Consumption Of Isoflavones Research Articles

Revealing anti-inflammatory mechanisms of soy isoflavones by flow: modulation of leukocyte-endothelial cell interactions

The antiatherogenic effects of soy isoflavone consumption have been demonstrated in a variety of studies. However, the mechanisms involved remain poorly defined. Adhesion of monocytes to vascular endothelial cells is a key step within the inflammatory cascade that leads to atherogenesis. Many factors, including the physical forces associated with blood flow, regulate this process. Using an in vitro flow assay, we report that genistein, a principal component of most isoflavone preparations, inhibits monocyte adhesion to cytokine (TNF-alpha)-stimulated human vascular endothelial cells at physiologically relevant concentrations (0-1 microM). This effect is absolutely dependent on flow and is not observed under static conditions. Furthermore, this inhibition was dependent on activation of endothelial peroxisomal proliferator-activated receptor-gamma. No significant role for other reported properties of genistein, including antioxidant effects, inhibition of tyrosine kinases, or activation of estrogen receptors, was observed. Furthermore, the antiadhesive effects of genistein did not occur via modulation of the adhesion molecules E-selectin, ICAM-1, VCAM-1, or platelet-endothelial cell adhesion molecule-1. These data reveal a novel anti-inflammatory mechanism for isoflavones and identify the physical forces associated with blood flow and a critical mediator of this function.

Phytoestrogens: Implications in Neurovascular Research

The early discontinuation of the Women's Health Initiative trial evaluating the effects of estrogen plus progestin due to concerns about the risk-benefit ratio of this steroid combination therapy emphasizes the need to examine alternative methods of estrogen replacement. One such alternative is isoflavone consumption of soy-derived dietary phytoestrogens that have received prevalent usage due to their ability to decrease age related disease (cardiovascular and osteoporosis), hormone-dependent cancers (breast and prostate), and peri- and postmenopausal symptoms. Differences in dietary phytoestrogen consumption result in large variations in somatic phytoestrogen content. These molecules affect estrogen and estrogen receptor function in several ways, including having both agonist and antagonist effects on estrogen receptors, as well as functioning like natural selective estrogen receptor modulators. Similar to estrogens, dietary phytoestrogens appear to affect certain aspects of vascular, neuroendocrine, and cognitive function. This article reviews health effects of estrogen, isoflavones and their hormonal mechanism of action, brain penetration by isoflavones, heath effects of isoflavones, and effects of isoflavones on vascular, neuroendocrine, and cognitive function. Because of their diverse health effects and widespread availability in soy foods, dietary phytoestrogens merit continued research into their effects on human health and cognitive function.

Isoflavone content of foods with soy additives

The health effects of consumption of isoflavones from soy products are being actively investigated, yet food composition tables often have incomplete values for commercial foods containing soy additives. Therefore, we analyzed 167 processed and fast foods with soy additives (e.g. isolated soy protein, soy protein concentrate, soy flour, and hydrolyzed soy protein) for the isoflavones daidzein, glycitein, and genistein. Total isoflavones were computed by adding these three isoflavones. Additionally, 12 traditional soybean products were analyzed, including soy flour and Asian condiments made from soybeans. Foods were sampled from food stores and fast-food restaurants in Hawaii. Concentrations of total isoflavones, expressed as aglycones, ranged from not detected (ND, for 49 foods) to 149.9 mg/100 g. Ranges within food groups were: bread and grain products (ND—93.9), gravies and sauces (ND—2.7), meat and poultry products (ND—15.9), meat substitutes (0.3–149.9), nutritional products (0.4–36.5), reduced fat peanut butters (0.1–4.0), seafood products (ND—1.6), and soybean products (2.9–109.3). For 56 foods on our food composition table, we calculated composite isoflavone values reflecting the mean of multiple brands and the proportion of brands containing soy additives. The resulting table now permits more accurate estimates of total isoflavone intakes by participants in our research studies.

