The Best Diet to Prevent Heart Disease?

Summarized by Robert W. Griffith, MD
December 5, 2005

Introduction

The low-carb diet fad has almost passed, and people still accept that one should eat less saturated fat. But what's to replace the saturated fat, - carbohydrate, protein, or other fats? The answer is," all of the above". But there still remains the question of the optimal proportions of each. A study called the OmniHeart Randomized Trial has now been reported in the Archives of Internal Medicine; it tries to answer this problem.

What was done

The aim of the study was to compare the effects of three healthful diets (each with a reduced saturated fat intake) on blood pressure and serum cholesterol levels. Volunteers were over 30, with so-called pre-hypertension (blood pressure readings 120-139 mmHg systolic or 80-89 mmHg diastolic) or stage 1 hypertension (140-159 or 90-99 mmHg, respectively), and who had no diabetes, cardiovascular disease, obesity, or severely raised lipid levels.

The 164 participants were divided into three groups, and each group received a different diet for one month, with an adequate 'wash-out' period (2-4 weeks) between each diet, when they ate their own food. During each month's diet period blood pressure was measured at frequent intervals, and again during the 'wash-out' period. Fasting blood lipids levels were measured before and after each monthly diet period.

Each diet was reduced in saturated fat, cholesterol, and sodium, and was rich in fruits, vegetables, fiber, and potassium; they all contained the same calories; the diets were named according to the main substitute for saturated fat that they contained. Below is the percentage composition of the three diets, expressed as % of total calories.

Diet	Carbohydrate	Protein	Total Fat	Mono-U Fat*
'carbohydrate'	58%	15%	27%	13%
'protein'	48%	25%	27%	13%
'unsaturated fat'	48%	15%	37%	21%
* monounsaturated fat

What the results showed

The average age of the participants was 54; 45% of them were women, and 55% were African Americans.

Compared with baseline values, blood pressures and all cholesterol levels were all lower on each diet. High-density lipoprotein cholesterol (the 'good' cholesterol) decreased with the carbohydrate and protein diets but remained unchanged on the unsaturated fat diet. Compared with the carbohydrate diet, both the protein and unsaturated fat diets significantly lowered systolic and diastolic blood pressures. Systolic pressures decreased by an average of 9.3 mmHg on the unsaturated fat, and 8.2 mmHg on the carbohydrate diets.

The changes for the stage 1 hypertensive patients were more pronounced. Systolic blood pressures fell from a baseline average of 146.5 mmHg by 12.9 mmHg on the carbohydrate, 16.1 mmHg on the protein, and 15.8 mmHg on the unsaturated fat diets.

What these findings mean

The reader might be forgiven for being unimpressed by the average blood pressure level changes, and say "So what? Not enough improvement to influence my choice of food." This would be a mistake. The small changes were significant, and each diet was only taken for a month. Treating (or preventing) high blood pressure requires a lifetime commitment to an improved lifestyle, and a healthful diet is a part of this.

The investigators used numbers from the Framingham Heart Study to calculate changes in the likely 10-year risk of having a coronary heart disease event (e.g. a heart attack). This risk was lowered by 16% with the carbohydrate diet, 21% by the protein diet, and 19.6% by the unsaturated fat diet.

There doesn't seem much to choose between the three diets, which were all 'healthful'. The important features to remember are: low saturated fat, low sodium, plenty of fruit, veggies and fiber, and where possible substitute carbohydrate with protein or unsaturated fat.

Source

• Effects of protein, monounsaturated fat, and carbohydrate intake on blood pressure and serum lipids. Results of the OmniHeart Randomized Trial. ALJ. Appel , SFM. Sacks , CVJ. Carey ,  et al., JAMA, 2005, vol. 294, pp. 2455--2464

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