Some phytoestrogens do have significant activities at estrogen receptors. When phytoestrogens are used in combination with estrogen-containing products, the risk of estrogen-related side effects (nausea, bloating, breast fullness, or tenderness) may be increased. Patients should consult a physician or pharmacist before taking estrogen and phytoestrogens.
It is not known whether phytoestrogens decrease effectiveness of tamoxifen (Nolvadex), used for breast cancer treatment, or raloxifene (Evista), used for the prevention of postmenopausal osteoporosis.
Antihypertensive drugs including Trandolapril (Mavik) or verapamil (Isoptin), when taken concurrently with a bioflavonoid preparation containing naringin (a flavonoid present in grapefruit and soy) may cause serious adverse reactions. Consult with a physician or pharmacist before taking this combination.
Soy products containing tyramine can cause a hypertensive reaction in patients currently taking monoamine oxidase inhibitors (MAOIs), such as phenelzine (Nardil). Soy foods to avoid during MAOI therapy are fermented soybean products and soybean pastes that contain a significant amount of tyramine. Soy products that should be used with caution include soy sauces.
Soy products decrease absorption of L-thyroxine in infants with congenital hypothyroidism. This becomes a problem if the infant is receiving soy-based formula. The L-thyroxine dose should be increased if an infant with congenital hypothyroidism is fed soy-formula. After soy-formula is discontinued, the L-thyroxine dose should be decreased. For adults, staggering doses by at least 2 hours is recommended. Soy may inhibit thyroid hormone synthesis but research is lacking in this area.
Information on the relationship between substances and disease is provided for general information, in order to convey a balanced review of the scientific literature. In many cases the relationship between a substance and a disease is tentative and additional research is needed to confirm such a relationship.
Soy isoflavones are phytoestrogens found in soybeans (Glycine max). Phytoestrogens are plant-derived nonsteroidal compounds that possess both weak estrogenic and anti-estrogenic effects.
The three main isoflavones soy contains are genistein, daidzein and glycitein and their glycosylated forms.
• The most abundant isoflavones in soybeans are genistin (approximately 50%), daidzin (approximately 40%), and glycitin (approximately 5-10%).
Of the glycosylated forms of genistein (genistin, 6’-O-malonylgenistin, and 6’-O-acetylgenistin), the malonyl glycosides are the major forms of isoflavones found in soybeans.
Soy isoflavones can bind to both alpha- and beta-estrogen receptors, however, binding affinity to the beta-receptor is higher.
The FDA has approved a cardiovascular health claim for foods that contain at least 6.25 grams of soy protein per serving. Isoflavone concentrations are generally 0.5 to 3.0 mg/g of soy flours and concentrates.
Various observational and epidemiological studies suggest that more than 30 mg daily of soy isoflavones approximates the dose that may have health benefits.
Soy isoflavones have estrogenic activity and may have antioxidant, anticarcinogenic, anti-atherogenic, hypolipidemic, and anti-osteoporotic activities.
Phytoestrogens bind to estrogen receptors and act as estrogen agonists or antagonists. Their actions at the cellular and molecular level are influenced by many factors including, but not limited to, concentration, receptor status, presence or absence of endogenous estrogens, and type of target organ or cell.
Genistein, the most studied of the soy isoflavones, has been found to have antioxidant activities. It is a scavenger of reactive oxygen species, inhibiting lipid peroxidation and superoxide formation.
In animal experiments genistein has been found to increase activities of the antioxidant enzymes superoxide dismutase, glutathione peroxidase, catalase, and glutathione reductase.
Possible anticarcinogenic activities of genistein include upregulation of apoptosis, inhibition of angiogenesis, and inhibition of DNA topoisomerase II and protein tyrosine kinases.
Possible activity against prostate cancer includes inhibition of nuclear factor (NF)-kappa B in prostate cancer cells, downregulation of transferring growth factor (TGF)-beta, and inhibition of epidermal growth factor (EGF)-stimulated growth.