Antioxidant activities and lipid lowering effects of isoflavone in male rabbits

Dietary flavonoids appear to play a role in the prevention of a number of chronic diseases such as cancer and cardiovascular disease and the soy isoflavones have been the focus of particular. Consumption of soy isoflavones may reduce the risk of cardiovascular disease both through reduction in serum lipids and by the antioxidant properties. We have therefore investigate the effects of either 2.5 or 5 mg/kg B.W. doses of isoflavones on the levels of free radicals, lipids and lipoproteins in male New Zealand White rabbits. Animals were orally given 2.5 or 5 mg/kg B.W. doses of isoflavones. The tested doses were given to rabbits every other day for 13 weeks. Treatment with isoflavones caused significant ( P<0.05) decrease in the concentrations of free radicals in plasma by 33% and 35%, liver by 18% and 27%, brain by 12% and 33%, testes by 40% and 21%, and kidney by 38% and 20% for 2.5 or 5 mg/kg B.W. doses, respectively, as compared to the control. On the other hand, the activity of glutathione S-transferase (GST) did not change in treated animals as compared to control. Also, results showed that isoflavones caused a significant decrease ( P<0.05) in the levels of plasma total lipids (TL) by 16% and 19%, total cholesterol by 20% and 20%, triglyceride (TG) by 18% and 23%, low density lipoprotein (LDL) by 19%, 22%, very low density lipoprotein (VLDL) by 18% and 23%, and LDL:HDL ratio by 36% and 39% for 2.5 or 5 mg/kg B.W. doses, respectively, as compared to the control. While the level of high density lipoprotein (HDL) increased by 29% and 32%. The present results showed that the 5 mg/kg dose of isoflavone seemed to be related to a better plasma lipid and lipoprotein profiles and antioxidant activity.

Critical Issues in R&amp;D of Soy Isoflavone‐enriched Foods and Dietary Supplements

ABSTRACT: Increasing consumer awareness of soy as a healthy food ingredient has led to a tremendous growth in sales of soy isoflavone‐enriched foods and dietary supplements in the past 3 y. Because of their weak estrogenic activity, isoflavones are believed to have preventive effects for several hormone‐dependent diseases. This article discusses some critical issues to be considered in any R&amp;D program of novel soy and isoflavone products for the healthy‐food market, such as (1) Which isoflavone compounds should be in the product? (2) Which individual isoflavones are the most beneficial ones? (3) Can certain isoflavone groups be targeted to specific disease prevention goals? (4) Are there any safety concerns in isoflavone consumption? (5) How are isoflavones affected by thermal processing and storage? (6) How should isoflavones be analyzed? These questions may affect the choice of isoflavone source, processing conditions, quality control procedures, and marketing considerations.

Regulative actions of dietary soy isoflavone on biological antioxidative system and lipid metabolism in rats.

Male Sprague-Dawley rats, 4 weeks of age, were fed purified diets either with or without 0.2% soy isoflavones rich powder for 5 weeks to elucidate their direct functions such as antioxidative action and regulation of lipid metabolism. Dietary soy isoflavones decreased serum lipid peroxide level in rats. Levels of liver and serum alpha-tocopherol were higher in the rats fed isoflavone than in those fed isoflavones-free diet. Thus, dietary soy isoflavones exhibited mild antioxidative function in this animal experiment. Isoflavone metabolites from diet may act as scavengers of reactive oxygen species. Dietary soy isoflavones lowered hepatic 3-hydroxy-3-methylglutaryl CoA reductase activity, although liver cholesterol level was not modulated. However, the levels of serum cholesterol and triglyceride decreased by consumption of soy isoflavones. Therefore, dietary soy isoflavones may exhibit hypocholesterolemic and hypolipidemic functions. Moreover, dietary soy isoflavones lowered hepatic Delta6 desaturase activity. Reflecting this observation, Delta6 desaturation indices ((18:2(n = 6) + 18:3(n = 6))/20:4(n = 6)) of tissue lipids tended to be lower in rats fed isoflavones than in those fed isoflavones-free diet. This action may contribute to the prevention of inflammatory response by imbalance of eicosanoids. These observations suggest that the positive intake of soy isoflavones may reduce the risk of some cardiovasucular diseases through their radical scavenging function and hypocholesterolemic action.

