Skip to main content Accessibility help
×
Home

Information:

  • Access
  • Open access
  • Cited by 12

Actions:

      • Send article to Kindle

        To send this article to your Kindle, first ensure no-reply@cambridge.org is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below. Find out more about sending to your Kindle. Find out more about sending to your Kindle.

        Note you can select to send to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be sent to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.

        Find out more about the Kindle Personal Document Service.

        The impact of equol-producing status in modifying the effect of soya isoflavones on risk factors for CHD: a systematic review of randomised controlled trials
        Available formats
        ×

        Send article to Dropbox

        To send this article to your Dropbox account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your <service> account. Find out more about sending content to Dropbox.

        The impact of equol-producing status in modifying the effect of soya isoflavones on risk factors for CHD: a systematic review of randomised controlled trials
        Available formats
        ×

        Send article to Google Drive

        To send this article to your Google Drive account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your <service> account. Find out more about sending content to Google Drive.

        The impact of equol-producing status in modifying the effect of soya isoflavones on risk factors for CHD: a systematic review of randomised controlled trials
        Available formats
        ×
Export citation

Abstract

Recent studies suggest that the ability to produce equol, a metabolite of the soya isoflavone daidzein, is beneficial to coronary health. Equol, generated by bacterial action on isoflavones in the human gut, is biologically more potent than dietary sources of isoflavones. Not all humans are equol producers. We investigated whether equol-producing status is favourably associated with risk factors for CHD following an intervention by dietary soya isoflavones. We systematically reviewed randomised controlled trials (RCT) that evaluated the effect of soya isoflavones on risk factors for CHD and that reported equol-producing status. We searched PubMed, EMBASE, Ovid Medline and the Cochrane Central Register for Controlled Trials published up to April 2015 and hand-searched bibliographies to identify the RCT. Characteristics of participants and outcomes measurements were extracted and qualitatively analysed. From a total of 1671 studies, we identified forty-two articles that satisfied our search criteria. The effects of equol on risk factors for CHD were mainly based on secondary analyses in these studies, thus with inadequate statistical power. Although fourteen out of the forty-two studies found that equol production after a soya isoflavone intervention significantly improved a range of risk factors including cholesterol and other lipids, inflammation and blood pressure variables, these results need further verification by sufficiently powered studies. The other twenty-eight studies primarily reported null results. RCT of equol, which has recently become available as a dietary supplement, on CHD and its risk factors are awaited.

CHD is the leading cause of morbidity and mortality in the USA( 1 ) and worldwide( 2 ). Nutrition is an important determinant for the risk of developing CHD; poor dietary habits are estimated to account for 20 % of CHD cases in the US adult population( 1 ). Soya foods are a potential nutritional source for modifying biomarkers of CHD( 3 , 4 ). One of the main components of soya that may exert protective cardioprotective effects are isoflavones, bioactive phyto-oestrogens found in soyabeans( 3 ). The predominant soya isoflavones are genistein, daidzein and glycitein. Isoflavones may reduce the risk of CHD by: (1) their action via oestrogen receptor β, due to their structural similarity to oestradiol, leading to decreased vasodilation and inflammation( 4 7 ); (2) their antioxidant activity, which may prevent the oxidative damage to LDL-cholesterol (LDL-C) that contributes to atherogenesis( 8 ); and (3) modulating the vascular system, reducing atherosclerotic lesions and improving vascular reactivity and vascular stiffness( 9 , 10 ).

Although there are clear cardiovascular benefits of isoflavones in vitro and in animal studies( 9 , 11 ), the evidence in humans is conflicting( 12 14 ). A growing hypothesis is that the ability of humans to metabolise daidzein to equol, referred to as ‘equol producers’, may contribute to the protective effects of soya( 15 , 16 ). Equol has a greater affinity for oestrogen receptors than its precursor daidzein( 17 ), a longer half-life and bioavailability in plasma than daidzein and genistein( 3 , 18 ), and more potent antioxidant activity than any other isoflavone( 3 ). Therefore, the potential beneficial effects of soya isoflavones for CHD and its risk factors may be greater among equol producers. While all tested animals, including rodents and monkeys, can produce equol, not all humans have the gut microflora required to convert daidzein to equol, a bioactive metabolite( 15 , 19 ).

Equol is a promising candidate for hindering the initiation and progression of atherosclerosis due to its ability to induce vasorelaxation and its anti-inflammatory and antioxidant activity( 20 ). Specifically, it induces vasorelaxation through enhancing the production of endothelium nitric oxide synthase-derived NO( 21 ). It can also inhibit NO derived by inducible nitric oxide synthase, expressed by immune cells during host defence, which is linked to atherosclerosis development( 22 ). Furthermore, equol prevents lipid and lipoprotein peroxidation, a crucial process in the pathogenesis of atherosclerosis( 23 , 24 ).

The purpose of the present review is to examine if there is a difference in the cardioprotective effect of soya isoflavones in humans based on the hosts’ ability to produce equol. No previous reviews have thoroughly examined the impact of equol-producing status on risk factors for CHD. We conducted a comprehensive search of the scientific literature to identify randomised controlled trials (RCT) that evaluated the effects of soya isoflavones on risk factors for CHD and selected studies that included analyses based on equol producer status.

Methods

Literature search

The systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines( 25 ). We initially searched PubMed (1950 to April 2015), EMBASE through Embase.com (1966 to April 2015), Ovid Medline (1946 to April 2015) and the Cochrane Library (Cochrane Central Register of Controlled Trials, 1999 to April 2015) for papers in any language using one or more textual or medical subject heading (MESH) terms for isoflavones (isoflavones, isoflavonoids, genistein, daidzein, equol), risk factors for CHD (cardiovascular disease, coronary heart disease, myocardial infarction, lipids, low-density lipoprotein-cholesterol, triglyceride, lipoproteins, hypercholesterolemia, lipid metabolism, blood pressure, glucose, vital signs, arterial stiffness, vascular stiffness, intima-media thickness, inflammation, endothelial function, endothelium, adipocytes) and RCT (randomised control study, clinical trial, placebo, intervention studies, pilot projects, sampling studies, twin studies, prospective studies, double blind study, single blind study, epidemiologic research design). We reviewed the reference lists of the collected articles to identify additional potentially relevant papers not identified by the original keyword search.