Anti-atherogenic activity of soy isoflavones includes antioxidant abilities, which may inhibit lipid peroxidation and LDL oxidation. Isoflavones may also lower serum cholesterol concentrations. Other cholesterol-lowering components associated with soy protein include trypsin inhibitors, phytic acid, saponins and fiber.
Women with estrogen receptor-positive tumors should
use caution when taking soy isoflavones and only use when recommended and
monitored by your healthcare professional.
According to a recent in-depth review, the available scientific evidence to date supports the safety of isoflavones as typically consumed in diets based on soy, or soy-containing products.10
Twenty-four healthy postmenopausal women ingested a single dose of purified isoflavone preparations (from soybeans), delivering a genistein dose of 2, 4, 8, or 16 mg/kg body weight. A single dose administration of purified isoflavones at amounts exceeding normal dietary intakes had minimal clinical toxicity in healthy postmenopausal women.11
Effects of dietary intake of soy protein and isoflavones on cardiovascular disease risk factors in high risk, middle-aged men in Scotland
Sixty-one men (aged 45 to 59 years) with high blood pressure and high total cholesterol levels consumed 20 grams of soy protein with 80 mg of isoflavones or a placebo diet for 5 weeks. A significant decrease in systolic (p=0.001) and diastolic (p=0.002) blood pressure, total cholesterol (p=0.018), and non-HDL cholesterol (p=0.002)was found in the soy supplemented group. HDL cholesterol concentrations increased in both the soy supplemented and placebo groups (p=0.005, p=0.004, respectively). These data suggest that 20 grams of soy protein with 50 mg isoflavones per day could significantly reduce cardiovascular disease risk.31
Effects of high- and low-isoflavone soyfoods on blood lipids, oxidized LDL, homocysteine, and blood pressure in hyperlipidemic men and women
Forty-one hyperlipidemic men and postmenopausal women consumed either a low-fat dairy food diet (control), a high isoflavone soy diet (50 grams soy protein, 73 mg isoflavones/day), or a low isoflavone soy diet (52 grams soy protein, 10 mg isoflavones/day) for 1 month. Fasting blood samples were drawn and blood pressure was measured at the beginning and end of each diet. Results from both soy diets showed a significant reduction in total blood cholesterol concentrations and estimated coronary artery disease (CAD) risk (10.1 +/-2.7%). Both soy diets resulted in lower ratios of total to HDL cholesterol, LDL to HDL cholesterol, and apolipoprotein B to A-1. No significant differences were seen between high- and low-isoflavone diets. Substituting soy-containing foods for animal-based products may help reduce CAD risk through modest reductions of blood lipid concentrations and reduction of oxidized LDL, homocysteine and blood pressure.14
Moderate intakes of intact soy protein rich in isoflavones compared with ethanol-extracted soy protein increase HDL but do not influence transforming growth factor-beta(1) concentrations and ...
A randomized crossover trial was conducted to compare the effects of moderate intakes of soy protein on risk factors for coronary heart disease. The high isoflavone diet included 56 mg phytoestrogens and the low isoflavone diet contained 2 mg isoflavones, both with soy protein. The effects on active transforming growth factor beta(1) [TGF-beta(1)] concentrations and plasma lipid and hemostatic risk factors for coronary heart disease (CHD) were monitored. Twenty-two young, healthy subjects (5 men 17 women) with normal blood lipid concentrations consumed one of the soy diets for 17 days followed by a 25-day washout period. Fasting blood samples were obtained on days 13 and 14 of each treatment. Plasma isoflavone concentrations were significantly increased in the high isoflavone diet compared to the low isoflavone diet (100-999 times, P<0.05). Plasma HDL-cholesterol and apolipoprotein A-1 concentrations were 4% and 6% higher, respectively, after the high isoflavone diet than after the low isoflavone diet (p<0.01). Soy protein with intact phytoestrogens increased HDL-cholesterol and apolipoprotein A-1 concentrations (4%, 95% CI: 1%, 8%; 6%, 95% CI: 3%, 10%; P<0.01 for both), but did not influence LDL cholesterol or TGF-beta(1) concentrations in healthy, normolipidemic subjects.15
The effects of a soy rich diet on serum lipids: the Menfis randomized trial
