Dietary supplements of soya flour lower serum testosterone concentrations and improve markers of oxidative stress in men

Abstract

Objective: We examined the effects on serum sex steroids, lipids and markers of oxidative stress of supplementing the diets of healthy male volunteers with scones made with soya flour.

Design: A randomized placebo controlled cross-over trial.

Setting: University Hospital of Wales.

Subjects: Twenty volunteers recruited by advertisement.

Interventions: Male volunteers ate three scones a day in addition to their normal diet for a period of 6 weeks. The scones were made with either wheat or soya flour (containing 120 mg/day of isoflavones). Blood was analysed for sex steroids (testosterone, dihydro-testosterone, oestradiol, oestrone, sex hormone binding globulin, albumin and the concentration of non-protein bound sex steroids were calculated), lipid profile (total cholesterol, high density lipoprotein cholesterol and triglycerides) and measures of oxidative stress (hydroperoxides, susceptibility of LDL to oxidation with copper and myeloperoxidase).

Results: The volunteers' mean age was 35.6 (s.d. 11.2) y. Total serum testosterone fell in volunteers taking the soya scones (19.3–18.2 nmol/l; 95% CI 1.01, 1.12; P=0.03). No significant changes were seen in the concentrations of the other serum sex steroids, albumin or sex hormone binding globulin throughout the study. Significant improvements in two of the three markers of oxidative stress were seen in volunteers taking soya scones. Lag time for myeloperoxidase rose from 55.0 to 68.0 min (95% CI −16.0, −3.5; P=0.009) and the presence of hydroperoxides decreased from 2.69 to 2.34 µmol/l (95% CI 0.12, 0.71; P=0.009). There were no changes seen in serum triglycerides or cholesterol.

Conclusion: We have shown that soya supplements reduce serum testosterone and improve markers of oxidative stress. These findings provide a putative mechanism by which soya supplements could protect against prostatic disease and atherosclerosis. Further dietary studies with clinical end points are warranted.

Sponsorship: The Mason medical research foundation.

Introduction

Half of the male population over the age of 50 will experience symptoms due to benign prostatic hypertrophy or prostatic carcinoma (Griffiths et al, 1998). The incidence of prostatic carcinoma varies dramatically according to geographical location, with the highest rates seen in North America and Northern Europe, and the lowest in China, Japan and India (Griffiths et al, 1998; Zaridze & Boyle, 1987). Migrant populations assume the risk of acquiring prostatic carcinoma of their destination community (Shimizu et al, 1991). Large differences between American and Asian diets have been implicated in the pathogenesis of prostatic carcinoma (Griffiths et al, 1998).

Role of steroid hormones in prostatic disease

Throughout life the steroid milieu of prostatic cells is under constant change, and interactions between the cells and their environment result in dynamic alterations in cell biology. Androgens are regarded as the most important hormones in prostatic physiology, but others including oestrogens, retinoids, vitamin D-related compounds and growth factors exert significant influence (Griffiths et al, 1996; Wilding, 1995). Prostatic stromal hyperplasia occurs due to the increased oestrogen to androgen ratio, seen with declining androgen concentration after the age of 50 (Griffiths et al, 1998). However, the exact role of oestrogens and their relationship with androgens and transforming growth factors in the aetiology of prostatic tumorigenesis remains unclear (Bosland et al, 1994; Henderson et al, 1988; Ricciardelli et al, 1994).

Soya

Interest in the role of the soybean in the prevention of human cancers is increasing. Soybeans are a rich source of many biologically active components, including isoflavones (Setchell et al, 1987). Flavonoids are a large group of polyphenolic compounds found in many fruits, vegetables and beverages such as tea and wine. Over 4000 flavonoids have been identified and they are divided into several groups according to their chemical structure, including flavonols (quercetin and kaempherol), flavanols (the catechins), flavones (apigenin) and isoflavones (daidzein and genistein). Isoflavones are of interest because of their phytoestrogenic and antioxidant properties. Lignans have similar properties but are found more commonly in cereals, vegetables and berries.