Study selection

Studies were selected for the systematic review if they met the following criteria: (1) RCT; (2) full-text was published in English; (3) analysed adult subjects who ingested soya with isoflavones or isolated isoflavones as an intervention; (4) analysed traditional risk factors for CHD (including lipids, inflammatory, blood pressure, glycaemic and body composition variables) as outcome measurements; (5) determined the equol producer status of the participants; and (6) stratified the outcome measurements by equol producer status. The exclusion criteria included reviews or commentaries.

Data synthesis and quality assessment

Searching, data extraction and the quality assessment were completed by two authors independently according to the inclusion criteria. Discrepancies were resolved by consensus. For each RCT, extracted data included sample size, baseline characteristics of the participants (sex, mean age, health status, demographics, equol producer status), study design, treatment regimen (dose, duration, isoflavone content, and type of soya intervention), and the assessment of the risk factor(s) for CHD.

The quality of the RCT methodology was graded using a fourteen-point evaluation tool for controlled clinical trials developed by the National Heart, Lung, and Blood Institute( 26 ). Questions were answered with a ‘yes’, ‘no’, ‘not reported’, ‘cannot determine’ or ‘not applicable’. The evaluation was based on the primary outcome measurements of the RCT. The RCT were given an overall rating of ‘good’, ‘fair’ or ‘poor’ at the discretion of the reviewers based on the guidelines provided by this tool.

Results

Search results

A total of 1671 papers were collected and, of these, 829 were excluded because they were not RCT, did not measure the traditional risk factors for CHD, or were not published in English (Fig. 1). Of the remaining 247 papers screened, forty-two met the selection criteria for this review. An outline of our search strategy using PubMed is provided in Supplementary Table S1.

Fig. 1. Study flow diagram of screened, excluded and analysed publications.

Study characteristics

Study characteristics are summarised in Tables 1 and 2, and Supplementary Table S2. Thirty studies included only female participants( 14 , 27 55 ), eleven studies included both males and females( 56 66 ), and one study had only male participants( 67 ). Of the forty-one studies involving women, thirty-four included postmenopausal women only( 14 , 27 , 28 , 30 55 , 60 , 63 , 65 , 66 ). The age of the participants ranged from 27 to 73 years. Participants were hypercholesterolaemic in seven studies( 56 , 57 , 59 , 62 , 64 66 ), hyperlipidaemic in two studies( 58 , 66 ), prehypertensive or hypertensive in five studies( 39 41 , 55 , 59 ), had type 2 diabetes in two studies( 30 , 61 ), had the metabolic syndrome in two studies( 27 , 63 ), and considered healthy in twenty-three studies( 14 , 29 , 31 , 34 38 , 42 54 , 60 , 67 ). Diet interventions in nineteen studies used soya protein isolate with isoflavone flour, or powder, or tablets( 31 33 , 37 42 , 48 51 , 57 , 58 , 61 , 62 , 65 , 67 ), fifteen used soya- and isoflavone-enriched milk or foods( 14 , 27 , 30 36 , 46 , 47 , 55 57 , 59 , 63 66 ), and nine used isolated isoflavone tablets or capsules( 28 , 29 , 42 45 , 52 54 ), with Gardner et al. ( 57 ) using interventions that covered two categories. Isoflavone doses ranged from approximately 40 to 120 mg/d, with one dose particularly high at 900 mg/d( 44 ). Twenty-three studies examined cholesterol markers( 27 , 28 , 31 , 32 , 35 , 38 , 39 , 42 , 43 , 48 , 52 , 56 67 ), twenty-one examined other lipid variables( 27 , 28 , 31 , 32 , 35 , 38 , 39 , 42 , 43 , 47 , 48 , 52 , 56 , 58 , 59 , 61 , 62 , 64 67 ), eighteen examined blood pressure and vascular variables( 14 , 27 , 30 , 34 37 , 39 , 40 , 43 , 48 , 49 , 51 , 55 , 56 , 59 , 63 , 66 ), seventeen examined inflammatory markers( 27 , 33 , 34 , 42 , 44 46 , 48 , 50 , 53 , 54 , 56 , 62 , 63 , 65 67 ), ten examined glucose and insulin variables( 27 , 29 , 35 , 39 , 43 , 50 , 57 , 62 , 63 , 65 ) and five examined body composition variables( 27 , 41 , 43 , 65 , 66 ). There were numerous methods and standards used to distinguish equol producers from non-equol producers, including sampling from urine or serum, different threshold levels for differentiation, and various analytical techniques.

Table 1. Demographic and clinical characteristics of the participants in the randomised controlled trials (RCT) employing soya interventions and examining the effect of equol producer (EP) status on risk factors for CHD

M, male; F, female; NEP, non-equol producer; MetS, metabolic syndrome; CO, crossover; DB, double-blind; P, parallel; B, blinded; SB, single-blinded; HRT, hormone replacement therapy.

* The quality of the RCT were evaluated based on the main outcomes reported. RCT were given a score of ‘good’, ‘fair’ or ‘poor’ after appraising the degree to which flaws in the study designs could affect the validity of the results.

Studies that are or potentially using shared study participants.

Studies that are or potentially using shared study participants.

§ Studies that are or potentially using shared study participants.

Studies that are or potentially using shared study participants.

Studies that are or potentially using shared study participants.

** Studies that are or potentially using shared study participants.

†† Studies that are or potentially using shared study participants.

Table 2. Characteristics of the soya isoflavone interventions used in the randomised controlled trials examining the effect of equol producer (EP) status on the risk factors for CHD

TLC, therapeutic lifestyle changes; AG, aglycone; SPI, soya protein isolate; WB, whole bean soya; MPI, milk protein isolate; N/A, not applicable.