One hundred eighty-seven healthy asymptomatic post-menopausal women aged 39 to 60 years were randomized to a soy-rich diet group (40-60 mg isoflavones per day), a hormone replacement therapy (HRT) group, or a control group. The soy-rich diet group consumed an ad libitum diet, along with one soy food serving daily (e.g. soy milk, miso soup, tempeh, soy beans). This group was also required to exchange two meals twice a week with two meals from the study menu, consisting of phytoestrogen-rich foods. Serum lipids, along with blood pressure, endometrial thickness, uterine artery resistance thickness (RI), and body mass index were measured at baseline, after six months, and at the end of the study. The blood lipid profile improved in the soy rich diet group after six months, similar to what was observed in the HRT group. However, compliance to the soy diet was low. Soy products and supplements may be used to help support a healthy cardiovascular system in postmenopausal women. Strategies to improve compliance with consuming soy should be considered and further research is needed to confirm these findings.16
Meta-analysis of the effects of soy protein containing isoflavones on the lipid profile
Meta-analysis of 38 clinical trials with soy protein confirms the beneficial effects of soy on blood lipid profiles. Soy protein with intact isoflavones significantly reduced serum total cholesterol (3.77%), LDL cholesterol (5.25%), and triglycerides (7.27%) during the initial treatment period. HDL cholesterol levels were increased by an average of 3.03%, but only after treatment for at least twelve weeks. Daily intake of soy protein with 80 mg or more of isoflavones was the most beneficial; benefits were greatest for men. Isolated soy isoflavone tablets, however, had no significant effect on blood lipid parameters. Thus, soy protein containing isoflavones was found to have a beneficial effect on blood lipid parameters.13
Soy protein favorably affects LDL size independently of isoflavones in hypercholesterolemic men and women
To investigate the affect of soy protein versus animal protein on LDL cholesterol particle size, thirty-six hypercholesterolemic men and women consumed each of four diets for six weeks: 16-17 grams/d soy protein or 15-17 grams/d animal protein with or without 46-52 mg of added isoflavones. Soy protein diets increased LDL particle size (P<0.01) and shifted LDL particle distributions to less atherogenic patterns independent of isoflavone content. Specifically, soy protein diets decreased cholesterol levels in LDL particles with diameter less than 25.5 nm by 12.3% (P<0.001) and increased cholesterol in particles larger than 26.0 nm in diameter by 14.3% (P<0.05). These data suggest that incorporation of soy protein in a healthy diet may help to improve lipid profiles.32
Soy in hypercholesterolaemia: a double-blind, placebo-controlled trial
A double-blind, placebo controlled trial was conducted in Finland to examine the effects of a soy protein product with isoflavones versus placebo in subjects with hypercholesterolemia. Thirty subjects were randomized to the soy group and 30 to the placebo initially; however, six subjects were withdrawn from the active group and two from the placebo group due to gastrointestinal symptoms. In addition to their regular diets, subjects were given either the soy protein product with isoflavones (26 grams isolated soy protein standardized with 3.7 mg isoflavones per gram of soy protein) or control (calcium caseinate, whole milk powder and cellulose) for six weeks. Although an unexpected reduction in blood lipid concentrations was found in the placebo group (5.1%), a statistically significant lipid-lowering effect (8.3%) was found in the soy product group. LDL cholesterol concentrations were decreased 13.2% in the soy group versus 8.0% in the placebo. The conclusion from this study was that addition of soy protein and isoflavones to a regular diet, could positively impact plasma LDL cholesterol and total cholesterol concentrations.17
The effects of soy-derived phytoestrogens on serum lipids and lipoproteins in moderately hypercholesterolemic postmenopausal women
Thirty-six mildly hypercholesterolemic, postmenopausal women aged 69±4 years participated in a placebo-controlled clinical trial to examine the effect of phytoestrogens from soy on serum lipids and lipoproteins. Participants were randomized to receive either 150 mg of soy-derived phytoestrogen supplement or placebo (maltodextrin with 10% caramel color) daily. Serum samples were analyzed at baseline and 2 months for total triacylglycerol, total cholesterol, and high-density lipoprotein cholesterol using standard Lipid Research Clinic procedures. Total triacylglycerol and cholesterol were also measured after 6 months of treatment. After 2 months of treatment, no significant differences were found in total triacylglycerol, total cholesterol, or high-density lipoprotein cholesterol concentrations between the phytoestrogen and placebo groups. Total triacylglycerol and total cholesterol concentrations remained unchanged after 6 months. Soy phytoestrogen supplementation at 150 mg/day over 6 months did not alter serum lipoprotein concentrations in postmenopausal elderly women. Thus, this population may not effectively reduce their risk for coronary artery disease with soy phytoestrogen supplementation.18
