Maren Hegsted, J. Samuel Godber, Zhimin Xu and Jack N. Losso

Soy flour and more highly purified soy proteins contain a number of constituents that can be used in combating a variety of diseases. Soy isofla-vones may prevent diseases associated with post-menopausal women such as osteoporosis and coronary heart disease. A peptide found in soy flour is a potential anti-carcinogen. LSU AgCenter research has been directed at extraction, purification, stability testing and functional activity of these compounds.

Soybeans have been a major food source in Asian cuisines for centuries. Soybean cultivation as a crop began in northern China more than 5,000 years ago and slowly spread into southern China, Korea, Japan and Southeast Asia. Early Chinese writings by the Emperor Sheng-Nung in 2838 BC include a description of soybeans as one of the five sacred crops along with rice, wheat, barley and millet. Later poets celebrated the benefits of soybeans and their service to humanity in China. Soybeans were processed into many food items such as tofu, miso, tempeh and soy sauce, which were part of the daily diet. Fermented soy products were used medicinally in many parts of Asia, and moldy soybean curds have been used for more than 3,000 years to treat skin infections.

Today, soybean consumption is still much higher in Asian countries than in the United States. Because lower rates of heart disease, some cancers and osteoporosis are associated with higher intakes of soy products, scientists have started to examine soy as a functional food. The relationship between soy intake and heart disease was the first focus of research because heart disease rates are much lower in soy-consuming countries. Because of evidence supporting the benefits of soy in lowering cholesterol, the Food and Drug Administration has approved a nutrition health claim on soy-containing food products stating that “foods rich in soy protein as part of a low-fat diet may help reduce the risk of heart disease.”

Isoflavones

Soy contains many phytochemicals, and scientists are just now identifying the roles they may play in human health. Among these phytochemicals are the isoflavones—genistein, daidzein and glycitein—which may act as estrogen analogs in the body, affecting cells that contain estrogen receptors. LSU AgCenter researchers are studying the purification and characterization of soy isoflavones.

The chemical structural differences of soy isoflavones may result in variable bioavailability in biological systems. The structures of soy isoflavones are not consistent during routine food processing. Factors induced in the food processing, such as enzymes in raw soy flour, heating and additives, can affect the stability of soy isoflavones. The isoflavones genistein, daidzein and glycitein, as found in soy foods, are usually bound to a glucose molecule forming the glucosides—genistin, daidzin and glycitin. In LSU AgCenter research, high purity genistin, daidzin and glycitin were prepared from soy flour and observed for their stability during heating. The results are useful in understanding the thermal stability of soy isoflavones. Overall, the stability of daidzein was found to be higher than that of glycitein or genistein. 

Many reports have indicated that soy isoflavones lower plasma cholesterol and may reduce the risk of cancer. The detailed mechanism for this capability is not fully understood. Oxidation products of cholesterol are harmful to many cells in the vascular system, which contribute to plaque formation and cancers. Because soy isoflavones contain phenolic groups, they may possess antioxidation properties that offer protection against oxidation of cholesterol and oxidative damage to blood vessel cells. In LSU AgCenter research, both genistein and daidzein demonstrated significant antioxidant activity in the inhibition of cholesterol oxidation. Since soy contains both genistein and daidzein, the combined antioxidant benefits of both isoflavones could be important in reducing oxidative damage to body tissues.

Bowman-Birk inhibitor

Another health benefit of soy intake may be through inhibition of angiogenesis by a peptide found in many soy products, referred to as Bowman-Birk inhibitor (BBI). Angiogenesis is the formation of new blood vessels to supply blood and nutrients and to remove metabolic wastes from living tissue. Physiologic angiogenesis is a tightly regulated process important to embryonic development, reproduction and wound healing. Pathologic angiogenesis is a feature of many inflammatory diseases and contributes to the spread of several chronic diseases and may involve excessive or inadequate angiogenesis. Prevention of excessive angiogenesis involves using drugs that inhibit the enzymes required for new blood vessel formation. Stimulation of new blood vessel growth with growth factors is being considered for therapeutic treatment of insufficient angiogenesis for patients with poor wound healing of skin ulcers.