* Studies that are or potentially using shared study participants.

Studies that are or potentially using shared study participants.

Studies that are or potentially using shared study participants.

§ Studies that are or potentially using shared study participants.

Studies that are or potentially using shared study participants.

Studies that are or potentially using shared study participants.

** Studies that are or potentially using shared study participants.

Synthesis of results

We categorised both the effects of soya isoflavones and equol producer status on the examined CHD risk factors as beneficial, negligible, or adverse (Tables 3–8). We analysed each risk factor independently; therefore the RCT were potentially categorised more than once. Twenty-two studies found statistically significant improvements in the risk factors for CHD after the soya isoflavone intervention compared with placebo. Of these, equol producer status further improved risk factors for CHD in six studies (including LDL-C, TAG, systolic blood pressure, diastolic blood pressure, flow-mediated dilation, soluble intercellular adhesion molecule-1, platelet-selectin and C-reactive protein). Equol producer status was comparable to the soya intervention in sixteen studies (including total cholesterol, LDL-C, HDL-cholesterol (HDL-C), TAG, apoB, systolic blood pressure, diastolic blood pressure, nitrate and nitrite, systemic arterial compliance, peak flow velocity, aortic augmentation index and IL-6).

Table 3. Randomised clinical trial results reporting the effect of soya isoflavone interventions and equol producer (EP) status on cholesterol and other lipid parameters*

MetS, metabolic syndrome; NEP, non-equol producer; LDL-C, LDL-cholesterol; sdLDL-C, small dense LDL-C; Lp, lipoprotein; TC, total cholesterol; HDL-C, HDL-cholesterol; WB, whole bean soya; SPI, soya protein isolate; ABCA1, adenosine triphosphate-binding cassette A1; CD, conjugated diene formation; OxLDL-C, oxidised LDL-C.

* Results are first stratified by the impact of EP status and then the impact of the soya isoflavone interventions on each of the lipid risk factors.

+, Beneficial effect of soya isoflavones on risk factors of CHD; 0, negligible effect of soya isoflavones on risk factors of CHD; –, adverse effect of soya isoflavones on risk factors of CHD.

+, Beneficial effect of EP status on risk factors of CHD after soya intervention; 0, negligible effect of EP status on CHD risk factors after soya intervention; –, adverse effect of EP status on risk factors of CHD after soya intervention.

Table 4. Randomised clinical trial results reporting the effect of soya isoflavone interventions and equol producer (EP) status on blood pressure and vasculature parameters*

DBP, diastolic blood pressure; MetS, metabolic syndrome; NEP, non-equol producer; FMD, flow-mediated dilation; SBP, systolic blood pressure; LDL-C, LDL-cholesterol; BP, blood pressure; MAP, mean arterial pressure; PWV, carotid to femoral pulse wave velocity; NMD, nitroglycerine-mediated endothelium-independent vasodilation; NOx, nitrate and nitrite; ET-1, endothelin-1; SAC, systemic arterial compliance; AIx, augmentation index; PFV, peak flow velocity; vWF, von Willebrand factor; IAC, isobaric arterial compliance; SVR, systemic vascular resistance; CIMT, carotid artery intima-media thickness; HDL-C, HDL-cholesterol; EFI, endothelial function index.

* Results are first stratified by the impact of EP status and then the impact of the soya isoflavone interventions on each of the lipid risk factors.

+, Beneficial effect of soya isoflavones on risk factors of CHD; 0, negligible effect of soya isoflavones on risk factors of CHD; –, adverse effect of soya isoflavones on risk factors of CHD.

+, Beneficial effect of EP status on risk factors of CHD after soya intervention; 0, negligible effect of EP status on CHD risk factors after soya intervention; –, adverse effect of EP status on risk factors of CHD after soya intervention.

Table 5. Randomised clinical trial results reporting the effect of soya isoflavone interventions and equol producer (EP) status on inflammation and DNA damage parameters*

CRP, C-reactive protein; sICAM-1, soluble intercellular adhesion molecule-1; MetS, metabolic syndrome; NEP, non-equol producer; hsCRP, high-sensitivity C-reactive protein; MDA, malondialdehyde; P-selectin, platelet selectin; Hcy, homocysteine; E-selectin, endothelial selectin; VCAM-1, vascular cell adhesion molecule 1; ICAM-1, intracellular adhesion molecule-1; MCP-1, monocyte chemoattractant protein-1; AP-site, apurinic/apyrimidinic site; TUNEL, terminal deoxynucleotidyl transferase dUTP nick end labelling; ADMA, asymmetric dimethylarginine.

* Results are first stratified by the impact of EP status and then the impact of the soya isoflavone interventions on each of the lipid risk factors.

+, Beneficial effect of soya isoflavones on risk factors of CHD; 0, negligible effect of soya isoflavones on risk factors of CHD; –, adverse effect of soya isoflavones on risk factors of CHD.

+, Beneficial effect of EP status on risk factors of CHD after soya intervention; 0, negligible effect of EP status on CHD risk factors after soya intervention; –, adverse effect of EP status on risk factors of CHD after soya intervention.

Table 6. Randomised controlled trial results reporting the effect of soya isoflavone interventions and equol producer (EP) status on glucose and insulin parameters*

IGF, insulin-like growth factor; IGF-BP1, insulin-like growth factor binding protein-1; SHBG, sex hormone binding globulin; IGF-BP3, insulin-like growth factor binding protein-3.

* Results are first stratified by the impact of EP status and then the impact of the soya isoflavone interventions on each of the lipid risk factors.

+, Beneficial effect of soya isoflavones on risk factors of CHD; 0, negligible effect of soya isoflavones on risk factors of CHD; –, adverse effect of soya isoflavones on risk factors of CHD.

+, Beneficial effect of EP status on risk factors of CHD after soya intervention; 0, negligible effect of EP status on CHD risk factors after soya intervention; –, adverse effect of EP status on risk factors of CHD after soya intervention.

