Studies suggest that diets high in soy may protect the cardiovascular system and reduce the risk of cardiovascular disease (CVD). However, many researchers are unsure what part of the soybean is responsible for its beneficial effects. While some scientists point to soy protein, isoflavones, or other phytonutrients, others think that only whole soybeans offer protection. Some experts have also suggested that people who eat soy have a lower overall risk of CVD because they eat less animal protein, not because of soy itself.
Soy may have a beneficial effect on blood
pressure. Photo by Steven Foster.
Two randomized, crossover studies recently explored this issue, producing differing results. The first, conducted by researchers from the Human Nutrition Research Center on Aging at Tufts University (Boston), found that the source of protein had little impact on CVD risk factors. The second, conducted by researchers at Beth Israel Deaconess Medical Center (Boston), found that the source of protein may make a difference, after all. Both studies were of a relatively short duration and used small samples that aren’t necessarily representative of the entire population. However, they do offer some new information about soy’s effects on blood pressure.
DOES PROCESSING MATTER?
In the Tufts study, which was published in the April 2007 issue of the American Journal of Clinical Nutrition, 28 people aged 50 and over followed four different diets over consecutive six-week periods. Energy from the diets consisted of 55% carbohydrates, 30% fat, and 15% protein, with 7.5% coming from an experimental source of protein. While the cholesterol, fiber, and fatty acid profiles of all four diets were the same, the protein was derived from either whole soybeans, soy flour, soy milk, or animal protein.
The Tufts researchers found no significant differences among the different protein sources in CVD risk factors such as total and very low-density lipoprotein (VLDL) cholesterol, triacylglycerol, apolipoprotein B (apoB), C-reactive protein (CRP), blood pressure, and vascular function—although they did find that soy milk produced a modest decrease in LDL cholesterol compared with animal protein and soy flour, and a slight increase in high-density lipoprotein (HDL) cholesterol compared with whole soybeans and soy flour.
Because the amount of soy protein used in the study was about 1.5 times the amount recommended by FDA in its soy health claim, the researchers concluded that the findings “may have important implications for the management of CVD.”
A LITTLE TLC
In the second study, which appeared in the May 28, 2007, issue of the Archives of Internal Medicine, the Beth Israel Deaconess Medical Center team recruited 60 postmenopausal women, including 12 hypertensive women and 48 women with normal blood pressure, who each followed one of two diets for eight weeks and then switched to the other diet for an additional eight weeks.
Ultrasonic Processing Boosts Soy Protein and Sugar Yields
Bombarding soybeans with high-frequency sound waves could greatly improve soy protein processing operations, according to Iowa State (Ames, IA) researchers.
According to Samir Khanal, PhD, Iowa State research assistant professor of civil, construction, and environmental engineering, ultrasonic processing breaks the bonds that connect sugars to the soy proteins, enhancing the release of soy proteins by 46% and raising the sugar content of the soy whey created during processing by 50%.
Like conventional dairy whey, the high-sugar soy whey can nourish lactic acid bacteria. One bacteria of interest to food manufacturers is nisin, a natural food preservative used in products such as mouthwash and toothpaste.
While Khanal’s team has been able to increase protein and sugar content on a batch-by-batch basis, it now plans to test the technology on a larger scale, such as that used in a full-sized soy protein processing facility.
Because the costs and power requirements for ultrasonic processing are small and because the treatments last for only a few seconds, the benefits to the food processing industry could be substantial. The team estimates that the technology could generate up to $230 million in yearly revenue from a typical plant that produces 400 million lb of soy protein isolate.
In the first diet, modeled after the Therapeutic Lifestyle Changes (TLC) regimen, calories consisted of 55% carbohydrates, 30% fat, and 15% protein. The diet also provided 1200 mg of calcium per day, less than 200 mg of cholesterol, and two servings of fatty fish per week. The second diet was almost identical, except the women substituted some of their protein with a half cup per day of soy nuts containing 25 g of soy protein and 101 mg of aglycone isoflavones.
By the end of the study, the researchers had discovered some interesting effects from the soy protein. First, the soy nuts appeared to significantly reduce systolic and diastolic blood pressure in all of the hypertensive women and in all but eight of the other women. Second, the soy nuts seemed to lower LDL cholesterol and apoB in the hypertensive women. “Compared with the TLC diet alone, the TLC diet plus soy nuts lowered systolic and diastolic blood pressure 9.9% and 6.8%, respectively, in hypertensive women and 5.2% and 2.9%, respectively, in normotensive women,” wrote the authors.
Because experts estimate that a 12-mm decrease in systolic blood pressure for 10 years can prevent one death for every 11 patients with stage-one hypertension, added the authors, the study’s average reduction of 1 mg of mercury in systolic blood pressure in hypertensive women could have “significant implications” for CVD.
“This study was performed in the free-living state,” wrote the researchers. “Therefore, dietary soy may be a practical, safe, and inexpensive modality to reduce blood pressure. If the findings are repeated in a larger group, they may have important implications for reducing cardiovascular risk in postmenopausal women on a population basis.”