Usual dietary isoflavone intake and body composition in postmenopausal women.

Previous studies suggest an association between isoflavone supplementation and improved body composition. The purpose of this study was to investigate the cross-sectional association between isoflavone consumption from a typical Western diet and measures of total body fat, body fat distribution and lean body mass. This study examined 208 healthy, postmenopausal women, aged 45 to 74 years. At screening, demographic and behavioral characteristics and medical history were obtained. One month later, dietary intake for the past year was assessed with a standardized questionnaire. Anthropometric measurements were obtained, and bioelectric impedance was used to measure total body fat (lb) and lean body mass (lb). Isoflavone consumption did not vary by age, smoking, exercise, or daily alcohol intake. A significant inverse relation was shown between genistein consumption and weight, body mass index, waist circumference, and total body fat mass by bioelectric impedance analysis (P < 0.05). These findings remained significant after adjustment for potential confounders. No association was seen, however, between genistein, daidzein, or total isoflavone intake and body fat distribution or lean body mass. This study suggests that usual dietary isoflavone intake is associated with reduced total body fat and may consequently have a role in the prevention of chronic disease.

Relation between soy-associated isoflavones and LDL and HDL cholesterol concentrations in humans: a meta-analysis.

Differences in isoflavone content of soy protein may explain the absence of a dose-response relation between soy protein intake and blood cholesterol concentrations. To study specifically the effect of soy-associated isoflavones on cholesterol concentrations in well-controlled trials substituting soy protein with dairy or animal protein. Studies were identified by MEDLINE searches (1995 - 6 June 2002) and reviewing reference lists. Studies were included if they had a control group or treatment, experimental diets only differed in the amounts of soy protein and isoflavones and were each fed for at least 14 days. A total of 10 studies met these criteria, providing 21 dietary comparisons. : Studies comprised 959 subjects (336 men and 623 women), average age ranged from 41 to 67 y and baseline cholesterol concentration from 5.42 to 6.60 mmol/l. The intake of soy-associated isoflavones increased by 1-95 mg/day and the intake of soy protein increased by 19-60 g/day. Feeding daily 36 g soy protein with 52 mg soy-associated isoflavones on average decreased low-density lipoprotein (LDL) cholesterol by -0.17+/-0.04 mmol/l (mean+/-s.e.) and increased high-density lipoprotein (HDL) cholesterol by 0.03+/-0.01 mmol/l. There was no dose-response relation between soy-associated isoflavones and changes in LDL cholesterol (R=-0.33, P=0.14) (Pearson correlation coefficient) or HDL cholesterol (R=-0.07, P=0.76) or their ratio. Consumption of soy-associated isoflavones is not related to changes in LDL or HDL cholesterol.

Soy, isoflavones, and breast cancer risk in Japan.