Data from animal models and cell culture show that angiogenesis can be inhibited by naturally occurring physiologically active compounds, such as BBI. BBI is stable to gastrointestinal digestion and maintains biological activity even after boiling for 10 minutes. BBI is used in the soybean as a defense mechanism against insects, predators and bacterial and viral infections. BBI is a natural inhibitor of several metalloprotease enzymes that lead to the onset and progression of angiogenesis and diseases related to excess production of new blood vessels.

LSU AgCenter research has shown that BBI at concentrations found in a cup of soy milk can completely prevent the activation of metalloproteinases by binding to the enzymes. These enzymes can be maintained in an inactive form by the presence of low concentrations of BBI. The prevention of metalloprotein-ase activation preserves the integrity of the basal membrane and other tissues and may delay the onset and progression of diabetes complications, periodontal diseases, cancer, arthritis, osteoporosis, psoriasis and AIDS complications. Because treatments for many of these conditions may be costly and ineffective, good nutrition may be the key to prevention. Two advantages for promoting BBI as an inhibitor of angiogenesis are 1) BBI is not perceived as a medicine and has no known side effects in Asian populations who have consumed soy products for generations and appear to have low rates of chronic diseases and 2) food fortification with soybean BBI would be a relatively inexpensive way to deliver BBI over a lifespan.

BBI is a protein with health-enhancing properties that encompass a wide range of human ailments. Consumption of soy milk by the general population may provide health benefits in the intestinal tract and should be encouraged. A cup of soy milk a day may help keep the doctor away.

Soy and osteoporosis

The incidence of osteoporosis in this country is increasing as the population ages, leading to more hip fractures, pain, disability and death. One commonly recommended treatment is hormone replacement therapy for women at menopause. However, many women choose not to use hormone replacement because of the potential for side effects such as increased risk for breast cancer and leg thrombosis. Asian populations with a high soy intake have a substantially lower incidence of osteoporosis, suggesting that increasing our soy intake may be a beneficial alternative to hormone replacement therapy. Soy isoflavones are phyto-estrogens and may bind to estrogen receptors to maintain bone integrity after menopause without the detrimental side effects seen with hormone replacement treatments. Ovariectomized rats are being used as a model for postmenopausal women to study the potential of soy protein for maintaining bone mineral.

Several studies in the LSU AgCenter’s School of Human Ecology have confirmed that soy protein containing natural soy isoflavones can reduce the vertebral bone loss that normally occurs in ovariectomized rats. This research has shown that the protective effect of soy is dose responsive to the level of soy protein in the diet and is most beneficial when soy is part of a low-fat diet. In the most recent soy study, ovariectomized rats were fed a low-fat diet with 5 percent, 10 percent or 15 percent soy protein and compared to casein-fed rats with and without ovariectomy. The density of the vertebrae was measured at the end of the study. The benefits of soy protein on vertebral bone density were clearly seen in 10 percent and 15 percent soy fed rats. As the level of soy protein increased in the diets from 5 percent to 10 percent to 15 percent, the vertebral bone mineral density increased by 3 percent, 15 percent and 18 percent respectively, when compared to the ovariectomized control rats. This confirms the benefits of soy in reducing bone loss and osteoporosis.

In summary, soybeans are a powerful functional food that contains phytochemicals that can reduce the risk of chronic diseases. Work at the LSU AgCenter has focused on purifying and describing the characteristics of the active components of soybeans that can benefit health. Researchers are continuing to study soy products and the isolation of active components that can benefit all of us. Throughout history, people living in Asia have relied heavily on soybean products to prevent or fight disease. It is time for the western world to learn how to use soybeans effectively in our diets to maintain a healthier life.

(This article appear in the fall 2002 issue of Louisiana Agriculture.)