A randomized trial comparing the effect of casein with that of soy protein containing varying amounts of isoflavones on plasma concentrations of lipids and lipoproteins
A double-blind randomized parallel trial included 156 healthy men and women with mild hypercholesterolemia examined the effects of isoflavones and protein sources on plasma lipids and lipoproteins. Participants consumed diets containing 25 g of casein or 25 g of isolated soy protein with varying amounts of isoflavones for 9 weeks. Isolated soy protein with 62 mg of isoflavones lowered total and LDL cholesterol levels by 4% and 6% compared with casein (P=0.04 and P=0.01, respectively). For participants with LDL cholesterol levels in the top half of the population studied (>4.24 mmol/L [>164 mg/dL]), total cholesterol was reduced by 9% (P<0.001) and LDL cholesterol was reduced by 10% (P=0.001). Isolated soy protein with 37 mg of isoflavones reduced total and LDL cholesterol (P=0.007 and P=0.02, respectively) cholesterol levels by 8% for people with LDL cholesterol levels in the top half. The researchers concluded that isolated soy protein with isoflavones reduced plasma concentrations of total and LDL cholesterol in mildly hypercholesterolemic individuals.5
Soy isoflavones improve plasma lipids in normocholesterolemic and mildly hypercholesterolemic postmenopausal women
A randomized, crossover design study investigated the effects of isolated soy proteins containing isoflavones on measures of cardiovascular health. Eighteen postmenopausal women participated in the trial; study diets were consumed for 93 day periods. The diets contained isolated soy protein with varying amounts of isoflavones: control, 7.1±1.1; low isoflavone, 65±11; high isoflavone, 132±22 mg isoflavones/day. For those consuming the high-isoflavone diet, plasma LDL cholesterol concentration was 6.5% lower (P<0.02) compared to the control diet. The ratio of LDL to HDL cholesterol was 8.5% and 7.7% lower, respectively, during the low- and high-isoflavone diets (P<0.02). There was a dose-response effect based on increasing amounts of isoflavones on plasma total and LDL cholesterol concentrations. Soy protein with isoflavones did not change plasma HDL cholesterol or triacylglycerol concentrations.6
Usual dietary isoflavone intake is associated with cardiovascular disease risk factors in postmenopausal women
In a cross-sectional study, the association between dietary isoflavone intake and cardiovascular disease risk factors such as lipoprotein concentrations and insulin were measured. Dietary intake of isoflavones over the preceding year was assessed among 208 postmenopausal women aged 45 to 74 years. High genistein intake correlated with lower BMI (P-trend=0.05), lower waist circumference (P-trend=0.05), and fasting insulin (P-trend=0.07) when compared to those who did not consume genistein daily. Positive associations were found for genistein, daidzein, and total isoflavone intake with HDL cholesterol (P=0.05). Genistein, daidzein, and total isoflavone intake were inversely associated with postchallenge insulin (P=0.05). These results suggest a protective role for isoflavone against cardiovascular disease in this population of postmenopausal women.7
Lack of effects of isoflavones on the lipid profile of Brazilian postmenopausal women
A small double-blind, placebo-controlled clinical trial investigated the effects of isoflavones on lipid profile in postmenopausal women. Forty-seven women aged 47 to 66 years participated in the six week study. Participants were randomized to receive either 40 mg isoflavones or 40 mg of casein daily. Blood lipids were assessed at baseline and after six weeks. Total cholesterol and LDL cholesterol decreased similarly in both groups. HDL cholesterol increased similarly in both groups. Both very low density lipoprotein and triacylglycerols increased in both groups during the trial, but did not reach significance. The results of this study suggest that soy isoflavones do not have a beneficial effect for the blood lipid profiles of postmenopausal women; the changes seen are potentially attributable to placebo effect, bias, or unrelated behavioral changes (such as an increase in exercise or dietary modifications).12
Hormonal response to diets high in soy or animal protein without and with isoflavones in moderately hypercholesterolemic subjects
To evaluate the effects of soy protein or soy isoflavones versus animal protein on hormone levels, forty-two men and women with high cholesterol consumed diets with 25 grams soy protein or animal protein or 50 mg of soy isoflavones for 6 weeks in a crossover format. For women, daily soy protein intake increased circulating levels of estrone. For men, concentration of dehydroepiandrosterone was decreased after supplementation with isoflavones but increased after both soy and animal protein. The authors conclude that soy protein and soy isoflavones have small, sex-specific influence on hormone concentrations. The cumulative effects of these changes over longer timescales is not known.31
