Soy & Diabetes

The U.S. Centers for Disease Control (CDC) estimates that 18 million Americans have diabetes. With obesity reaching epidemic proportions in this country, that number continues to increase.

Studies show soy may have potential positive effects on diabetes and obesity. In addition, soy appears to positively impact cardiovascular health and kidney health, organs known to be particularly vulnerable to diabetes.

Glucose/Insulin Regulation

Maintaining optimal control of glucose and insulin levels is viewed as essential for overall good health, and specifically important for diabetics. Some evidence suggests that soy foods may be valuable in this regard. For example, a recent prospective study found that, among Chinese postmenopausal women with a body mass index of <25 kg/m², the risk of glycosuria was reduced by about two-thirds in high- versus low-soy consumers.¹

These findings are consistent with those from Ho and Chen, who found that among 173 postmenopausal women from Hong Kong habitual soy protein intake was inversely related to fasting serum glucose levels in women with baseline fasting glucose levels above the median.²

These observations may be related to the in vitro finding that the isoflavone genistein inhibits alpha glucosidase activity, which could aid in carbohydrate regulation and reduce serum glucose levels.³ However, no in vivo work verifying this finding has yet been published. A few human intervention studies suggest that soy protein or isoflavones increase insulin sensitivity, but many other studies do not.^4-11 At this point, no firm conclusions can be made.¹²

Still, it is important to note that soy foods have a low glycemic index.¹³ The glycemic index refers to the relative blood glucose response to carbohydrate-containing foods. Some evidence indicates that foods with a high glycemic index, because they cause greater rises in serum glucose and/or insulin levels, increase risk for variety of chronic diseases, including diabetes and obesity.¹⁴ Thus, the low glycemic index of soy foods suggests that they have a role to play in helping control diabetes and obesity.

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Diabetes Complications – Renal Disease

Increased rates of diabetes have resulted in a dramatic rise in the incidence of kidney, or renal, disease,^{15, 16} which is often a complication of diabetes. Studies have shown that the type and amount of protein, such as soy protein, can play an important role in renal function.

The impact of dietary protein on overall renal function and glomerular filtration rate (GFR) has been recognized for more than two decades; for this reason protein restriction is one approach to preventing further decline in renal function in renal disease patients.^17,18

Findings from the Nurses' Health Study support the benefits of protein restriction. Among women with mild renal insufficiency, those who consumed the most protein were more than threefold as likely to suffer at least a 15 percent decline in renal function over the 11 years of follow-up compared to women consuming the least amount of protein.¹⁹ (In healthy individuals, however, high-protein diets are unlikely to adversely affect kidney function.²⁰) In addition to total protein intake, there is considerable interest in the possibility that the type of protein consumed impacts kidney disease and, in this regard, there is considerable interest in soy protein.

One of the first human studies to suggest that soy protein might favorably affect renal function was published by Kontessis et al. in 1990.²¹ They found that when healthy subjects were challenged with an equivalent amount (80 g) of soy protein or meat protein, the soy protein (relative to meat) produced favorable effects on kidney function, including changes in GFR, renal plasma flow and the fractional clearance of albumin.

Two years later, when nephrotic patients were placed on a vegetarian diet in which essentially all of the protein was derived from soy, urinary protein and serum LDL-cholesterol levels decreased relative to the usual diet consumed.²² The results from this study are in agreement with those from a latter, similarly designed study by this group.²³ However, these studies have been criticized because the overall level (1.1 vs. 0.7 grams/day) of protein intake on the soy-vegetarian diet was lower than in the control diet.

