Hypertension

Substituting proteins and unsaturated fats for carbohydrates can reduce blood pressure, boost lipid profiles, OMNIHEART shows

Shelley Wood

November 15, 2005

Nov 15, 2005


Dallas, TX - Turning conventional dietary wisdom on its head, results of the OMNIHEART study indicate that substituting proteins or unsaturated fats for carbohydrates within the context of a healthy diet can reduce blood pressure and improve lipid profiles. Dr Lawrence J Appel (Johns Hopkins University, Baltimore, MD) presented the results of the "feeding study" here during the late-breaking sessions of the American Heart Association Scientific Sessions 2005 meeting; the results were published simultaneously in the Journal of the American Medical Association [ 1 ].



Dr Lawrence J Appel



"In the setting of a healthy diet, partial replacement of carbohydrate with protein, of which about half is from plant sources, or with monounsaturated fat can further lower blood pressure, improve lipid profiles, and reduce estimated coronary heart disease [CHD] risk," Appel said.

OMNIHEART was designed to extend the findings from the Dietary Approaches to Stop Hypertension (DASH) trials, which found that a carbohydrate-rich diet that emphasized fruits, vegetables, and low-fat dairy products, while minimizing sodium, saturated fat, total fat, and cholesterol, could substantially lower blood pressure. The diet that arose out of DASH has been widely advocated in health guidelines, including the 2005 Dietary Guidelines Scientific Advisory Committee, even while it appears to lower HDL cholesterol and has no effect on triglycerides, the investigators note. In the Optimal Macronutrient Intake Trial to Prevent Heart Disease (OMNIHEART) study, Appel et al set out to establish whether replacing some of the carbohydrates in the DASH diet with other macronutrients?namely either protein-based foods or unsaturated fats?while keeping weight constant could lower blood pressure and alter these additional lipid parameters.


At the heart of OMNIHEART

Designed as a three-phase crossover study, OMNIHEART assigned 164 adults with prehypertension or hypertension to one of three diets (rich in carbohydrates, proteins, or unsaturated fats, with this last made up primarily of monounsaturated fats) for six weeks, before each group switched to a different diet for the same number of weeks.

Analysis of the effects of each diet pointed to striking benefits in all groups; however, additional specific improvements were seen the fat and protein groups. Compared with participants eating the carbohydrate-rich diet, those eating the protein-rich diet had greater reductions in blood pressure, LDL, and triglycerides, although unfortunately HDL was also significantly reduced. Likewise, those eating the diet rich in unsaturated fats saw significant reductions in blood pressure and triglycerides, but actually experienced a significant increase in HDL levels and no effects on LDL, compared with those eating the carbohydrate-based diet. Ten-year CHD projections, as estimated by Framingham equation, were lower than baseline for all three groups but were lower, and similar, for both the protein and unsaturated fat groups than for the carbohydrate group.

BP and lipid changes in the protein and fat groups, beyond those achieved in the carbohydrate diet

End point
Protein vs carb
p
Fat vs carb
p
Systolic BP (mm Hg): All patients
-1.4
0.002
-1.3
0.005
Systolic BP (mm Hg): Hypertensive patients
-3.5
0.006
-2.9
0.02
Diastolic BP (mm Hg)
-1.2
<0.001
-0.8
.02
LDL (mg/dL)
-3.3
0.01
-1.5
0.22
HDL (mg/dL)
-1.3
0.02
1.1
0.03
Triglycerides (mg/dL)
-15.7
<0.001
-9.6
0.02
Estimated 10-year risk of CHD (%)
-5.8
-
-4.2
-


To download table as a slide, click here


To renalwire , Appel said that it is not possible to advocate either the protein-rich diet or the unsaturated-fat rich diet over its counterpart at this stage, even if they appeared to have different effects on lipid parameters. "We didn't test it, but one could possibly consider a hybrid of the two diets that's modestly increased in protein and unsaturated fat at the expense of carbohydrates, and that might be the best, but that's speculation."

Also commenting on the results for renalwire , study coauthor Dr Eva Obarzanek (National Heart, Lung, and Blood Institute, Bethesda, MD) agreed that it is not possible to "advocate one diet over the other," particularly since the estimated risk scores were similar between the protein and fat diets. Nor should the carbohydrate diet be dismissed entirely, since it, too, had important beneficial effects. "Actually, all three diets were healthful. If a person could even adhere to just one of these diets, because the overall dietary pattern was the same for each, emphasizing fruits, vegetables, low-fat dairy products, and whole grains. And just getting the population up to that level of fruits and vegetables would be a major benefit. And at that point you could be guided by your personal preferences and what's easier to follow, or even switch between the two."

Dr Barbara Howard (MedStar Research Institute, Hyattsville, MD) who was not involved in the study, pointed out to renalwire that it is the effects on blood pressure that were the most striking findings of OMNIHEART. Other than the DASH study, physicians have very little randomized clinical-trial results on dietary interventions for hypertension.

That said, Howard continued, "we don't really have any trials on long-term outcomes, and just looking at short-term markers of blood pressure and lipids is not the same as outcome studies."

Howard also took issue with the study's focus on monounsaturated fats, saying she would have preferred a study emphasizing polyunsaturated fats, which are known to have a better effect on cardiovascular risk than monounsaturated fats.

Still, she said, "This study helps us to understand what to increase in the diet when one is looking to lower saturated fats." In recent years, diets cutting back on saturated fats have tended to replace these nutrients with carbohydrates. "This gives a little bit more information on a diet that replaces carbohydrates with good fats," she said.


Knowledge gaps

In an editorial accompanying the OMNIHEART study in JAMA, Dr Myron H Weinberger (Indiana University, Indianapolis) observed that the higher-protein diet appeared to be associated with reduced physical activity, reduced appetite, and bloating, warranting further studies in this group [ 2 ]. For example, he points out, participants in OMNIHEART were maintained at the same weight, whereas future studies may want to also focus on weight loss, he notes. He also emphasized the need for further studies, following patients to test for cardiovascular outcomes rather than just estimating from surrogates. After all, says Weinberger, "longer trials¿.¿.¿.¿will be needed to convince a skeptical public of the benefit of yet another unique and difficult-to-achieve dietary regimen."

Additional points were raised by the study's scheduled discussant during the late-breaking session. Dr Ira Goldberg (Columbia University, New York, NY) pointed to questions left unanswered by OMNIHEART, including the role of different types of fiber, animal vs plant proteins, and simple vs complex carbohydrates. For example, the "somewhat-detrimental" effects of the carbohydrate diet in OMNIHEART might have been due to the fact that a lot of the carbohydrates were free carbohydrates, from fruit and simple sugars, which would have produced a fructose effect, not a carbohydrate effect, Goldberg observed.

"Obviously the long-term effects of this diet and the long-term effects and short-term effects on insulin sensitivity and inflammatory status are of interest to all of us," Goldberg concluded.



Sources

  1. Appel JL, Sacks FM, Carey VJ et al. Effects of protein, monounsaturated fat, and carbohydrate intake on blood pressure and serum lipids. JAMA 2005; 294:2455-2464. Abstract

  2. Weinberger MH. More novel effects of diet on blood pressure and lipids. JAMA 2005; 294:2497-2498. Abstract

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