Although isoflavones, such as those found in soy, have been shown to inhibit breast cancer in laboratory studies, associations between consumption of isoflavone-containing foods and breast cancer risk have been inconsistent in epidemiologic studies. We evaluated the relationship between isoflavone consumption and breast cancer risk among women in the Japan Public Health Center-Based Prospective Study on Cancer and Cardiovascular Diseases (JPHC Study). In January 1990, 21 852 Japanese female residents (aged 40-59 years) from four public health center areas completed a self-administered questionnaire, which included items about the frequency of soy consumption. Through December 1999 and 209 354 person-years of follow-up, 179 women were diagnosed with breast cancer. Cox proportional hazards regression was used to estimate the relative risks (RRs) and 95% confidence intervals (CIs) for breast cancer in relation to consumption of miso soup, soyfoods, and estimated isoflavones. All statistical tests were two-sided. Consumption of miso soup and isoflavones, but not of soyfoods, was inversely associated with the risk of breast cancer. The associations did not change substantially after adjustment for potential confounders, including reproductive history, family history, smoking, and other dietary factors. Compared with those in the lowest quartile of isoflavone intake, the adjusted RRs for breast cancer for women in the second, third, and highest quartiles were 0.76 (95% CI = 0.47 to 1.2), 0.90 (95% CI = 0.56 to 1.5), and 0.46 (95% CI = 0.25 to 0.84), respectively (P(trend) =.043). The inverse association was stronger in postmenopausal women (P(trend) =.006). In a population-based, prospective cohort study in Japan, frequent miso soup and isoflavone consumption was associated with a reduced risk of breast cancer.

Role of isoflavones in the hypocholesterolemic effect of soy.

Epidemiologic data suggest an inverse relationship between the consumption of soy isoflavones and cardiovascular disease risk. The aims of this review are to determine if isoflavones play a role in the hypocholesterolemic effect of soy and whether the studies realized with that scope were adequately designed. In humans, most studies have been performed in postmenopausal women. The results are inconsistent, however; some studies show a decrease in total cholesterol and low-density lipoprotein concentrations, and an increase in high-density lipoprotein levels, and other investigations fail to show any beneficial effect of soy isoflavones on lipid profiles. In most studies, beneficial effects could not be attributed with certainty to soy isoflavones. If these components have any health-protecting effect in humans, it is small in comparison with the effect of soy protein itself. There are currently not enough data to recommend the consumption of isoflavone supplements to lower plasma cholesterol levels.

Dietary Soy Protein Maintains Some Indices of Bone Mineral Density and Bone Formation in Aged Ovariectomized Rats

Hormone replacement therapy (HRT) has been used to prevent osteoporosis in postmenopausal women. However, HRT may increase the incidence of some cancers and has other side effects. There is considerable interest in dietary alternatives that include the consumption of soy and isoflavones derived from soy. The purpose of this study was to determine the effects of dietary soy protein on bone density, formation and resorption in cortical and cancellous bone in aged, ovariectomized rats. Specific emphasis was placed on indices of bone formation. Rats were assigned to the following groups: baseline; sham surgery + casein diet; sham + soy protein diet; ovariectomy (Ovx) + casein diet; Ovx + soy protein diet. The diets were fed for 3 mo. The Ovx Soy group had a greater bone mineral density (BMD) than the Ovx Casein group. There was a trend (P < 0.10) for greater periosteal bone formation rates in the Sham Soy compared with the Sham Casein group. In the Ovx Soy group, indices of endocortical bone formation were greater than those of the Ovx Casein group. There were no significant differences in resorption indices or endochondral growth (bone elongation) rates with soy in either the Sham or Ovx groups. In cancellous bone, the double-labeled surface and bone formation rates were greater in the Ovx Soy group than in the Ovx Casein group. These results show that dietary soy had a beneficial effect on the preservation of BMD associated with estrogen deficiency bone loss in aged rats. These data also show that at the tissue level, soy functions in a manner different from estrogen by increasing or sustaining elevated bone formation rates after ovariectomy.

Isoflavone Consumption Does Not Increase the Bone Mass in Osteopenic Obese Female Zucker Rats

Some controversy exists in the literature concerning the effects of leptin on bone metabolism. Thus we have compared femoral bone density and biochemical markers of bone metabolism in male and female fatty (leptin-resistant) Zucker rats and their lean homozygous controls at 3 and 6 months of age. At 3 months, no differences concerning total, diaphyseal (cortical bone), and distal metaphyseal (trabecular bone) femoral bone densities, plasma osteocalcin concentrations, and urinary deoxypyridinoline excretion were observed between fatty and lean rats. On the opposite, at 6 months of age, in both males and females, total, diaphyseal, and distal metaphyseal femoral bone densities and plasma osteocalcin concentrations were lower in Zucker than in lean rats. Soybean isoflavone consumption (40 µg/g body weight/day for 90 days, a dose which prevents osteopenia following ovariectomy both in lean Zucker homozygous controls and in Wistar rats) by obese female Zucker rats had no significant effect upon their bone mass.