Phytoestrogens

Phytoestrogens are compounds such as isoflavones and lignans which are weak oestrogens (Setchell et al, 1987). Phytoestrogens compete with endogenous oestrogens for oestrogen receptor sites (Shutt & Cox, 1972), acting as partial agonists. Phytoestrogens stimulate the liver to produce sex hormone binding globulin and thus indirectly reduce the amount of biologically active oestrogen (Martin et al, 1978). In vitro, isoflavonoids inhibit the enzymes 5α-reductase and 17β hydroxysteroid dehydrogenase (both of which are required for androgen synthesis; Evans et al, 1995; Mäkelä et al, 1998), inhibit angiogenesis, DNA topoisomerases and tyrosine-specific protein kinases (Markovits et al, 1989). Isoflavonoids inhibit tumour development and growth in cell lines (Peterson & Barnes, 1991,1993) and in animal models (Mäkelä et al, 1995; Pollard & Luckert, 1997; Zhou et al, 1999). In Asian populations, prostatic carcinoma is less aggressive than in Western populations (Breslow et al, 1977) and the decreased incidence of the disease correlates with increased soya intake (Severson et al, 1989).

Soya intake and cardiovascular disease

It has been proposed that a high soya consumption is responsible for the low rate of cardiovascular disease seen in the orient (Setchell & Cassidy, 1999). An inverse relationship is seen between serum cholesterol and soya intake in Japan (Nagata et al, 1988). Consumption of flavonoids is also inversely correlated with coronary heart disease in most, but not all, studies (Hollman & Katan, 1999). The mechanism by which soya may protect against cardiovascular disease is far from clear and may involve reduction in cholesterol or in oxidative stress or the action of phyto-oestrogens (Adlercreutz & Mazur, 1997).

Aims of the study

The potential to use dietary manipulation as protection against prostatic carcinoma is clearly an important and enticing goal. We aimed to examine the effect of soya protein supplementation at the levels seen in Asian diets, in a cohort of Western men. In addition to examining hormonal profiles we assessed parameters of oxidative stress and lipid profiles.

Methods

Experimental design

Volunteers were recruited through adverts placed in the hospital, and were accepted only if they were omnivorous, healthy and non-smokers. They were rejected if obese (body mass index (BMI) >30 Kg/m2) or if they had taken any antibiotics or vitamin supplements within the previous month. After acceptance, the volunteers were interviewed and advised to avoid soy-containing products for the 4 weeks preceding the start of the study. At the start volunteers were weighed and measured to calculate their BMI and waist–hip ratio. Blood was taken before 9 a.m. after an overnight fast. Serum and plasma was stored frozen at −70°C until analysis. A dietary record was kept for 2 week days and 2 weekend days.

After baseline assessment volunteers were asked to eat three scones a day (one at 08:00 then at 13:00 and 18:00) in addition to their normal diets for a period of 6 weeks. The scones were made either with wheat flour or soya flour (Nutrisoy flour, ADM Europort, Netherlands). The two scone types were well matched for energy, fat and fibre content (wheat flour scones 1568 kJ, 11.8 g, 3.64 g vs soya flour scones 1625 kJ, 12.6 g, 3.67 g). In those volunteers taking the soya scones the additional daily intake of isoflavones was 120 mg/day (daidzein 75 mg/day and genistein 45 mg/day). After a 6 week washout period the volunteers switched to the alternative scone type for a further 6 weeks (randomized double blind placebo controlled cross-over trial). Four-day dietary records were recorded during weeks 6 and 12 and fasting blood was taken at the end of these periods before 9 am. Volunteers were reviewed weekly to monitor their progress, provide encouragement and check compliance by counting the number of scones they had not eaten.

The study was approved by the South Glamorgan Local Research Ethics Committee.

Dietary records

Subjects were asked to write down the type and amount of food eaten, using scales or household measures to gauge portion sizes where possible. Where insufficient information was given the subjects were contacted by telephone for a fuller description. Volunteers were specifically asked to record alcohol intake. Volunteers were encouraged to keep their diets, alcohol intake and exercise patterns unchanged during the experiment. The records were analysed for individual nutrients (total energy, total dietary fibre (Southgate), insoluble non-starch polysaccharide (NSP), soluble NSP, total NSP, total fat, saturated fat, polyunsaturated fat, protein, carbohydrate, extrinsic sugar and alcohol) using a computer program based on McCance and Widdowson's The Composition of Foods (Paul & Southgate, 1978) and on published values for NSP (Englyst et al, 1988,1989).

Serum analysis

All the steroid hormone concentrations were assayed in duplicate by laboratory staff blinded to the exposure status of the samples.

