한국콩연구회

Effects of Soybean Protein on Human Health

John W. Erdman, Jr., Ph.D

A goal of Nutritional, medical and food sciences is to encourage, promote and provide for the consumption of nutritionally-balanced diets for all people. Clearly, the approach to achieve this goal must be different for populations of well-fed and for those who are under-nourished. That soy protein-based foods fit well into a balanced diet concept for both these distinctly different socioeconomic groups is the theme of this paper. a more detailed discussion of this subject has been published (Erdman & Fordyce, 1989).

Soy protein and soy-based foods contain a high quantity of protein with a very good amino acid profile. They also can contain significant quantities of digestible and indigestible(fiber) complex carbohydrates an array of minerals and vitamins. In full-fat products, which are appropriate for those populations that are short on calories and/or fat, soybean oil is high in polyunsaturated fatty acid content and is cholesterol-free. Poorly digested oligosaccharides that may cause flatulence can be removed by appropriate processing steps. Adequate thermal processing will obviate the growth depressing effects of trypsin inhibitor and other protease inhibitors present in uncooked soybeans. The possible long term effects of low intakes of residual trypsin inhibitors from foods by humans is not really known but is under active study(Friedman, 1986).

The heat stable phytic acid in soy and other legumes and cereals is of some concern, but soyfoods can be supplemented with certain trace minerals to override phytic acis’s mineral chelation. This is only needed in those cases where a soy protein food constitutes a high proportion of the diet.

The amino acid profile of soy protein is unusually well-rounded for a plant protein. In mosproducts, only the combined amount of the essential sulfur amino0acids( methionine plus cystine) falls below recommended patterns for humans. Moreover, the true digestibility of the protein can be expected to reach 92~100% in adequately processed soy products.

Various human studies have been performed to evaluate the quality of soy protein for adults, children and infants where soy (usually soy concentrate or soy isolates) was the only source of protein in the diet. For example, Kies & Fox(1971) compared the nitrogen balance of adults consuming textured soy protein (with or without methionine supplementation) with adults consuming ground beef. They found that the protein quality of beef appeared to be better than the supplemented and the non-supplemented soy protein at a low level of nitrogen intake (about 0.4g protein/kg body weight/day), while at higher total protein intake (about 0.7g protein/kg/day, a level considered appropriate for adults), both soy products and beef resulted in essentially the same nitrogen balances. A number of other studies also suggest that unsupplemented commercially available soy products can serve as the sole source of protein for adults.

Torum (1981) suggested that well-processed soy protein products were comparable to milk in protein quality for preschoolers and older children. Moreover, it is generally accepted that modern soy protein-based infant formulas promote growth to the same extent as cows milk-based formulas, and may be less allergenic (Brady et al., 1986). Premature infants, however, experience relative malsorption of fats and other nutrients secondary to immaturity. Soy-based formula is not recommended for low birth weight infants for periods longer than 2-3 weeks (Torum, 1981).

Plant protein sources in contrast to animal protein sources have generally been shown to be hypercholesterolemic in controlled feeding studies with animals and humans. Of the plant proteins studies, soy protein (usually an isolate, concentration or textured vegetable protein(TVP)) has received by far the greatest interest. For example, Sirtori and coworkers from the University of Milano, Italy in 1972 began studies on the effects of TVP feeding on serum lipids of type Ⅱ hyperlipoproteinemic patients. They found that when patients were switched from a standard low-lipid diet containing animal protein to a similar diet containing a TVP in place of animal protein, most patients demonstrated striking cholesterol lowering (Sirtori et al., 1983). Furthermore, the most marked reductions of plasma cholesterol were observed with the most severally hypercholesterolemic patients(Sirtori, 1983).

It can be concluded from these and other studies that soy protein substitution for animal protein sources generally does not affect blood lipid levels in persons with normal levels of cholesterol, but usually will reduce cholesterol when the patient have high blood levels of this steroid. The mechanism of soy’s cholesterol lowering is not yet clear. A variety of hypotheses have been suggested (Erdman & Fodyce, 1981) and are under extensive study.

Soybean polysaccharides derived from soy hull or cotyledon have also been reported to decrease plasma lipids in humans. Shorey and associates (1985) found that the consumption of soybean polysaccharide significantly reduced plasma cholesterol concentrations in individuals with mild to moderate hyper- cholesterolemia. Lo et al. (1986) further found that daily consumption of 25g of soy fiber resulted in significant reductions in plasma total and LDL-cholesterol in comparison to a standard low fat/low cholesterol diet.

Legumes have been shown to be particularly useful in decreasing blood glucose response in diabetic persons (Jenkins et al., 1980). Slow starch digestion in legumes is thought to contribute to this benificial effect. Although, soybean do not contain significant quantities of starch, soy fiber supplementation significantly reduced insulin response to oral glucose challenge by 20% in type Ⅱ-A hyper- cholesterolemic and by 16.5% in type Ⅳ hypertriglyceridic patients (Lo et al., 1986).

Much like other foods of plant origin, the bioavailability of minerals such as zinc and iron from many soy products is less than desirable. The relatively high abundance of minerals in soy partially overcomes this bioavailability concern.

Coearly, however, if a soy products (such as an infant formula) makes a significant caloric contribution to the overall diet, consideration should be given to mineral fortification and/or utilization of those processing techniques that result in optimal mineral bioavailability. This subject has been more extensively reviewed elsewhere(Forbes and Erdman, 1983, Erdman and Fordyce, 1989, Fordyce et al., 1987).

A variety of nutritious, low-cost soy protein products are available for the world market. When included as part of the diet, many of these products promote accepted concepts of good eating practices: they are low in saturated fat and can be low in total fat( if needed), they are low in sugar, devoid in cholesterol, and can be low in sodium. Appropriately processed soy foods have a good overall profile of amino acids and other nutrients. Other than for long-term feeding of premature infants, soy protein is of such high quality that it could serve as the sole protein source in the human diet. Soy protein and soy fiber have been shown to be beneficial for persons suffering from hypercholesterolemia and diabetes mellitus. Thus it appears that the nutrient profiles of a variety of adequately- processed soy protein products are appropriate for, and indeed are, a healthful addition to the human diet.

출처; 한국콩연구회 제 34호(1989.9.20.)