Effect of soy protein isolate and conjugated linoleic acid on the growth of Dunning R-3327-AT-1 rat prostate tumors.

Epidemiologic and animal model studies suggest that consumption of soy isoflavones may be associated with reduced risk of prostate cancer (PC). In addition, animal model studies suggest that conjugated linoleic acid (CLA), a natural positional isomer of linoleic acid, inhibits tumor growth in various models, including models of PC. Based on the above-mentioned data, the objective of the present study was to test the hypothesis that supplementation of the diet with combinations of isoflavone-rich soy protein isolate and CLA would act to inhibit the growth of androgen-independent R-3327-AT-1 rat prostate tumor cells inoculated ectopically into male Copenhagen rats. The results of this study indicate that neither an isoflavone-rich soy protein isolate (SPI), nor CLA inhibit the in vivo growth and development of prostate tumor cells when administered in the diet either singly or in combination. Moreover, at the highest concentrations SPI and CLA (i.e., 20% SPI, 1% CLA), there was a statistically significant increase in tumors volume over controls. Administration of SPI at 10% in the diet also enhanced tumor growth, whereas at 5%, SPI exerted no measurable effect. CLA administration alone had no observable effects on AT-1 tumor growth. These results, in an established rat model, suggest caution in using isoflavone-rich SPI in human studies involving advanced hormone-refractory prostate cancer until further investigation of these effects are completed.

Flavonoid and botanical approaches to prostate health.

Benign prostatic hyperplasia (BPH) is a common problem among aging men that produces significant morbidity and health care costs. Contention exists as to whether currently available surgical and pharmacologic options for BPH are appropriate for men in the watchful-waiting stage. Recently, the possible benefits of phytotherapies (plant-derived preparations) in treating BPH and prostate cancer are being considered. Several phytotherapies, including saw palmetto, Pygeum africanum, curbicin, and isoflavone-containing supplements (red clover [Trifoleum pratense] and soy), are widely used in patients with BPH. Evidence suggests that the consumption of isoflavones found in legumes is related to lower rates of BPH and prostate cancer among Asian men. When evaluating natural therapies, the physician should look for a product that relieves symptoms and is safe, contains a health-conferring ingredient with a defined mechanism of action, and is standardized for that ingredient. Phytotherapies, particularly isoflavone-containing supplements, are likely to have an important role in the management of patients in the watchful-waiting stage of BPH.

Isoflavones protect against diesel engine exhaust injury in organotypic culture of lung tissue

The wide range of potential health beneficial effects of isoflavones, including a chemoprentive action, have prompted us to study the potential benefits of genistein and daidzein in an experimental model of environmental pollution impact on lung tissue. A diesel engine placed was used to generate reproducible emissions including both gaseous and particulate matters that are commonly found in urban atmospheres. Isoflavones were added to culture medium of rat lung slices 2 h prior to their exposure to pollutants for 3 h. Intracellular ATP and GSH levels, TNFα production, nucleosome assay and TUNEL labeling were monitored. Isoflavones showed almost total in vitro protection against inflammatory and pro-apoptotic responses in lung slices. Isoflavones 0.3 and 1 μmol/l protected against exhaust induced GSH depletion. Isoflavones 0.3 μmol/l appeared to exert the most beneficial effects. In conclusion, this study points out the potential interest of soy isoflavones consumption in polluted areas. Further studies should be undertaken to verify that similar effects could be obtained after in vivo administration of isoflavones.

