Omniheart Trial

Effects of the three different diets on blood pressure and serum lipids

Appel LJ, Sacks FM, Carey VJ, et al. Effects of protein, monounsaturated fat, and carbohydrate intake on blood pressure and serum lipids. Results of the OmniHeart Randomized Trial. JAMA 2005;294:2455-64.

There is widespread consensus that a reduced intake of saturated fat lowers cardiovascular disease risk, but what optimal type of macronutrient (protein, unsaturated fat, or carbohydrate) should replace saturated fat?
The DASH (Dietary Approaches to Stop Hypertension) is a carbohydrate-rich diet with fruits, vegetables, low-fat dairy products, reduced in saturated fat, total fat, and cholesterol. It lowers blood pressure, lowdensity lipoprotein cholesterol (LDL-C), but also high-density lipoprotein cholesterol (HDL-C).
Diets rich in mono-unsaturated fat lower triglycerides (TG) and raise HDL-C but their effect on blood pressure (BP) is unknown, and protein-rich diets have been shown to lower BP. The Optimal Macronutrient Intake Trial to Prevent Heart Disease (OmniHeart) was a feeding study with a randomised 3-period crossover design, funded by the National Heart, Lung, and Blood Institute. Its objective was to compare the effects on BP and serum lipids of three diets (one rich in carbohydrate, one rich in protein, and one rich in unsaturated fat, mainly mono-unsaturated), all reduced in saturated fat. The participants had a 6-day run-in period, in which they ate 2 days of meals from each study diet, then were randomly assigned to 1 of 6 sequences of the 3 diets (Carb-Protein-Unsat fat, Carb-Unsat fat-Protein, Protein-Carb-Unsat fat, Protein-Unsat fat-Carb, Unsat fat-Carb- Protein, Unsat fat-Protein-Carb). A wash-out period of 2-4 weeks separated the 6-week feeding periods. The food was prepared in the research kitchen, using commonly available food. The main meal was eaten on-site on week days, other meals were eaten off-site. For each day participants had to indicate whether they ate non-study food, or whether they did not eat all study food. The main interest was to examine the difference between the carbohydrate diet on the one hand, and the protein and unsaturated fat diets on the other hand. Primary outcomes were systolic blood pressure (SBP) and LDL-C, secondary outcomes were diastolic blood pressure (DBP), triglycerides (TG), and HDL-C. BP and biological parameters were measured at baseline and end-of-period. Of 191 subjects assigned to 1 of 6 diet sequences, 164 completed at least 2 feeding periods and were included for analysis. Adherence was high: all study food was consumed and no non-study food was eaten in 95-96% of person-days on each diet. Compared with baseline, SBP, DBP, LDL-C, total cholesterol, and HDL-C were lower on each diet. HDL-C decreased from baseline on the carbohydrate diet and the protein, but were unchanged on the unsaturated fat diet.

The protein diet, compared with the carbohydrate diet, reduced SBP and DBP in all participants and in hypertensive subjects, and lowered LDL-C, HDL-C, total cholesterol, non-HDL-C, and TG.
The unsaturated fat diet, compared with the carbohydrate diet, reduced SBP and DBP in all participants and in hypertensive subjects, total cholesterol, non-HDL-C, and triglycerides, but not LDL, and increased HDL-C (Table I).

Changes in serum lipids by diet are shown in Table II.

The 10-year risk of CHD was lower on each study diet by 16.1% to 21.0%.
In conclusion, a diet where carbohydrates were partially replaced by protein or unsaturated fat (predominantly mono-unsaturated), has beneficial effects on blood pressure and serum lipid levels, and reduces the cardiovascular risk. These results are of importance for clinical and public health.