Emerging evidence on the role of soy in reducing prostate cancer risk
A review of in vitro and clinical trials suggests that incorporation of soy protein in the diet may reduce risk of prostate cancer. In vitro studies indicate that genistein inhibits growth of prostate cancer cells. In animal studies, soy protein with isoflavones and isolated isoflavones inhibit growth of prostate tumors. Evidence from clinical trials is lacking, but incorporation of 15 grams of soy protein with at least 50 mg of isoflavones into a healthy diet may help to reduce risk for prostate cancer.30
It remains inconclusive whether a diet containing soy reduces the risk of breast cancer for adult women. Some research has suggested that consuming soy early in life (i.e. before puberty and during adolescence) may be beneficial in modulating the risk of breast cancer in premenopausal women. Further research is needed.19
Phytoestrogens and breast cancer risk. Review of the epidemiological evidence
A recent review analyzed epidemiological data on the effect of phytoestrogens on breast cancer risk. A total of 18 studies were included and 13 studies assessed the direct relation between individual dietary intake of soy products and the risk for breast cancer. Although some limitations were noted in the studies included, overall it was concluded that soy does not offer protective effects against breast cancer for adult women. However, it was established that soy may be beneficial for young women to reduce breast cancer risk during adolescence. Four prospective studies did not find a statistically significant difference in reduction of breast cancer. Three case control studies assessed urinary isoflavone excretion after breast cancer occurrence, which limits the interpretation of these results. In conclusion, few prospective studies were conducted to assess the effects of phytoestrogens on breast cancer risk. The existing prospective studies did not focus on the age at consumption of soy, an important factor that is based on the results from previous dietary case control studies.20
Ecological study of the association between soy product intake and mortality from cancer and heart disease in Japan
A study conducted in 47 prefectures in Japan examined soy product intake (obtained from National Nutritional Survey Reports) and mortality rates from several types of cancer. After adjusting for confounding factors such as age, smoking and total energy intake, isoflavone intake was significantly related to mortality rate of colorectal cancer in men and women. Only in women did isoflavone significantly reduce the mortality rate of heart disease. No abstract available at this time.8
Topic: Premenopause
Effect of consumption of soy isoflavones on behavioural, somatic and affective symptoms in women with premenstrual syndrome
The affect of a daily soy protein supplement containing 68 mg of soy isoflavones versus placebo on premenstrual symptoms was determined in 23 young women (18 to 35 years) over seven menstrual cycles. Total symptoms and physical symptoms were significantly decreased in both the soy and placebo groups. However, cramps and swelling were significantly reduced in the soy group compared to the placebo group. These results suggest that soy protein with isoflavones may offer some relief for premenstrual symptoms.28
Hormonal effects of soy in premenopausal women and men
Consumption of soy (32-200 mg/day of isoflavones) has been suggested by numerous intervention studies to have cancer protective benefits in premenopausal women, as shown by increased menstrual cycle length and sex hormone-binding globulin (SHBG) levels, and decreased estrogen levels.21
Dietary soy isoflavones and bone mineral density: results from the study of women’s health across the nation
The influence of dietary soy isoflavones on bone mineral density, baseline data from the Study of Women’s Health Across the Nation were analyzed. The study included 1,927 women aged 42 to 52 years. Intake of genistein and daidzein by African American and Caucasian participants were too low for analysis, so only Chinese and Japanese participants were included in the final analysis (median genistein/daidzein intakes 3,511 mcg/d and 7,151 mcg/d, respectively). For Chinese women, no associations were bound between isoflavones and bone mineral density. For Japanese women, premenopausal women with higher genistein/daidzein intakes had higher spine and femoral neck bone mineral density (highest vs. lowest tertile: spinal, 7.7% higher, p=0.02; femoral neck, 12% higher, p=0.0001). For some premenopausal women, this study found evidence for a beneficial effect from soy isoflavone intake for bone mineral density. Additional studies are needed to evaluate these effects.22
Effects of the phytoestrogen genistein on bone metabolism in osteopenic postmenopausal women: a randomized trial.