Table 7. Randomised controlled trial results reporting the effect of soya isoflavone interventions and equol producer (EP) status on body composition variables*

BW, body weight.

* Results are first stratified by the impact of EP status and then the impact of the soya isoflavone interventions on each of the lipid risk factors.

+, Beneficial effect of soya isoflavones on risk factors of CHD; 0, negligible effect of soya isoflavones on risk factors of CHD; –, adverse effect of soya isoflavones on risk factors of CHD.

+, Beneficial effect of EP status on risk factors of CHD after soya intervention; 0, negligible effect of EP status on CHD risk factors after soya intervention; –, adverse effect of EP status on risk factors of CHD after soya intervention.

Table 8. Results of the randomised clinical trials examining the effect of soya isoflavone interventions on the risk factors for CHD in equol producers (EP) only

LDL-C, LDL-cholesterol; HDL-C, HDL-cholesterol; hsCRP, high-sensitivity C-reactive protein; TC, total cholesterol; CIMT, carotid artery intima-media thickness; DBP, diastolic blood pressure; SBP, systolic blood pressure; MAP, mean arterial pressure; FMD, flow-mediated dilation; BW, body weight; NEP, non-equol producers; TLR4, Toll-like receptor 4; TIRAP, toll–interleukin 1 receptor domain-containing adaptor protein.

* +, Beneficial effect of EP status on risk factors of CHD after soya intervention; 0, negligible effect of EP status on CHD risk factors after soya intervention; –, adverse effect of EP status on risk factors of CHD after soya intervention.

Forty studies found no association between soya isoflavones and risk factors for CHD compared with placebo. Of these, equol producer status significantly improved risk factors for CHD in seven studies (including total cholesterol, LDL-C, TAG, apoA-I, apoB, lipoprotein (a), blood pressure, diastolic blood pressure, mean arterial pressure, carotid to femoral pulse wave velocity). As with the soya isoflavone intervention, equol producer status was insignificant in thirty-two studies and was adverse in one study.

Three studies found that soya isoflavones had a negative effect on the risk factors of CHD. Of these, equol producer status was negligible in two studies and magnified the adverse outcomes of the soya isoflavone intervention in one study (isoprostane excretion). Equol producer status was also associated with the adverse outcome of an increase in insulin-like growth factor binding protein-3.

Five studies were comprised of participants who were all equol producers (Table 8); two of the studies found statistically significant beneficial effects of the isoflavone interventions on risk factors of CHD (including LDL-C, high-sensitivity C-reactive protein, TAG, inflammatory gene expression) while four studies observed negligible effects.

The RCT varied in quality, with the overall scores provided in Table 1 and the ratings summarised in Supplementary Table S3. Failure to report sample size calculations, details on the randomisation and allocation concealment procedures, and lack of intention-to-treat analyses or other suitable statistical method of dealing with participant drop-out were the most frequent flaws. Six RCT were given a ‘good’ rating, twenty were given a ‘fair’ rating and sixteen were given a ‘poor’ rating.

The heterogeneity of the studies in terms of populations, treatment regimens, intended duration, and outcomes prevented us from quantitatively synthesising the evidence in the form of a meta-analysis. Besides, the total number of participants included in all forty-two of the studies together was 3796, which, along with varying interventions and populations, probably provides insufficient statistical power to quantitatively measure the effect of dietary interventions. Further, most of these forty-two studies were small and had fewer than fifty participants, and only eighteen out of the forty-two studies qualified to be ‘fair’ or ‘good’ quality. The six ‘good’-quality papers (Hodis et al. ( 14 ); Liu et al. ( 39 41 ); van der Velpen et al. ( 53 , 54 )) come from three different trials – while the Hodis study examined carotid artery intima-media thickness progression among equol producers and non-producers, Liu et al. and van der Velpen et al. examined their intervention only among equol producers. Liu et al. examined the effect of soya on risk factors such as lipid markers( 39 41 ), while van der Velpen et al. ( 53 , 54 ) examined the effect of soya on the expression of inflammatory genes. Given these varying outcomes, we have chosen to not perform a meta-analysis in our present review.

Discussion

Though the overall effect of equol producer status during a dietary soya intervention on risk factors of CHD is inconclusive, we found evidence of a favourable effect of equol producer status in fourteen of the forty-two studies( 27 , 30 , 35 , 39 , 42 , 45 , 50 , 54 56 , 59 , 61 , 66 , 67 ) regardless of the success of the soya intervention. Equol production was associated with positive changes in cholesterol( 35 , 39 , 42 , 56 , 59 , 61 , 66 , 67 ) and other lipid variables( 27 , 35 , 39 , 56 , 59 , 61 , 66 ), blood pressure measurements( 27 , 30 , 55 , 56 ) and inflammatory markers( 27 , 39 , 45 , 50 , 54 , 56 ). The effect of equol producer status was insignificant on CHD risk factors in forty studies( 14 , 27 46 , 48 67 ) and adverse in two studies( 29 , 47 ). We did not find consistent evidence of equol production affecting specific risk factors for CHD. The heterogeneity of the CHD risk factors analysed, sample size, study designs and quality, and definition of equol producers prevented quantitative synthesis of the results.

The majority of the studies in the present review retrospectively categorised study participants by equol producer status and conducted a secondary analysis of the effect of equol on the risk factors for CHD. Therefore, these RCT were very unlikely to be sufficiently powered to detect a difference in CHD risk factors between equol producers and non-equol producers. We identified ten studies with study designs that included enrolment criteria based on equol producer status( 28 , 39 41 , 49 54 ). Of these, three found equol producer status improved several CHD risk factors (LDL-C, LDL-C:HDL-C, TAG, platelet-selectin and inflammatory gene expression) after the soya intervention( 39 , 50 , 54 ) while the remaining associations measured in the RCT were negligible.