Oestradiol and oestrone

These oestrogens were measured separately by selective radio-immunoassay after extraction from the sample with organic solvent (Lewis et al, 1997). Precision (as coefficient of variation) of the oestrone assay was 8% at 273 pmol/l and 13% at 593 pmol/l; for oestradiol it was 13% at 167 pmol/l and 14% at 335 pmol/l.

Testosterone

After extraction from serum into diethyl ether, radio-immunoassay was performed using an antibody raised to testosterone-3-(o-carboxymethyloxamine)-bovine serum albumin conjugate and tritiated testosterone as a labile.

Dihydro-testosterone (DHT)

After extraction from serum into diethyl ether, potassium permanganate was added (which oxidizes testosterone such that it does not bind to the antibody). Dihydro-testosterone remains intact and competes with tritiated DHT for binding sites. A radio-immunoassay was done, and the unbound DHT was absorbed onto dextran-coated charcoal, leaving the bound fraction in solution. After centrifugation, the supernatant was decanted into scintillation vials and counts obtained in a liquid scintillation counter. The serum DHT concentrations were determined by interpolation of the counts bound on a standard curve.

Sex hormone binding globulin, albumin and non-protein bound sex steroids

Sex hormone binding globulin ISHDG was adsorbed from the sample with Concanavalin A-Sepharose. Measurement was as described by Whittaker et al (1992). The serum albumin concentration was determined using reagents supplied by Boehringer Mannheim Gmbh on a Hitachi 747 automated analysis system. The concentrations of non-protein bound DHT, testosterone, oestrone and oestradiol in each sample were calculated using the measured values for sex steroids, SHBG and albumin (Speight et al, 1979).

Cholesterol and triglycerides

Cholesterol and triglycerides were measured using Roche/BM reagents by standard enzymatic assays on a BM/Hitachi 747 analyser (Roche Diagnostic Ltd, Lewes, UK). High-density lipoprotein (HDL) cholesterol was measured by precipitation of other non-HDL cholesterol fractions using phospho-tungstate and then measuring remaining cholesterol in the supernatant by the previously described cholesterol method. Low-density lipoprotein (LDL) cholesterol was calculated using the formula: [LDL-cholesterol]=[total cholesterol]/[HDL choles-terol]−([triglycerides]/2.2).

Oxidative stress

Susceptibility of LDL cholesterol to oxidation. Low-density lipoprotein cholesterol (LDL-cholesterol) was isolated from the plasma by rapid ultracentrifugation as described by McDowell et al (1995), then purified by size-exclusion chromatography. The protein concentration was standardized to 50 mg/l and oxidation was initiated by the addition of Cu2+ (final concentration 2 µmol/l), or a combination of myeloperoxidase (0.25 U/ml) and H2O2 (100 µM) at 37°C. Conjugated diene formation was monitored at 234 nm. The lag phase prior to the onset of rapid LDL-cholesterol oxidation is seen as a slope with a slow increase in absorbency. The duration of the lag phase is indicative of the resistance to oxidation of the LDL-cholesterol being examined, and was measured as the time intercept between the line of maximum slope of the propagation phase and the absorbency baseline at time t=0. Inter-assay CV was <5% and intra-assay CV was <1%.

Lipid hydroperoxides

The ‘FOX 1’ (Ferrous oxidation in xylenol orange, version 1) assay was used for the direct measurement of lipid hydroperoxides in plasma (Wolff, 1994). The FOX 1 reagent is made up with xylenol orange, ammonium ferrous sulphate and sulphuric acid. The addition of sorbitol leads to chain amplification in the ferrous oxidation step. Samples are spiked with catalase to discriminate between authentic hydroperoxides reacting with ferrous ions and H2O2, which may be present in the sample. The concentration of hydroperoxides in the sample was determined from the absorbency at 560 nm using a standard curve. Intra-assay CV was 2% and inter-assay CV 4%.

Compliance

Volunteers were given a known number of scones in excess of their need. Compliance was determined from the number of scones returned. Most volunteers ate their scones in a semi-supervised manner.