A randomized, double-blind, placebo-controlled trial investigated the effects of genistein supplementation on bone mineral density. Three-hundred-eighty-nine women with osteopenia participated in the two-year trial. Participants were randomized to receive either 54 mg genistein or a placebo daily. Both included calcium and vitamin D. Bone mineral density increased in the genistein group and decreased in the placebo group at the anteroposterior lumbar spine at 24 months (difference, 0.10 g/cm2 (CI, 0.08 to 0.12); P<0.001) and the femoral neck (difference, 0.062 g/cm2 (CI, 0.049 to 0.073]; P<0.001). Urinary excretion of pyridinoline and deoxypyridinoline were significantly decreased in the genistein group compared to the placebo group. Levels of bone-specific alkaline phosphatase and insulin-like growth factor I increased in the genistein group versus the placebo group. Endometrial thickness was not changed compared with placebo. The results of this study suggest that genistein may be beneficial for bone health in postmenopausal women.20
Isoflavone-rich soy protein isolate attenuated bone loss in the lumbar spine of perimenopausal women
A randomized, double-blind, placebo controlled clinical trial investigated the effects of soy protein and soy isoflavones on bone loss in perimenopausal women. Sixty-nine women (aged 41 to 62 years) participated in the trial and were randomly assigned to receive one of three diets: isoflavone-rich soy (SPI+; 80.4 mg isoflavones/d; n=24), isoflavone-poor soy (SPI-; trace isoflavones; n=24), or whey (control; n=21) protein for 24 weeks. For women consuming soy diets (with or without isoflavones), the percentage change in lumbar spine bone mineral density and bone mineral content, respectively, did not differ from zero (-1.28%, P=0.0041). Loss of bone mineral density and bone mineral content occurred in the control group (-1.73%, P: = 0.0037). It was determined that the SPI+ treatment had a positive effect on change in BMD (5.6%;P=0.023) and BMC (10.1%; P=0.0032). These results indicate that soy isoflavones attenuated bone loss from the lumbar spine in perimenopausal women.9
Benefits of soy isoflavone therapeutic regimen on menopausal symptoms
A double-blind, placebo-controlled study examined the effect of a soy isoflavone regimen on menopausal symptoms and cardiovascular risk factors in postmenopausal women. Eighty women were randomly assigned to either an isoflavone treatment (100 mg/day) or a placebo for 4 months. A significant decrease in menopausal symptoms (p<0.01), and a decrease in total and LDL cholesterol concentrations (p<.001) were noted in the isoflavone group, compared with baseline or the placebo group. These results suggest that soy isoflavone treatment at 100 mg/day could be a safe and effective alternative for treating menopausal symptoms, and provide cardiovascular benefits as well.23
Effect of soy phytoestrogens on hot flashes in postmenopausal women with breast cancer: a randomized, controlled clinical trial
A randomized, double-blind, placebo-controlled trial examined the benefits of soy phytoestrogens as a treatment for hot flashes in postmenopausal women with breast cancer. Women with moderate hot flashes, who were previously treated for early-stage breast cancer, were stratified for tamoxifen use and randomized to a soy beverage containing 90 mg isoflavones or to a placebo rice beverage. During the treatment period, women used a daily menopause diary to record the number and severity of hot flashes at baseline and for every 4 weeks over a 12-week period. No significant differences were found between the soy and placebo groups in the number of hot flashes or hot flash scores. However, both groups had reductions in hot flashes, presumably due to the strong placebo effect. These results concur with other previous studies.24
Soy protein containing isoflavones and mammographic density in a randomized controlled trial in postmenopausal women
