Soybean Protein Supplementation May Reduce Systolic, Diastolic Blood Pressure

Laurie Barclay, MD

July 05, 2005

July 5, 2005 -- Soy protein supplements can significantly reduce blood pressure in hypertensive patients, according to the results of a randomized, double-blind study published in the July 5 issue of the Annals of Internal Medicine. The editorialists suggest that recommending soy protein would be premature because of incomplete evidence, but they do advocate the Dietary Approaches to Stop Hypertension (DASH) diet.

"Epidemiologic studies suggest that vegetable protein intake is inversely related to blood pressure," write Jiang He, MD, PhD, from Tulane University in New Orleans, Louisiana, and colleagues. "The effect of dietary macronutrients on blood pressure has not been well studied, although clinical trials indicate that diets rich in fruits, vegetables, and low-fat dairy products and with reduced saturated and total fat (DASH diet) lower blood pressure."

In three communities in the People's Republic of China, 302 participants with an initial untreated systolic blood pressure of 130 to 159 mm Hg, diastolic blood pressure of 80 to 99 mm Hg, or both were randomized to receive 40 g of isolated soybean protein supplements daily or complex carbohydrate control for 12 weeks. Age range was 35 to 64 years. Mean systolic blood pressure was 135.0 ± 10.9 mm Hg, and diastolic blood pressure was 84.7 ± 6.9 mm Hg at baseline.

Of the 302 participants, 91.4% completed the 12-week trial. Neither group reported significant adverse effects. Compared with the control group, the net blood pressure changes in the soy group after the intervention were -4.31 mm Hg systolic (95% confidence interval [CI], -2.11 to -6.51 mm Hg; P < .001) and -2.76 mm Hg diastolic (95% CI, -1.35 to -4.16 mm Hg; P < .001), respectively.

In participants with hypertension, the net changes in systolic and diastolic blood pressure were -7.88 mm Hg (95% CI, -4.66 to -11.1 mm Hg) and -5.27 mm Hg (95% CI, -3.05 to -7.49 mm Hg), respectively. In participants without hypertension, the net changes were -2.34 mm Hg (95% CI, 0.48 to -5.17 mm Hg) and -1.28 mm Hg (95% CI, 0.52 to -3.07 mm Hg).

Study limitations were inability to determine whether the blood pressure reduction was related to protein or isoflavones in soybean and failure to control dietary nutrient intake as strictly as is possible in a feeding study.

"Soybean protein supplementation resulted in a reduction in systolic and diastolic blood pressure," the authors write. "These findings suggest that increased intake of soybean protein may play an important role in preventing and treating hypertension."

However, they note that 40 g of soybean protein is approximately equivalent to one soy burger plus one to two cups of soy milk, which may be difficult to consume daily.

The Tulane University Health Sciences Center; National Heart, Lung, and Blood Institute in Bethesda, Maryland; and the National Ninth Five-Year Plan Key Program from the Ministry of Science and Technology of the People's Republic of China supported this study. The authors report no potential financial conflicts of interest.

In an accompanying editorial, Jeffrey A. Cutler, MD, and Eva Obarzanek, PhD, from the National Heart, Lung, and Blood Institute, cite some evidence of an association between soy protein and increased risk for bladder cancer. They wonder if a mixed vegetable protein product would produce the same blood pressure lowering effect without increased bladder cancer risk. An ongoing large trial should help address some of these issues, with results anticipated later this year.

"We should require good evidence of benefit and safety before recommending soybean protein to the much larger population that is at risk for hypertension," Drs. Cutler and Obarzanek write. "However, higher protein intake may prove to be healthful, and we agree with current blood pressure control guidelines that recommend diets higher in total protein, such as the DASH diet."

Ann Intern Med. 2005;143:1-9, 74-75

Reviewed by Gary D. Vogin, MD

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