Statistics

Volunteers were randomly allocated to receive either soya or wheat flour scones first. The randomization protocol was generated using Microsoft, Excel computer software and the results entered into sealed envelopes. Pre-intervention data were assessed as parametrically or non-parametrically distributed using histograms and Ryan-Joiner tests. The differences between pre- and post-intervention readings were then analysed using two-tailed Student t-tests (results expressed as means and standard deviation (s.d.)), or Wilcoxon tests (results expressed as medians and interquartile ranges (IQ range), as appropriate). Serum oestrogen and androgen concentrations were analysed using log10 transformed data, with results expressed as geometric means and 95% confidence interval of the ratio of the geometric means.

Results

Nineteen of the 20 volunteers completed the study; one withdrew for personal reasons. At entry into the study the average age was 35.6 (s.d. 11.2) y, BMI was 25.6 (s.d. 3.1) kg/m2 and the waist–hip ratio was 0.87 (s.d. 0.06). No changes in weight, BMI or waist–hip ratios were noted throughout the study. Compliance with scone consumption was a median of 98.7% (IQ range 98, 100).

The dietary record data are given in Table 1. There were no changes in dietary records (excluding the scones) throughout the study. Total serum testosterone fell in volunteers taking the soya scones. Whilst the concentrations of the other sex steroids reduced with soya supplements, all these changes (for both total and non-protein bound) were non-significant (Table 2). There were no changes seen in fasting serum triglycerides or cholesterol throughout the study (Table 3). Significant improvements in two of the three markers of oxidative stress were detected in volunteers taking soya scones (Table 4).

Table 1 Dietary records at baseline and at end of active and placebo interventional periods (mean, s.d. or median, IQ range as appropriate)
Table 2 Total and non-protein bound (NPB) dihydrotestosterone, testosterone, oestradiol and oestrone and sex hormone binding globulin and albumin concentrations at baseline and end of the interventional period (geometric mean, 95% CI) and the 95% CI of the ratio of the geometric means and P-value
Table 3 Serum total cholesterol, high density lipoprotein cholesterol, low density lipoprotein cholesterol and triglycerides (all mmol/l) at baseline and end of the interventional period (median, IQ range)
Table 4 Markers of oxidative stress at baseline and end of the interventional period (median, IQ range)

Discussion

Consumption of soya scones led to a reduction in serum testosterone and an improvement in the markers of oxidative stress. This is the first study to demonstrate that soya flour can protect LDL against oxidation by myeloperoxidase. No significant changes in the concentrations of the other measured parameters were seen.

Testosterone is the most potent androgen secreted by the testis, the others (androstenedione and dehydroepiandrosterone) are produced in reduced concentration. The biologically active form of testosterone is dihydrotestosterone. A reduction in plasma testosterone concentrations offers a putative mechanism by which a soya diet may influence prostatic biology. The mechanism by which serum concentrations of testosterone were reduced is not apparent. The concentrations of the other steroid hormones measured (dihydrotestosterone, oestradiol and oestrone) reduced but not significantly. Cassidy et al (1998) gave middle-aged male volunteers 60 g a day of soya protein, no changes in androgen concentrations were seen. Nagata et al (2001) gave men 400 ml of soya milk a day, which lead to reductions in serum oestrone but not oestradiol or androgens.

Diets in Asia have an isoflavone content of 50–100 mg/day which compares with a Western diet of less than 5 mg/day (Coward et al, 1993; Jones et al, 1989). A regular daily consumption of soy protein isolates (130 mg/day isoflavones) raises plasma concentrations of the isoflavones genistein and daidzein above the levels of the average Japanese male (Gooderham et al, 1996). Isoflavonoids inhibit tumour development and growth in cell lines (Peterson & Barnes, 1991,1993). It is possible that any health benefit from soya products may be steroid independent (Nakhla et al, 1994). Isoflavones are found in higher concentrations in prostatic fluid than serum (Morton et al, 1997). Reduction in serum oestrogen concentration is seen in women taking soya supplements (Lu et al, 1996). In studies in women serum oestrogen concentrations can be reduced by diets low in fat or high in fibre (Rose et al, 1987,1991). In men, diets high in fat and low in fibre reduced the urinary excretion of oestradiol and oestrone while increasing the mean plasma concentrations of testosterone (Dorgan et al, 1996). It may be expected that if dietary soya replaced the animal produce seen in Western diets instead of just being given as a supplement then an even greater reduction in sex steroid hormones would be seen. Only a large randomized study with clinical endpoints could definitively show benefit from soya supplementation.