Hormonal Effects of Soy in Premenopausal Women and Men

Over the past few years, there has been increasing interest in the possible hormonal effects of soy and soy isoflavone consumption in both women and men. Soy consumption has been suggested to exert potentially cancer-preventive effects in premenopausal women, such as increased menstrual cycle length and sex hormone-binding globulin levels and decreased estrogen levels. There has been some concern that consumption of phytoestrogens might exert adverse effects on men's fertility, such as lowered testosterone levels and semen quality. The studies in women have provided modest support for beneficial effects. One cross-sectional study showed serum estrogens to be inversely associated with soy intake. Seven soy intervention studies controlled for phase of menstrual cycle. These studies provided 32-200 mg/d of isoflavones and generally showed decreased midcycle plasma gonadotropins and trends toward increased menstrual cycle length and decreased blood concentrations of estradiol, progesterone and sex hormone-binding globulin. A few studies also showed decreased urinary estrogens and increased ratios of urinary 2-(OH) to 16alpha-(OH) and 2-(OH) to 4-(OH) estrogens. Soy and isoflavone consumption does not seem to affect the endometrium in premenopausal women, although there have been weak estrogenic effects reported in the breast. Thus, studies in women have mostly been consistent with beneficial effects, although the magnitude of the effects is quite small and of uncertain significance. Only three intervention studies reported hormonal effects of soy isoflavones in men. These recent studies in men consuming soyfoods or supplements containing 40--70 mg/d of soy isoflavones showed few effects on plasma hormones or semen quality. These data do not support concerns about effects on reproductive hormones and semen quality.

Comparative metabolism of genistin by human and rat gut microflora: detection and identification of the end-products of metabolism

1. Biotransformations by gut microflora play a pivotal role in determining the biological activity of isoflavones that occur in soya-based foods predominantly as β -glycosyl conjugates. Microflora prepared from rat caecae and human faeces were used to investigate the metabolic fate of genistein β -glycosides extracted from soya flour. The end-products of such metabolism were determined by parallel incubations of microflora with [2',3',5',6'-3H] and [4-14C]-labelled genistein. 2. Quantitative analysis by LC-MS/MS indicated very rapid and complete degradation of genistin, which was associated with a transient increase in genistein. Qualitative studies indicated that the malonyl and acetyl glycosides of genistein were also degraded by the microflora. 3. Incubation of caecal and faecal microflora with [3H] and [14C]genistein yielded similar radiolabelled metabolites, which were identified by radio-LC-MSn as the intermediates dihydrogenistein and 6'-hydroxy-O-desmethylangolensin and end-product 4-hydroxyphenyl-2-propionic acid. This profile of genistein metabolites indicated selective hydrolysis of 6'-hydroxy-O-desmethylangolensin between carbon atoms 1' and 1 to yield the end-products 4-hydroxyphenyl-2-propionic acid and 1,3,5-trihydroxybenzene. 4. The biological significance of the products of genistein metabolism warrant further investigation since they may play an important role in mediating the beneficial antioxidant health effects associated with the consumption of isoflavones in food.

Usual dietary isoflavone intake, bone mineral density, and bone metabolism in postmenopausal women.

