Appel LJ, Sacks FM, Carey VJ, Obarzanek E, Swain JF, Miller ER, Conlin PR, Erlinger TP, Rosner BA, Laranjo NM, Charleston J, McCarron P, Bishop LM
JAMA. 2005;294:2455-2464.
Background:
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. In particular, dietary recommendations to lower blood pressure and serum lipids-two major targets for CVD risk reduction-are debatable.
Objective:
The aim of this study was to compare the effects of 3 healthful diets, each with reduced saturated fat intake (a carbohydrate-rich diet, a diet rich in protein, or a diet rich in unsaturated fat), on blood pressure and serum lipids.
Methods:
The Optimal Macronutrient Intake Trial to Prevent Heart Disease (Omni-Heart) was a randomized, crossover feeding study. Each feeding period lasted 6 weeks and was separated by a washout period of 2 to 4 weeks, during which time the participants ate their own food. Trial participants were generally healthy adults, aged 30 years and older, with prehypertension or stage 1 hypertension. Because of the disproportionate burden of CVD in African Americans, a recruitment goal was to achieve a cohort of 50% African Americans. The primary distinguishing feature of the 3 diets was their macronutrient composition. The carbohydrate-rich diet used in this trial is similar to the DASH diet. The protein-rich diet had approximately half of the protein from plant sources. The diet rich in unsaturated fat used predominantly monounsaturated fat, including olive, canola, and safflower oils, as well as a variety of nuts and seeds, to meet its target fatty acid distributions. Systolic blood pressure and LDL cholesterol were coprimary outcomes. Weight was measured each weekday and was kept stable (goal of within 2% of baseline) by adjusting caloric levels.
Results:
A total of 164 persons completed at least 2 feeding periods, and 159 completed all 3 periods. Adherence was high (95%-96% of person-days), and rates were similar for all 3 diets. Compared with baseline, systolic and diastolic blood pressure and levels of LDL, total, and non-HDL cholesterol were lower on each diet. HDL cholesterol levels decreased from baseline on the carbohydrate and protein diets but were unchanged on the unsaturated fat diet. Compared with baseline, triglyceride levels were lower on the protein and unsaturated fat diets but not on the carbohydrate diet. With regard to between-diet comparisons, both the protein and unsaturated fat diets significantly lowered systolic and diastolic blood pressure compared with the carbohydrate diet. Only the protein diet (not the unsaturated fat diet) significantly lowered LDL cholesterol levels compared with the carbohydrate diet. However, HDL cholesterol levels also significantly decreased on the protein diet. Conversely, HDL cholesterol levels significantly increased in the unsaturated fat diet when compared with the carbohydrate diet. This pattern of results was similar in subgroups defined by sex and race. Poor appetite, bloating or fullness, and dry mouth were reported more often in the protein diet compared with the carbohydrate or unsaturated fat diets.
Discussion:
This study extends previous observations on the effects of protein and unsaturated fat on blood pressure and provides evidence that, in addition to salt, potassium, weight, alcohol, and the DASH diet, macronutrients also affect blood pressure. 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 and improve lipid risk factors.
Comment:
The outcomes chosen in this study were CVD risk factors, not clinical events. Hence, trial results need to be interpreted as such and extrapolation to CVD events and risk should be done cautiously. Nevertheless, this study places macronutrients to the forefront and provokes clinicians to think about macronutrient choices during dietary counseling. This study suggests that we can improve on the DASH diet and further individualize diet recommendations, based upon patient-specific CVD risk targets.
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