We found that two of the three measures of oxidative stress showed benefit from soya supplementation. Flavonoids influence arachidonic acid metabolism to a considerable extent (Alcaraz & Ferrandiz, 1987). The parent compound flavone is a strong inhibitor of cyclo-oxygenase (Mower et al, 1984) and has free radical scavenging properties (Sweis et al, 1984). A probable mechanism of flavonoid interaction with prostoglandin synthetase may be through antioxidant or free radical scavenging (Buck, 1996), the antioxidant activity being related to inhibition of lipo-oxygenase (Middleton & Drzewiecki, 1982). There is evidence that products of cyclo- and lipo-oxygenase such as organic peroxides and free radicals are involved in the aetiology of malignancy (Cerutti, 1985). Reductions in antioxidant capacity and elevations in peroxidative damage with increasing age are seen in rat prostates (Ghatak & Ho, 1996). The inhibition of these enzymes by flavonoids may be a mechanism by which they could be protective against prostatic malignancy, and may also be relevant to protection against atherosclerosis.

Oxidative modification of LDL is an important step in the development of the atherosclerotic plaque (Heinecke, 1997; Witztum & Horkko, 1997). Oxidation of LDL is a process initiated and propagated by free radicals (Heinecke, 1997). All the cells of the vessel wall, including endothelial cells, smooth muscle cells, macrophages and lymphocytes, can modify LDL in vitro (Folcik et al, 1995; Henriksen et al, 1981; Morel et al, 1984). Several mechanisms are likely to be involved, including transition metal ion-mediated generation of hydroxyl radicals, production of reactive oxygen species by enzymes such as myeloperoxidase and lipoxygenase, and direct modification by reactive nitrogen species.

This is the first study to demonstrate that soya flour can protect LDL against oxidation by myeloperoxidase, although no effect was observed on copper-mediated oxidation. Interestingly soya supplements have previously been shown to protection against LDL oxidation by copper (Tikkanen et al, 1998). Myeloperoxidase (MPO), a haeme protein, is secreted from leukocytes in response to an inflammatory stimulus (Badwey & Karnovsky, 1980; Segal, 1989). The enzyme utilizes hydrogen peroxide released from activated leukocytes, generating a variety of reactive oxygen species including hydroxyl radical (Tauber & Babior, 1977; Weiss et al, 1977) and singlet oxygen (Krinsky, 1974). In physiological media MPO gives rise to two main radical species, hypochlorous acid and tyrosyl radical (Heinecke et al, 1993). Both species are potently pro-oxidant and have been shown to cause modification of LDL lipids and protein (Domigan et al, 1995; Hazen & Heinecke, 1997). The oxidative products of LDL-protein modification by either species are specific markers of the action of MPO and along with MPO have been isolated from human atherosclerotic lesion tissue (Domigan et al, 1995; Hazen & Heinecke, 1997). This is compelling evidence implicating MPO as a mediator of oxidative damage to lipoproteins in the arterial intima. Our finding that soya flour can inhibit MPO-mediated oxidation of LDL therefore provides a novel mechanism by which soya products might help prevent atherosclerosis.

We failed to find any changes in serum lipids with soya supplements. Previous studies have shown a variable ability of soya to reduce serum cholesterol. A meta-analysis has suggested a benefit of soya protein over animal protein (Anderson et al, 1995), but many of the soya products used in the studies analysed contained very little phytoestrogen, suggesting that other mechanisms may be relevant (Sirtori et al, 1997). Certainly there is evidence that if animal protein is replaced with soya protein, a reduction in serum cholesterol is seen (Anderson et al, 1995).

Conclusion

We have shown that soya supplements can reduce serum testosterone and improve markers of oxidative stress. Whilst these findings should not be over-interpreted, they provide a putative mechanism by which soya supplements can protect against prostatic disease and atherosclerosis. Further dietary studies with clinical end points are warranted.

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Acknowledgements

The technical assistance of Marion Cawood and the staff of the SAS Centre for Steroid Hormones in Leeds is gratefully acknowledged.

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Correspondence to S J Lewis.

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Gardner-Thorpe, D., O'Hagen, C., Young, I. et al. Dietary supplements of soya flour lower serum testosterone concentrations and improve markers of oxidative stress in men. Eur J Clin Nutr 57, 100–106 (2003). https://doi.org/10.1038/sj.ejcn.1601495

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Keywords

  • soya
  • androgens
  • oxidative stress

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