Nevertheless, several additional studies, although not all, indicate that soy protein favorably affects renal function relative to animal protein.^24-26 This research suggests that unlike animal protein, soy protein does not increase postprandial GFR or renal blood flow.^{21, 27, 28}

Also, some research indicates that soy protein, when substituted for animal protein, decreases proteinuria in individuals with chronic renal disease.^29-31 Finally, the decrease in serum cholesterol discussed below may represent an additional indirect benefit of soy protein for renal disease patients, as some evidence indicates that elevated cholesterol exacerbates renal function.^{32, 33}

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Diabetes Complications – Coronary Heart Disease

As noted previously, people with diabetes are at a greatly increased risk of developing coronary heart disease (CHD). For example, in the Nurses' Health Study, over a 20-year period, women with diabetes were more than threefold as likely to die from CHD as were women without diabetes.³⁴ Similar data exist for men.³⁵ Several established risk factors for CHD appear to be favorably affected by the consumption of soy foods.

For example, soy protein modestly lowers serum levels of low-density lipoprotein cholesterol (LDL) (~5 percent) and triglycerides (~7 percent), and modestly raises levels of high-density lipoprotein levels (~3 percent).³⁶ Each of these effects will reduce CHD risk. In fact, over the course of many years each 1 percent reduction in LDL reduces CHD risk by as much as 2-4 percent.^37,38

There is also evidence that soy protein makes LDL less atherogenic³⁹ and that the isoflavones in soy beans directly improve the health of the coronary arteries by improving systemic arterial compliance and vascular reactivity.^40-44 More recent evidence suggests that soy protein may lower blood pressure.⁴⁵

Although the conflicting data in each of these areas prohibit firm conclusions, a prospective epidemiologic study of nearly 65,000 Shanghai women found that soy protein intake was associated with an 86 percent reduction in the risk of non-fatal myocardial infarction, lending support to the notion that soy exerts multiple coronary benefits.⁴⁶

Finally, full-fat soy foods, which are rich in the essential fatty acid linoleic acid, can help to lower serum cholesterol by displacing higher-saturated fat foods in the diet. Furthermore, the soy bean is one of the few good plant sources of the essential fatty acid, alpha-linolenic acid, an omega-3 fatty acid that has independent coronary benefits.⁴⁷ Certainly, on the basis of the established and possible coronary benefits, isoflavone-rich soy protein-based foods warrant a role in heart-healthy diets.

Visit the American Diabetes Association at www.diabetes.org for additional information on diabetes research.

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References

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2. Ho S, Chen Y. Habitual soy food consumption improves glycemic control among postmenopausal Chinese women: A one-year follow-up study (abstract). Sixth International Symposium on the Role of Soy in Preventing and Treating Chronic Disease (November; Chicago, IL) 2005.
3. Wang Y, Ma L, Li Z, Du Z, Liu Z, Qin J, Wang X, Huang Z, Gu L, Chen AS. Synergetic inhibition of metal ions and genistein on alpha-glucosidase. FEBS Lett 2004;576:46-50.
4. Wagner JD, Cefalu WT, Anthony MS, Litwak KN, Zhang L, Clarkson TB. Dietary soy protein and estrogen replacement therapy improve cardiovascular risk factors and decrease aortic cholesteryl ester content in ovariectomized cynomolgus monkeys. Metabolism 1997;46:698-705.
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44. Lissin LW, Oka R, Lakshmi S, Cooke JP. Isoflavones improve vascular reactivity in post-menopausal women with hypercholesterolemia. Vasc Med 2004;9:26-30.
45. He J, Gu D, Wu X, Chen J, Duan X, Whelton PK. Effect of soy bean protein on blood pressure: a randomized, controlled trial. Ann Intern Med 2005;143:1-9.
46. Zhang X, Shu XO, Gao YT, Yang G, Li Q, Li H, Jin F, Zheng W. Soy food consumption is associated with lower risk of coronary heart disease in Chinese women. J Nutr 2003;133:2874-2878.
47. Brouwer IA, Katan MB, Zock PL. Dietary alpha-linolenic acid is associated with reduced risk of fatal coronary heart disease, but increased prostate cancer risk: a meta-analysis. J Nutr 2004;134:919-922.