There are numerous differences in the experimental design of the RCT that could explain the inconsistency in the outcomes. The isoflavone dose ranged in both quantity and consistency between RCT. In particular, the amount of daidzein in the intervention formulations, which indicates the magnitude of equol that could be metabolised from daidzein and bioavailable in equol producers, largely varied between studies. Additionally, the duration and frequency of exposure to the intervention were inconsistent. Curtis et al. ( 30 ) found that improvements in blood pressure, mean arterial pressure, and pulse wave velocity measures in equol producers were seen after 1 year but not at 6 months, suggesting that long-term exposure to isoflavones may be more beneficial.

The criteria used to define equol producers differed across the RCT included in our review, with variability in the biological samples used to measure equol, the concentration cut-offs selected to distinguish equol producers from non-equol producers, and the analytical methods used to measure equol. Setchell & Cole( 68 ) proposed classifying equol producers by a threshold log10-transformed ratio of S-(-)equol, a diastereoisomer of equol produced by the intestinal bacteria in humans, to its precursor daidzein of −1·75 in urine after a 3 d soya isoflavone challenge. This accounts for inconsistency in the technical measurements of equol and avoids classifying equol producers based on absolute measurements of equol, which exhibit greater variability( 68 ). Nine studies used this approach( 39 54 , 56 , 62 , 64 , 66 ), with four finding a beneficial effect of equol producer status on risk factors of CHD( 39 , 54 , 56 , 66 ) and eight finding a negligible effect( 39 41 , 53 , 56 , 62 , 64 , 66 ).

Further complicating the interpretation of the data are the potential sex differences in the metabolism of soya( 69 ), which could affect the bioavailability of isoflavone metabolites between males and females. In a meta-analysis examining the effects of soya isoflavones on lipids, subjects with hypercholesterolaemia had greater reductions in men than in women( 12 ). While there were studies of mixed sex (n 11) or of only males (n 1), the present review consisted primarily of female-only RCT, which may have masked the effects of equol producer status on the outcome measurements. Nestel et al. ( 60 ) found that LDL-C was significantly reduced after supplementation with biochanin (a precursor of genistein) compared with placebo (P = 0·026); when results were stratified based on sex, males showed a significant reduction in median LDL-C levels of 9·5 % while females had no measurable difference. Equol producer status did not further reduce LDL-C, which the authors speculated was due to the small sample size of fifteen equol producers, with seven included in the biochanin intervention group( 60 ).

The source of soya may also contribute to the variability in its effectiveness. The type of processing used for soya products during production can affect the isoflavone content( 13 ) and modify other components of soya( 70 ). Additionally, soya protein isolate primarily contains isoflavone glucosides while fermented soya foods contain isoflavones mainly in the aglycone form( 15 , 71 ). Isoflavone aglycones are absorbed more efficiently than isoflavone glucosides in humans and may therefore be more effective in CHD prevention( 72 ). Daidzein in the aglycone form is also more readily converted to equol( 15 ). Clerici et al. ( 56 ) found that pasta enriched in isoflavone aglycones significantly reduced total cholesterol, LDL-C, high-sensitivity C-reactive protein, and arterial stiffness compared with placebo in study participants, with effects more pronounced in equol producers. Of the fourteen RCT that found a positive association between equol producer status and CHD risk factors, seven used interventions of foods and milk enriched with soya( 27 , 30 , 35 , 55 , 56 , 59 , 66 ).

Furthermore, baseline age and the health status of the participants may contribute to variability in the outcome measurements. Oestrogen receptor β has been found to be enhanced in extracted arteries from postmenopausal CHD patients compared with normal subjects, with enhanced dilation in response to isoflavones( 73 ). Hodis et al. found that isoflavone supplementation failed to prevent the progression of subclinical atherosclerosis in healthy postmenopausal women overall; a subanalysis indicated, however, that healthy women within 5 years of becoming postmenopausal had a significantly reduced mean carotid artery intima-media thickness progression rate of 68 % compared with placebo( 74 ). Previous meta-analyses have also found lipid variables to be more positively affected by soya interventions in hypercholesterolaemic patients than in healthy subjects( 12 , 75 ). We identified thirty-five RCT that only used postmenopausal women; all of the studies that found a favourable association of equol producer status on risk factors of CHD had postmenopausal participants. There were a relatively equal number of RCT using healthy participants (n 20) v. participants with underlying health issues or a history of illness (n 22); of the fourteen studies that found equol producer status to improve risk factors for CHD, five had healthy participants( 35 , 42 , 45 , 54 , 67 ) while nine had participants with underlying health issues related to CHD( 27 , 30 , 39 , 50 , 55 , 56 , 59 , 61 , 66 ).

In the present systematic review, electronic databases were extensively searched following our defined set of guidelines and used to extract relevant data. Our results may imply that equol is beneficial on cardiovascular health, yet the interpretation is limited largely because of the secondary analysis of equol producers in RCT of dietary sources of isoflavones. Recently, equol itself has become available as a dietary supplement. Orally administered equol has greater plasma accumulation than other dietary sources of isoflavones( 76 ) and has the potential for enhanced therapeutic effects due to its more potent antioxidant properties and bioactivity among all isoflavones. In fact, one RCT of equol on risk factors of CHD has been conducted. Usui et al. ( 77 ) found a statistically significant improvement in LDL-C, glycated HbA1c levels, and cardio-ankle vascular index scores, a measure of vascular stiffness, in overweight and obese patients after dietary equol supplementation, particularly for non-equol producers. This study is limited by its small sample size and short duration of the intervention. We recommend additional RCT of equol itself as an intervention to directly assess its effects on CHD risk factors and potentially CHD.

Supplementary material

The supplementary material for this article can be found at http://dx.doi.org/10.1017/jns.2016.18

Acknowledgements

We thank Barb Folb for her expertise and guidance in designing the systematic review.

The present review was supported by the National Institutes of Health (R01 HL068200).