Clinical trials of isoflavone supplementation and bone density have been of relatively short duration and yielded inconsistent results. Few studies examined the effects of usual dietary isoflavone intake on bone density, and none examined the effects on markers of bone turnover. This cross-sectional study examines the association of usual, unsupplemented dietary soy intake with bone density at the lumbar spine and hip and markers of bone turnover in postmenopausal women. Participants were 208 postmenopausal Southern California women aged 45-74 years. Information on behavioral and lifestyle factors was obtained, and dietary intake of isoflavones over the past year was assessed with a standardized questionnaire. Bone density was measured at the spine and hip with dual energy x-ray absorptiometry (DEXA). Urinary type I collagen cross-linked N-telopeptides (N-Tx) and pyridinium cross-links (PYR), both markers of bone resorption, and bone alkaline phosphatase (BAP), a marker of bone formation, were assayed. After adjustment for age and obesity, women with the highest daily intake of dietary genistein had N-Tx concentrations 18% lower than those of women who reported no daily genistein consumption (mean 37.29 vs. 45.44, respectively, p = 0.01). After adjustment for all covariates, there were trends toward significant differences in N-Tx (p = 0.09) and spine bone density (p = 0.07), whereby women with the highest level of isoflavone consumption had greater bone density at the spine. These results suggest that usual, unsupplemented dietary isoflavone consumption may be protective against bone loss in postmenopausal women through a reduction in bone resorption.

Soya isoflavones: a new and promising ingredient for the health foods sector

Isoflavones are phytoestrogens, they have a structural/functional similarity to human estrogen and have been consumed by humans world-wide throughout history. Of all plant estrogens, soy isoflavones have been studied most. Because of the extensive consumption of soy-foods in Asia and Japan, there is reasonably knowledge on long-term safety aspects, as well as on possible interactions between soy isoflavone consumption and disease prevention. In Asian countries, soy-isoflavone intake is estimated to range from 20–100 mg/day. The Western population, however, consumes much less. A comparison of populations on a high isoflavone intake with the normal Western population highlights a lower mortality rate and lower incidence of heart disease, type II diabetes, osteoporosis and certain cancers. If it is true that regular consumption of isoflavones may help to reduce the risk of certain diseases, then this is a good reason to educate people to consume isoflavone-rich foods. Based on the availability of extracted soy isoflavones it is now possible to study the effect on relevant bio-markers in controlled clinical trials. A confirmation of safety and efficacy is important for the establishment and legal/public acceptance of compound related health claims. Recent studies on the effect of isoflavones on heart health, bone health and post-menopausal symptoms, indicate positive effects that are highly attractive to the functional foods market. Further independent studies are required to substantiate health benefit claims.

Soy Intake and the Maintenance of Peak Bone Mass in Hong Kong Chinese Women

Our previous study on bone health among premenopausal women showed that bone mass consolidation is attained by the early 30s, and small loss of spinal bone mineral density (SBMD) occurs soon after peak bone mass attainment. Recent interest has been shown in the potential beneficial effects of phytoestrogens on bone health. However, data are lacking, particularly in Asian women. This study aims to investigate the effect of soy isoflavones intake on the maintenance of peak bone mass in a cohort of 132 women aged 30-40 years who were followed up for 3 years. Baseline measurements of SBMD (L2-L4) were obtained using dual-energy X-ray densitometry, and dietary intake of soy foods and other key nutrients, including dietary calcium, were obtained through a quantitative food frequency method. Information on body measurements; physical activity (PA), weight-bearing activity in particular; age of menarche; and number of pregnancies were obtained at baseline. Repeated measurements of SBMD were obtained yearly for a further 3 years with an average follow-up time of 38 months. Analyses were performed on 116 subjects with at least three SBMD measurements (at baseline, 3-year follow-up, and at least one measurement during follow-up). The individual SBMD regression slope was computed for each of the subjects. Soy isoflavones consumption was categorized as quartiles of intake. We observed a significant difference in the SBMD individual regression slopes between women belonging to the fourth and first soy isoflavones intake quartiles. The positive effect of soy isoflavones on SBMD remained after adjusting for age and body size (height, weight, and bone area). Multiple linear regression analysis including the other known covariates (lean body mass, PA, energy adjusted calcium, and follow-up time) showed that soy isoflavones, together with these variables, accounted for 24% of the variances of the SBMD individual regression slope. This longitudinal study shows that soy intake had a significant effect on the maintenance of SBMD in women aged 30-40 years. The effects of phytoestrogens on bone health should be explored further in a population with habitual dietary soy but low calcium intake.