Dr. Prof. Mark Messina, PhD
Soyfoods Deserve a Larger Role in Western Diets
Soyfoods have played an important role in the diets of many Asian countries for centuries and have been embraced as a source of protein by vegetarians and other health-conscious individuals in non-Asian countries for several decades. But within the past 15 years, soyfoods and soybean constituents have attracted widespread research attention for a variety of purported health benefits, especially related to the prevention of several different chronic diseases. Much of this interest can be attributed to soyfoods being uniquely rich dietary sources of isoflavones, although the protein component of the soybean has also been the subject of much research.
It is however, important to recognize that on the basis of nutritional attributes alone – high-quality protein, low saturated fat content, good sources of both essential fatty acids and a variety of vitamins and minerals, it is clear that the soybean and foods made from this legume warrant a larger role in Western diets. This is especially true because essentially all nutritionists acknowledge the healthful properties of legumes. And yet, legume intake in Europe (1) and the United States (2) makes only a negligible contribution to overall protein intake. Among the various legumes, the soybean is an especially good choice because it is an extremely versatile legume that, as discussed below, may have some unique benefits.
The quality of soy protein is considered to be similar to animal proteins such as meat and milk protein although as discussed in the next section, unlike many animal foods soyfoods are low in saturated fat (3, 4). The protein in most soy products has a protein digestibility corrected amino acid score that approaches a score of 1.0, the highest possible rating, indicating that both amino acid pattern and digestibility of soy protein are excellent (5, 6). Furthermore, research suggests that digestibility of properly processed soy protein is likely to be good even in elderly subjects (7). Not unexpectedly, given the high quality of soy protein research shows that soy protein is able to promote muscle growth as well as animal proteins (8, 10). This is important because research suggests that muscle has an especially important role in maintaining overall health (9). Furthermore, some research suggests that soy protein may reduce oxidative stress, which may reduce fatigue and muscle injury associated with exercise (11, 12, 13, 14).
In Western countries average protein intake typically exceeds biologically requirements but often subsets of the populations consume inadequate amounts of this nutrient. For example, approximately one-third of women in the United States do not meet the recommended dietary allowance for protein; this figure is even higher for older women (15).
Finally, in addition to helping to meet the biological requirement for protein consuming soyfoods is an excellent way to increase the protein content of the diet as means of maintaining ideal weight. Research suggests that higher protein diets are more satiating and useful for weight management (16). Although high protein diets may increase risk of developing kidney disease in susceptible individuals (17), several studies indicate that soy protein favorably affects renal function in comparison to animal proteins (18, 19, 20, 21).
Fatty Acid Profile
The American Heart Association in the United States recently highlighted the important role that soyfoods can play in displacing higher-saturated-fat foods from the diet thereby helping to lower blood cholesterol levels (22). The soybean contains a heart-healthy balance of fatty acids, high in polyunsaturated fat (78% of total fat primarily in the form of linoleic acid) and low in saturated fat (15% of total fat) (23, 24). Of course, linoleic acid when substituted for saturated fat reduces blood cholesterol levels. Furthermore, the soybean is one of the few good plant sources of the essential fatty acid, alpha-linolenic acid (ALA) (25) an omega-3 fatty acid that may have independent coronary benefits (26). In a recent epidemiologic analysis conducted by Dutch researchers, high-consumers of ALA were 20% less like to die of coronary heart disease (CHD) than low-consumers of this fatty acid (26). Importantly, the difference in intake between the high and low ALA-consumers is an amount provided by just three cups of soymilk.
Chronic Disease Prevention
The estrogen-like effects of the soybean isoflavones prompted speculation about their skeletal benefits, although the nonhormonal properties of isoflavones may also be relevant to bone health (27). Epidemiologic studies generally show that soy intake is positively associated with bone mineral density (BMD) (28) among Asian women. Also, in the only epidemiologic study to include fractures as an end-point, relative risk was reduced by approximately one-third when comparing postmenopausal Chinese women in the 1st and 5th soy protein intake quintiles (29). Soy protein intake was especially beneficial among women in this study who were no more than 10 years postmenopausal.
More than 25 clinical trials have examined the impact of consuming soyfoods or soy isoflavones on BMD in postmenopausal women. The results from these trials, although somewhat inconsistent, generally show reduced bone loss in the soy or isoflavones groups in comparison to the control (28, 30, 31, 32, 33). The results from three large long-term US government-sponsored clinical trials may allow more definitive conclusions about the skeletal effects of isoflavones to be reached in the near future. Finally, aside from the possible skeletal benefits of isoflavones, soyfoods can contribute to bone health by providing high-quality protein. Evidence increasingly suggests that adequate protein intake is essential for optimal bone health (34, 35).
Coronary Heart Disease
Clinical research published over the past 40 years shows that soy protein directly lowers blood cholesterol. This effect is unrelated to the low saturated fat content of soyfoods. Official recognition of the cholesterol-lowering effects of soy protein came in 1999, when the US Food and Drug Administration (FDA) approved a health claim for soy protein and CHD (36). Health claims have subsequently been approved in eight other countries including the United Kingdom. Recent meta-analyses indicate that soy protein lowers plasma low density lipoprotein cholesterol between 3% and 5% (22, 37, 38, 39). According to British researchers, this degree of cholesterol reduction can reduce heart disease risk from 10-15% (40, 41). Furthermore, soy protein lowers serum triglycerides from 5 to 10% and raises high density lipoprotein cholesterol from 1 to 3% (22, 37, 38, 39). Both of these lipid effects can further help to reduce CHD risk (42-44).
In addition to these lipid effects, soyfoods may have a number of other coronary benefits. For example, there are speculative but intriguing data suggesting soyfoods reduce LDL particle size (45), enhance endothelial function (46, 47), and inhibit LDLC oxidation (48). The notion that soyfoods exert multiple coronary benefits is supported by results from a large prospective epidemiologic investigation involving ~65,000 postmenopausal women from Shanghai, China. After controlling for a host of dietary and lifestyle factors soy protein intake was associated with marked protection against nonfatal myocardial infarction (relative risk = 0.14; 95% confidence interval, 0.04-0.48); when comparing women in the highest vs. the lowest quartile of intake (P for trend = 0.001) (49). Especially worthy of note is that in this study the soy protein intake cutoff for the fourth quartile was only about 11 g/d, less than half the amount (25 g) established by the FDA as the threshold intake necessary for cholesterol reduction.
Mean daily adult Japanese soy protein intake is approximately 8 to 10 g (50), but there is epidemiologic evidence indicating the health status of Asian men and women who consume above average amounts of soy is superior to those who consume the average amount or less (51). On the basis of clinical and epidemiologic data and the importance of consuming a varied diet, a reasonable soy protein intake goal is 15 to 20 g per day. This amount of soy protein is provided by approximately two to three servings of traditional foods.
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