benefit on triglyceride levels, lowering them to a greater extent than both the carbohydrate diet and unsaturated fat diet.
"These novel effects of dietary protein on lipid risk factors merit further study," said the authors.
Overall they believe their findings have both public health and clinical importance, helping to reduce CVD risk in the general population and mitigate the need for drug therapy in people with risk factor levels above treatment thresholds, they say.
In an accompanying editorial however, Dr Myron H. Weinberger of Indiana University Medical Center, Indianapolis, raised doubts about the ability of a diet alone being able to control blood pressure, particularly given the lack of "commercial availability of these very carefully designed diets", which were made readily available to the study participants.
Also, 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".