DALLAS — The health benefit of the carbohydrate-rich diet that's endorsed by the U.S. Department of Agriculture was improved by shifting some of the diet's calories to either protein or unsaturated fat in a study with 164 people.
“In the setting of a healthy diet, partial replacement of carbohydrate with protein or with monounsaturated fat can further lower blood pressure, improve lipid profiles, and reduce estimated coronary heart disease risk,” Dr. Lawrence J. Appel said at the annual Scientific Sessions of the American Heart Association.
The benchmark diet that served as the baseline was a modified DASH diet, so named because it was first tested in the Dietary Approaches to Stop Hypertension (DASH) trials, a series of studies run by Dr. Appel and his associates. The efficacy of the DASH diet for lowering blood pressure was so well established that it was endorsed last year by the Dietary Advisory Committee of the U.S. Department of Agriculture, and it's also been recommended in other reports and guidelines.
But the DASH diet has certain flaws, such as reducing serum levels of HDL cholesterol and a failure to lower serum levels of triglycerides (TG). This prompted Dr. Appel and his associates to compare a carbohydrate-rich diet similar to the DASH diet with two investigational modifications: one boosted protein levels, especially plant protein, while reducing carbohydrates, and the other increased unsaturated fat, especially monounsaturated fat, while also reducing carbohydrates. Results from the study were published concurrent with the meeting report (JAMA 2005;294:2455–64).
The study enrolled healthy adults aged at least 30 years who had a systolic blood pressure of 120–159 mm Hg or a diastolic pressure of 80–99 mm Hg. Excluded were patients with diabetes, those with an active or a prior diagnosis of cardiovascular disease, and those with a serum level of LDL cholesterol of more than 220 mg/dL or a fasting TG level of more than 750 mg/dL.
The participants followed each diet for 6 weeks, followed by a 2-week washout period between diets. Each individual received standardized meals. The weight of each participant was monitored daily, and total daily calories were adjusted for each person daily to maintain their weight within 2% of baseline levels, so that any changes in blood pressure and serum cholesterol levels were attributable solely to diet. A total of 164 people completed at least two of the study diets.
The protein diet cut systolic pressure by an average of 1.4 mm Hg, compared with the DASH-like diet, a statistically significant difference. The fat diet dropped systolic pressure by 1.3 mm Hg, also a significant reduction, reported Dr. Appel, professor of medicine at Johns Hopkins University in Baltimore.
Serum levels of LDL cholesterol averaged 3.3 mg/dL lower with the protein diet compared with the higher carbohydrate diet, a statistically significant drop. The fat diet lowered LDL cholesterol by a mean of 1.5 mg/dL, not a significant difference.
The estimated 10-year risk of a coronary heart disease event based on the Framingham risk score averaged 4.0% when participants were on the protein diet, 4.3% on the unsaturated fat diet, and 5.1% on the DASH-like diet. Other measures showed that the protein diet cut TG levels by 15.7 mg/dL and the fat diet cut them by 9.6 mg/dL (see table). HDL-cholesterol levels were significantly reduced with the protein diet but were significantly increased with the unsaturated fat diet.
Source: JAMA 2005;294:2455–64