The authors’ responsibilities were as follows: A. S. designed the study; R. L. B. conducted the research; A. S., A. V. and R. L. B. analysed the data; A. S., V. A., A. V., R. W. E., Y. M., K. M., T. U. and R. L. B. drafted the manuscript; A. S., V. A. and R. L. B. had primary responsibility for the final content of the manuscript; and all authors read and approved the final manuscript.

None of the authors reported a conflict of interest related to the present review.

References

1. Dalen, JE & Devries, S (2014) Diets to prevent coronary heart disease 1957–2013: what have we learned? Am J Med 127, 364369.
2. World Health Organization (2014) The top 10 causes of death. http://www.who.int/mediacentre/factsheets/fs310/en/ (accessed April 2015).
3. Setchell, KD, Brown, NM & Lydeking-Olsen, E (2002) The clinical importance of the metabolite equol – a clue to the effectiveness of soy and its isoflavones. J Nutr 132, 35773584.
4. Matori, H, Umar, S, Nadadur, RD, et al. (2012) Genistein, a soy phytoestrogen, reverses severe pulmonary hypertension and prevents right heart failure in rats. Hypertension 60, 425430.
5. Tempfer, CB, Bentz, EK, Leodolter, S, et al. (2007) Phytoestrogens in clinical practice: a review of the literature. Fertil Ster 87, 12431249.
6. Deodato, B, Altavilla, D, Squadrito, G, et al. (1999) Cardioprotection by the phytoestrogen genistein in experimental myocardial ischaemia–reperfusion injury. Br J Pharmacol 128, 16831690.
7. Mahmoud, AM, Yang, W & Bosland, MC (2014) Soy isoflavones and prostate cancer: a review of molecular mechanisms. J Steroid Biochem Mol Biol 140, 116132.
8. Lukito, W (2001) Candidate foods in the Asia-Pacific region for cardiovascular protection: nuts, soy, lentils and tempe. Asia Pac J Clin Nutr 10, 128133.
9. Cano, A, Garcia-Perez, MA & Tarin, JJ (2010) Isoflavones and cardiovascular disease. Maturitas 67, 219226.
10. Erdman, JW Jr (2000) AHA Science Advisory: Soy protein and cardiovascular disease: a statement for healthcare professionals from the Nutrition Committee of the AHA. Circulation 102, 25552559.
11. Mann, GE, Bonacasa, B, Ishii, T, et al. (2009) Targeting the redox sensitive Nrf2-Keap1 defense pathway in cardiovascular disease: protection afforded by dietary isoflavones. Curr Opin Pharmacol 9, 139145.
12. Zhan, S & Ho, SC (2005) Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile. Am J Clin Nutr 81, 397408.
13. Sacks, FM, Lichtenstein, A, Van Horn, L, et al. (2006) Soy protein, isoflavones, and cardiovascular health: an American Heart Association Science Advisory for professionals from the Nutrition Committee. Circulation 113, 10341044.
14. Hodis, HN, Mack, WJ, Kono, N, et al. (2011) Isoflavone soy protein supplementation and atherosclerosis progression in healthy postmenopausal women: a randomized controlled trial. Stroke 42, 31683175.
15. Setchell, KD & Clerici, C (2010) Equol: history, chemistry, and formation. J Nutr 140, 1355S1362S.
16. Rowland, I, Wiseman, H, Sanders, T, et al. (1999) Metabolism of oestrogens and phytoestrogens: role of the gut microflora. Biochem Soc Trans 27, 304308.
17. Muthyala, RS, Ju, YH, Sheng, S, et al. (2004) Equol, a natural estrogenic metabolite from soy isoflavones: convenient preparation and resolution of R- and S-equols and their differing binding and biological activity through estrogen receptors α and β. Bioorg Med Chem 12, 15591567.
18. Kelly, GE, Joannou, GE, Reeder, AY, et al. (1995) The variable metabolic response to dietary isoflavones in humans. Proc Soc Exp Biol Med 208, 4043.
19. Atkinson, C, Frankenfeld, CL & Lampe, JW (2005) Gut bacterial metabolism of the soy isoflavone daidzein: exploring the relevance to human health. Exp Biol Med (Maywood) 230, 155170.
20. Setchell, KD & Clerici, C (2010) Equol: pharmacokinetics and biological actions. J Nutr 140, 1363S1368S.
21. Ohkura, Y, Obayashi, S, Yamada, K, et al. (2015) S-equol partially restored endothelial nitric oxide production in isoflavone-deficient ovariectomized rats. J Cardiovascular Pharmacol 65, 500507.
22. Kang, JS, Yoon, YD, Han, MH, et al. (2007) Equol inhibits nitric oxide production and inducible nitric oxide synthase gene expression through down-regulating the activation of Akt. Int Immunopharmacol 7, 491499.
23. Arora, A, Nair, MG & Strasburg, GM (1998) Antioxidant activities of isoflavones and their biological metabolites in a liposomal system. Arch Biochem Biophys 356, 133141.
24. Hodgson, JM, Croft, KD, Puddey, IB, et al. (1996) Soybean isoflavonoids and their metabolic products inhibit in vitro lipoprotein oxidation in serum. J Nutr Biochem 7, 664669.
25. Liberati, A, Altman, DG, Tetzlaff, J, et al. (2009) The PRISMA statement for reporting systematic reviews and meta-analyses of studies that evaluate health care interventions: explanation and elaboration. J Clin Epidemiol 62, e1e34.
26. National Heart, Lung, and Blood Institute, National Institutes of Health (2014) Quality assessment of controlled intervention studies. Systematic evidence reviews and clinical practice guidelines. http://www.nhlbi.nih.gov/health-pro/guidelines/in-develop/cardiovascular-risk-reduction/tools/rct (accessed July 2015).
27. Acharjee, S, Zhou, JR, Elajami, TK, et al. (2015) Effect of soy nuts and equol status on blood pressure, lipids and inflammation in postmenopausal women stratified by metabolic syndrome status. Metabolism 64, 236243.
