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# Effects of Soy on Health Outcomes

*Evidence Reports/Technology Assessments, No. 126*

Investigators: Ethan Balk, MD, MPH, Project Leader, Mei Chung, MPH, Research Associate, Priscilla Chew, MPH, Research Associate, Stanley Ip, MD, Investigator, Gowri Raman, MD, Research Associate, Bruce Kupelnick, BA, Research Associate, Athina Tatsioni, MD, Research Associate, Yannan Sun, DDS, Research Fellow, Brian Wolk, BA, Research Assistant, Deirdre DeVine, MLitt, Project Manager, and Joseph Lau, MD, Principal Investigator.

Agency for Healthcare Research and Quality (US)2005 Aug

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## Structured Abstract

#### Context:

Soy products, including both protein and isoflavones, have                been touted for a number of clinical benefits related to a putative estrogen-like                effect. However, potential risks of chronic soy consumption are also of concern.

#### Objectives:

Systematic review to describe the range of soy products and                outcomes that have been studied, to summarize the effects of soy consumption to                prevent a wide variety of medical conditions in healthy adults, and to summarize                adverse events related to soy consumption.

#### Data Sources:

We searched MEDLINE®, EMBASE, and the Commonwealth                Agricultural Bureau (CAB) databases. Additional studies were identified in                bibliographies of selected reviews and by technical experts.

#### Study Selection:

English-language, prospective studies of soy products                in adults, of at least 4 weeks' duration were included. We reviewed outcomes related                to cardiovascular health, menopausal symptoms and reproductive health, endocrine                function, tumor-related biomarkers, bone health, neurocognitive health, kidney                function, and glucose metabolism. Eligibility criteria were adjusted for several                outcomes.

#### Data Extraction:

Selected studies were extracted for study design,                demographics, amount of soy product, soy protein, and isoflavones, control,                outcomes. Based on these data, studies were graded for quality and                applicability.

#### Data Synthesis:

We screened almost 4,800 abstracts and retrieved 599                full text articles, of which 178 were eligible for review. Soy supplements                (including soy milk) were used in about three quarters of all the trials analyzed in                this report, with soy foods used in the remainder of the trials. Most used soy                protein with isoflavones, one-third used isoflavones alone, and a few used soy                protein without isoflavones. Textured soy protein and soy flour were the most common                soy foods investigated. Among studies with soy protein, the range of soy protein                consumed daily was 14 to 154 g, with a median of 36 g per day (equivalent to over a                pound of tofu daily). Among studies with soy isoflavones, the range of isoflavones                consumed daily was 10 to 185 mg, with a median of 80 mg per day. These ranges were                the same for all lipid profile studies. There is a large degree of heterogeneity                among the studies in terms of soy products evaluated, soy protein and isoflavone                doses, study durations, background diet, controls used, and study design. No study                evaluated clinical cardiovascular events. Meta-analysis indicates that consumption                of soy products appears to exert a small benefit on low density lipoprotein (LDL) -                the summary net change was -5 (95% confidence interval [CI] -8 to -3) mg/dL - and on                triglycerides - net change -8 (95% CI -11, -5) mg/dL. No significant effect was seen                on high density lipoprotein (HDL) - net change +0.6 (95% CI -0.5, +1.8). Across                studies, there is the possible suggestion that higher doses of soy protein are                associated with greater LDL reduction among those with elevated baseline LDL, but                not with HDL or triglycerides. Dose of isoflavones was not associated with effect                for any lipid. Higher baseline LDL or triglycerides may also be associated with net                effect for these 2 lipids; the effect of baseline HDL is unclear. In individual                studies, the effect of dose and baseline was generally inconsistent. Meta-analysis                of blood pressure (BP) found no effect of soy consumption. The net effect on                systolic BP was -1 (95% CI -3, +1) mm Hg, and on diastolic BP -1, (-2, +0) mm Hg. No                association was found between baseline BP, soy protein or isoflavone dose and effect                on BP. No significant effect of soy products was found for several markers of                inflammation, vascular function, or lipid oxidation. Although the effect of soy                products on menopausal symptoms are inconsistent across studies, the evidence of a                benefit was stronger from the randomized trials of soy isoflavone supplements, but                not of other soy products among post-menopausal women. This effect was not seen in                the few studies among peri-menopausal women or those treated for breast cancer. Soy                products do not appear to affect menstrual cycle length or estradiol level in                pre-menopausal women, thyroid stimulating hormone, bone markers, or glucose                metabolism. Small numbers of studies or inconsistency among studies precluded                drawing conclusions regarding the effect of soy protein on other endocrine markers                and other evaluated outcomes. For all outcomes, no soy protein or isoflavone                dose-effect response or soy product type difference in effect was apparent across                studies. The few studies that directly compared soy doses (generally isoflavone                doses) for the most part also found no difference in effect based on dose. In                general, soy products were well-tolerated, although study withdrawal due to aversion                was more common in soy treatment arms than control arms. The most common adverse                events reported were gastrointestinal or menstrual complaints although they were few                in number.