A double-blind, placebo-controlled randomized clinical trial investigated the effects of dietary soy supplementation on mammographic density, a risk factor for breast cancer. One-hundred-twenty-six postmenopausal women (aged 60 to 75 years) participated in the one year trial. Participants were randomized to receive either 99 mg isoflavones in soy protein or a casein placebo. Mammographic density decreased in both arms of the study between baseline and the end of the trial. No difference between the two treatments was in evidence. The results of this study indicate that soy isoflavones do not affect mammographic density.33
Effect of soy protein on endogenous hormones in postmenopausal women
One study examined the effects of soy protein with isoflavones on hormone concentrations in 73 free-living, hypercholesterolemic, postmenopausal women. Women were given 40 grams of protein as casein from non-fat dry milk (control), isolated soy protein (ISP) containing 56 mg isoflavones (ISP56), or ISP containing 90 mg of isoflavones (ISP90) over a 6-month period. Endogenous hormone concentrations were measured at baseline and at 3 and 6 months. Thyroxine concentration and free thyroxine index were higher in the ISP56 group, and the concentration of thyroid-stimulating hormone was higher in the ISP90 group than in the control group at 3 and 6 months (p<0.05). Triiodothronine was significantly higher in the ISP90 group only at 6 months. An inverse relationship was found between thyroxine, free thyroxine index, and thyroid-stimulating hormone and measures of baseline estrogenicity at 6 months. This particular study did not provide evidence that long-term consumption of soy protein alters steroid hormone values. However, these data suggest that soy protein may have some effect on thyroid hormone values, albeit not of clinical significance. There have been previous findings in animals of thyroid effects of soy protein, which demonstrates that future experimental and human research is needed to investigate the possible mechanisms of action.25
Effect of a phytoestrogen food supplement on reproductive health in normal males
There has been some concern about consuming soy and its effects on reproduction, especially for males. In this prospective study, healthy males consumed 40 mg isoflavones daily for 2 months. Blood and semen samples were collected 2 months before and 4 months after supplementation. Semen samples were analysed for ejaculate volume, sperm concentration, total sperm count, motility, and morphology; blood samples were analysed for sex hormone levels, gonadotrophin levels, and phytoestrogen concentrations; and testicular volume was measured using an orchidometer. Results showed that phytoestrogens had no effect on reproductive health in males. At this time, there is no strong evidence supporting reproductive problems in individuals consuming soy.26
Exposure to soy-based formula in infancy and endocrinological and reproductive outcomes in young adulthood
A recent cohort was conducted on adults 20-34 years of age who, as infants, were enrolled in controlled feeding studies at the University of Iowa. No significant differences were found in various physiological and reproductive measures (i.e. self-reported pubertal maturation, menstrual and reproductive history, height and usual weight, etc.) between men and women who received soy formula and those who received cow’s milk formula as infants. Consumption of infant soy formula over cow milk formula does not seem to negatively affect general health or reproductive health. Future studies should be conducted to further explore these findings.27
The dietary supplement information contained on this site has been compiled from published sources thought to be reliable, but it cannot be guaranteed. Efforts have been made to assure this information is accurate and current. However, some of this information may be purported or outdated due to ongoing research or discoveries. The authors, editors and publishers cannot accept responsibility for errors or omissions or for any consequences from applications of the information in this site and make no warranty, expressed or implied, with respect to the contents herein.
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