28. Badeau, R, Jauhiainen, M, Metso, J, et al. (2007) Effect of isolated isoflavone supplementation on ABCA1-dependent cholesterol efflux potential in postmenopausal women. Menopause 14, 293299.
29. Campbell, MJ, Woodside, JV, Honour, JW, et al. (2004) Effect of red clover-derived isoflavone supplementation on insulin-like growth factor, lipid and antioxidant status in healthy female volunteers: a pilot study. Eur J Clin Nutr 58, 173179.
30. Curtis, PJ, Potter, J, Kroon, PA, et al. (2013) Vascular function and atherosclerosis progression after 1 y of flavonoid intake in statin-treated postmenopausal women with type 2 diabetes: a double-blind randomized controlled trial. Am J Clin Nutr 97, 936942.
31. Gallagher, JC, Satpathy, R, Rafferty, K, et al. (2004) The effect of soy protein isolate on bone metabolism. Menopause 11, 290298.
32. Greany, KA, Nettleton, JA, Wangen, KE, et al. (2004) Probiotic consumption does not enhance the cholesterol-lowering effect of soy in postmenopausal women. J Nutr 134, 32773283.
33. Greany, KA, Nettleton, JA, Wangen, KE, et al. (2008) Consumption of isoflavone-rich soy protein does not alter homocysteine or markers of inflammation in postmenopausal women. Eur J Clin Nutr 62, 14191425.
34. Hall, WL, Vafeiadou, K, Hallund, J, et al. (2005) Soy-isoflavone-enriched foods and inflammatory biomarkers of cardiovascular disease risk in postmenopausal women: interactions with genotype and equol production. Am J Clin Nutr 82, 12601268; quiz 1365–1266.
35. Hall, WL, Vafeiadou, K, Hallund, J, et al. (2006) Soy-isoflavone-enriched foods and markers of lipid and glucose metabolism in postmenopausal women: interactions with genotype and equol production. Am J Clin Nutr 83, 592600.
36. Hallund, J, Bugel, S, Tholstrup, T, et al. (2006) Soya isoflavone-enriched cereal bars affect markers of endothelial function in postmenopausal women. Br J Nutr 95, 11201126.
37. Kreijkamp-Kaspers, S, Kok, L, Bots, ML, et al. (2005) Randomized controlled trial of the effects of soy protein containing isoflavones on vascular function in postmenopausal women. Am J Clin Nutr 81, 189195.
38. Kreijkamp-Kaspers, S, Kok, L, Grobbee, DE, et al. (2004) Effect of soy protein containing isoflavones on cognitive function, bone mineral density, and plasma lipids in postmenopausal women: a randomized controlled trial. JAMA 292, 6574.
39. Liu, ZM, Ho, SC, Chen, YM, et al. (2014) Whole soy, but not purified daidzein, had a favorable effect on improvement of cardiovascular risks: a 6-month randomized, double-blind, and placebo-controlled trial in equol-producing postmenopausal women. Mol Nutr Food Res 58, 709717.
40. Liu, ZM, Ho, SC, Chen, YM, et al. (2015) Effect of whole soy and purified daidzein on ambulatory blood pressure and endothelial function-a 6-month double-blind, randomized controlled trial among Chinese postmenopausal women with prehypertension. Eur J Clin Nutr 69, 11611168.
41. Liu, ZM, Ho, SC, Chen, YM, et al. (2013) A six-month randomized controlled trial of whole soy and isoflavones daidzein on body composition in equol-producing postmenopausal women with prehypertension. J Obes 2013, 359763.
42. Mangano, KM, Hutchins-Wiese, HL, Kenny, AM, et al. (2013) Soy proteins and isoflavones reduce interleukin-6 but not serum lipids in older women: a randomized controlled trial. Nutr Res 33, 10261033.
43. Nikander, E, Tiitinen, A, Laitinen, K, et al. (2004) Effects of isolated isoflavonoids on lipids, lipoproteins, insulin sensitivity, and ghrelin in postmenopausal women. J Clin Endocrinol Metab 89, 35673572.
44. Pop, EA, Fischer, LM, Coan, AD, et al. (2008) Effects of a high daily dose of soy isoflavones on DNA damage, apoptosis, and estrogenic outcomes in healthy postmenopausal women: a phase I clinical trial. Menopause 15, 684692.
45. Pusparini, Y & Hidayat, A (2015) Effect of soy isoflavone supplementation on endothelial dysfunction and oxidative stress in equol-producing postmenopausal women. Endocr Metab Immune Disord Drug Targets 15, 7179.
46. Reimann, M, Dierkes, J, Carlsohn, A, et al. (2006) Consumption of soy isoflavones does not affect plasma total homocysteine or asymmetric dimethylarginine concentrations in healthy postmenopausal women. J Nutr 136, 100105.
47. Sen, C, Morimoto, Y, Heak, S, et al. (2012) Soy foods and urinary isoprostanes: results from a randomized study in premenopausal women. Food Funct 3, 517521.
48. Steinberg, FM, Guthrie, NL, Villablanca, AC, et al. (2003) Soy protein with isoflavones has favorable effects on endothelial function that are independent of lipid and antioxidant effects in healthy postmenopausal women. Am J Clin Nutr 78, 123130.
49. Törmälä, R, Appt, S, Clarkson, TB, et al. (2008) Equol production capability is associated with favorable vascular function in postmenopausal women using tibolone; no effect with soy supplementation. Atherosclerosis 198, 174178.
50. Törmälä, R, Appt, S, Clarkson, TB, et al. (2008) Impact of soy supplementation on sex steroids and vascular inflammation markers in postmenopausal women using tibolone: role of equol production capability. Climacteric 11, 409415.
51. Törmälä, RM, Appt, S, Clarkson, TB, et al. (2007) Individual differences in equol production capability modulate blood pressure in tibolone-treated postmenopausal women: lack of effect of soy supplementation. Climacteric 10, 471479.