#### Conclusions:

A wide variety of soy products and formulations have been                investigated for a large number of conditions. However, a large proportion of the                studies suffer from poor reporting or study design, limiting conclusions. Soy                products appear to exert a small benefit on LDL and triglycerides; these effects may                be of small clinical effect in individuals, although possibly large enough to have a                population-wide effect. The inconsistent association between soy protein dose and                effect, and the lack of association between soy isoflavone dose and effect, limit                possible determination of an appropriate amount of soy product needed for lipid                reduction. Soy products may reduce menopausal symptoms in post-menopausal women. The                current literature does not support other effects of soy products. However, other                than menopausal- and menstrual-related symptoms, no clinical outcomes were                evaluated. The evidence from human studies does not suggest any worrisome adverse                events beyond mild gastrointestinal intolerance. Conclusions were often limited due                to small numbers of studies or heterogeneity across studies.

Given the large amount of heterogeneity and inadequate reporting, particularly                related to soy protein and isoflavone dose, many questions remain as to whether                specific soy products in adequate doses may be of benefit in specific populations.                Further, well-conducted studies are needed to clarify the effect of soy dose on                lipid parameters and to determine whether soy components other than protein or                isoflavones may be responsible for the lipid effects seen.

## Contents

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                        - Preface
                        - Acknowledgments
                        - Summary
                        - 1. Introduction                           - Metabolism and Mechanisms of Actions of Soy and Its Isoflavones
                          - Possible Roles of Soy and its Isoflavones in Influencing Diseases or					Conditions in Humans
                        - 2. Methods                           - Overview
                          - Key Questions Addressed in this Report
                          - Approach to Analyzing the Literature
                          - Literature Search Strategy
                          - Study Selection and Data Extraction
                          - Grading of the Evidence
                          - Reporting Results
                        - 3. Results                           - 3.1. Soy Products Used
                          - 3.2. Cardiovascular Events, Risk Factors, and Measures
                          - 3.3. Menopausal Symptoms
                          - Vasomotor Symptoms
                          - 3.4. Endocrine Function
                          - 3.5. Cancer and Tumor-Related Biomarkers
                          - 3.6. Osteoporosis and Osteoporosis Risk Factors
                          - 3.7. Reproductive Health
                          - 3.8. Miscellaneous Outcomes
                          - 3.9. Association of Dose and Product Type with Effects
                          - 3.10. Adverse Events Associated with Soy Intake
                          - Gastrointestinal Disturbances
                          - Menstrual Complaints
                          - Other Adverse Events
                          - Other Reports
                          - Drug Interactions
                        - 4. Discussion                           - Overview
                          - Main Findings
                          - Limitations
                          - Future Research
                        - Abbreviations
                        - Appendixes                           - Appendix A. Search Strategies
                          - Appendix B. Data Abstraction Form
                          - Appendix C. Evidence Tables
                          - Appendix D. Peer Reviewers
                        - References and Included Studies
                        - Bibliography
                        - List of Excluded Studies

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Prepared for: Agency for Healthcare Research and Quality, U.S. Department                of Health and Human Services.1                 Contract No. 290-02-0022.                Prepared by: Tufts-New England Medical                    Center Evidence-based Practice Center, Boston,                Massachusetts.

#### Suggested citation:

Balk E, Chung M, Chew P, Ip S, Raman G, Kupelnick B,                    Tatsioni A, Sun Y, Wolk B, DeVine D, Lau J. Effects of Soy on Health Outcomes.                    Evidence Report/Technology Assessment No. 126. (Prepared by Tufts-New England                    Medical Center Evidence-based Practice Center under Contract No. 290-02-0022.)                    AHRQ Publication No. 05-E024-2. Rockville, MD: Agency for Healthcare Research                    and Quality. August 2005.

This report is based on research conducted by the Tufts-New England Medical Center                Evidence-based Practice Center (EPC), under contract to the Agency for Healthcare                Research and Quality (AHRQ), Rockville, MD (Contract No. 290-02-0022). The findings                and conclusions in this document are those of the authors, who are responsible for                its contents; the findings and conclusions do not necessarily represent the views of                AHRQ. Therefore, no statement in this report should be construed as an official                position of AHRQ or of the U.S. Department of Health and Human Services.

The information in this report is intended to help health care decisionmakers,                patients and clinicians, health system leaders, and policymakers make well-informed                decisions and thereby improve the quality of health care services. This report is                not intended to be a substitute for the application of clinical judgment. Anyone who                makes decisions concerning the provision of clinical care should consider this                report as they would any medical reference and in conjunction with all other                pertinent information, i.e., in the context of available resources and circumstances                presented by individual patients.

This report may be used, in whole or in part, as the basis for development of                clinical practice guidelines and other quality enhancement tools, or as a basis for                reimbursement and coverage policies. Neither AHRQ's nor the U.S. Department of                Health and Human Services' endorsement of such derivative products may be stated or                implied.

                        - 1
                        - 
540 Gaither Road, Rockville, MD 20850. www​.ahrq.gov

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Bookshelf ID: NBK37881

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