52. Törmälä, RM, Nikander, E, Tiitinen, A, et al. (2006) Serum cholesterol efflux potential in postmenopausal women treated with isolated isoflavones. Menopause 13, 96101.
53. van der Velpen, V, Geelen, A, Hollman, PC, et al. (2014) Isoflavone supplement composition and equol producer status affect gene expression in adipose tissue: a double-blind, randomized, placebo-controlled crossover trial in postmenopausal women. Am J Clin Nutr 100, 12691277.
54. van der Velpen, V, Geelen, A, Schouten, EG, et al. (2013) Estrogen receptor-mediated effects of isoflavone supplementation were not observed in whole-genome gene expression profiles of peripheral blood mononuclear cells in postmenopausal, equol-producing women. J Nutr 143, 774780.
55. Welty, FK, Lee, KS, Lew, NS, et al. (2007) Effect of soy nuts on blood pressure and lipid levels in hypertensive, prehypertensive, and normotensive postmenopausal women. Arch Int Med 167, 10601067.
56. Clerici, C, Setchell, KD, Battezzati, PM, et al. (2007) Pasta naturally enriched with isoflavone aglycons from soy germ reduces serum lipids and improves markers of cardiovascular risk. J Nutr 137, 22702278.
57. Gardner, CD, Messina, M, Kiazand, A, et al. (2007) Effect of two types of soy milk and dairy milk on plasma lipids in hypercholesterolemic adults: a randomized trial. J Am Coll Nutr 26, 669677.
58. Ma, Y, Chiriboga, D, Olendzki, BC, et al. (2005) Effect of soy protein containing isoflavones on blood lipids in moderately hypercholesterolemic adults: a randomized controlled trial. J Am Coll Nutr 24, 275285.
59. Meyer, BJ, Larkin, TA, Owen, AJ, et al. (2004) Limited lipid-lowering effects of regular consumption of whole soybean foods. Ann Nutr Metab 48, 6778.
60. Nestel, P, Cehun, M, Chronopoulos, A, et al. (2004) A biochanin-enriched isoflavone from red clover lowers LDL cholesterol in men. Eur J Clin Nutr 58, 403408.
61. Pipe, EA, Gobert, CP, Capes, SE, et al. (2009) Soy protein reduces serum LDL cholesterol and the LDL cholesterol:HDL cholesterol and apolipoprotein B:apolipoprotein A-I ratios in adults with type 2 diabetes. J Nutr 139, 17001706.
62. Qin, Y, Shu, F, Zeng, Y, et al. (2014) Daidzein supplementation decreases serum triglyceride and uric acid concentrations in hypercholesterolemic adults with the effect on triglycerides being greater in those with the GA compared with the GG genotype of ESR-β RsaI. J Nutr 144, 4954.
63. Reverri, EJ, LaSalle, CD, Franke, AA, et al. (2015) Soy provides modest benefits on endothelial function without affecting inflammatory biomarkers in adults at cardiometabolic risk. Mol Nutr Food Res 59, 323333.
64. Thorp, AA, Howe, PR, Mori, TA, et al. (2008) Soy food consumption does not lower LDL cholesterol in either equol or nonequol producers. Am J Clin Nutr 88, 298304.
65. West, SG, Hilpert, KF, Juturu, V, et al. (2005) Effects of including soy protein in a blood cholesterol-lowering diet on markers of cardiac risk in men and in postmenopausal women with and without hormone replacement therapy. J Womens Health (Larchmt) 14, 253262.
66. Wong, JM, Kendall, CW, Marchie, A, et al. (2012) Equol status and blood lipid profile in hyperlipidemia after consumption of diets containing soy foods. Am J Clin Nutr 95, 564571.
67. McVeigh, BL, Dillingham, BL, Lampe, JW, et al. (2006) Effect of soy protein varying in isoflavone content on serum lipids in healthy young men. Am J Clin Nutr 83, 244251.
68. Setchell, KD & Cole, SJ (2006) Method of defining equol-producer status and its frequency among vegetarians. J Nutr 136, 21882193.
69. Ahn-Jarvis, J, Clinton, SK, Riedl, KM, et al. (2012) Impact of food matrix on isoflavone metabolism and cardiovascular biomarkers in adults with hypercholesterolemia. Food Funct 3, 10511058.
70. Gianazza, E, Eberini, I, Arnoldi, A, et al. (2003) A proteomic investigation of isolated soy proteins with variable effects in experimental and clinical studies. J Nutr 133, 914.
71. Clerici, C, Nardi, E, Battezzati, PM, et al. (2011) Novel soy germ pasta improves endothelial function, blood pressure, and oxidative stress in patients with type 2 diabetes. Diabetes Care 34, 19461948.
72. Pan, W, Ikeda, K, Takebe, M, et al. (2001) Genistein, daidzein and glycitein inhibit growth and DNA synthesis of aortic smooth muscle cells from stroke-prone spontaneously hypertensive rats. J Nutr 131, 11541158.
73. Cruz, MN, Agewall, S, Schenck-Gustafsson, K, et al. (2008) Acute dilatation to phytoestrogens and estrogen receptor subtypes expression in small arteries from women with coronary heart disease. Atherosclerosis 196, 4958.
74. Balk, E, Chung, M, Chew, P, et al. (2005) Effects of soy on health outcomes. Evid Rep Technol Assess (Summ) 126, 18.
75. Anderson, JW, Johnstone, BM & Cook-Newell, ME (1995) Meta-analysis of the effects of soy protein intake on serum lipids. New Engl J Med 333, 276282.
76. Legette, LL, Prasain, J, King, J, et al. (2014) Pharmacokinetics of equol, a soy isoflavone metabolite, changes with the form of equol (dietary versus intestinal production) in ovariectomized rats. J Agric Food Chem 62, 12941300.
77. Usui, T, Tochiya, M, Sasaki, Y, et al. (2013) Effects of natural S-equol supplements on overweight or obesity and metabolic syndrome in the Japanese, based on sex and equol status. Clin Endocrinol 78, 365372.