Estrogen plays an important role in maintaining bone density by regulating the formation and resorption of bone [20
]. Since lower circulating estradiol-1 levels are found during menopause, calcium is lost from the bone into blood plasma, leading to osteoporosis [21
]. One of the aims of hormone replacement therapy (HRT) is to prevent or lower the incidence of osteoporosis in postmenopausal women.
Prior to the turn of the twentieth century it was assumed that estrogens were exclusively produced by animals. However, the principle that plants can also produce estrogen-like molecules was established by 1966 [22
]. Now, it is recognized that certain plants and plant products contain phytoestrogens. One group of such compounds reported to have estrogenic activity is the flavonoids [23
Flavonoids are a large chemical class that are formed through the phenylpropanoid-acetate biodemial pathway via chalcone synthase and condensation reactions with malonyl CoA. Isoflavonoids are a subclass of flavonoids, where one phenolic ring has migrated from C-3 to C-2. The isoflavonoids from legumes, including genistein 2 and daidzein, are the most studied phytoestrogens. They can exist as glucosides or as aglycones, the glucosides being readily hydrolyzed in the gut to their aglycones. The aglycones are easily transported across intestinal epithelial cells [13
]. Genistein 2 has one-third the potency of estradiol 1 when it interacts with ERβ
, and one thousandth of the potency of estradiol 1 when it interacts with ERβ
as determined by expression of luciferase reporter gene construct in kidney cells that had been cotransfected with ERα
]. Genistein 2 can induce similar responses in breast, ovarian, endometrial, prostate, vascular, and bone tissues and cell lines as estradiol-1 [19
]. Genistein 2 can act as an estrogen antagonist in some tissues.
Soy isoflavones have an influence not only on sex hormone metabolism, but also possess other biological activities including cholesterol-lowering properties, anti-carcinogenic effects and more recently their protective role in bone health [27
Results of many studies on the protective role of soy protein, its isoflavones, or their combination on bone in ovarian hormone deficient models of osteoporosis are inconsistent. In these studies, the rate of bone formation and bone resorption, as assessed by biochemical markers are unchanged, decreased, or increased. Analogous to animal findings, the effect of soy and its isoflavones on bone in humans are also conflicting. Furthermore, it is not clear whether the bone protective effect of soy protein is due to its amino acid composition [29
], nonprotein constituents such as isoflavones [30
], or a combination of these factors [31
]. Animal studies using the ovariectomized rat model performed by Arjmandi et al. [30
] showed that the soy diet with isoflavones was more effective in preventing ovariectomy-induced loss of bone density than either the casein or the soy protein, depleted of its isoflavones, diet. Using an osteopenic rat model, Arjmandi et al. [31
] demonstrated a slight reversal of the ovarian hormone deficiency-induced loss of bone with soy protein diets with either normal or reduced isoaflvone content. Although animal findings [30
], so far, indicate the importance of isoflavones in preserving bone, it is still not clear whether isoflavones can exert similar bone protective effects independently of soy protein.
Findings by Picherit et al. [32
] support the bone protective role of isoflavones independent of soy protein. These authors reported that isoflavones dose-dependently prevented ovariectomy-induced bone loss in a rat model. However, the same group of investigators did not find similar beneficial effects of isoflavones in reversing bone loss in ovariectomized osteopenic rats [33
], even though the rate of bone turnover was reduced. Hence, one can speculate that a longer treatment period with isoflavones would have reversed bone loss. Whether the magnitude of effects of isoflavones on bone in these animal studies by Picherit [32
] would have been greater had isoflavones been given in conjunction with soy protein still remains to be answered?
On the other hand, studies investigating the effects of individual soy isoflavones, genistin and daidzin, support the important role of these naturally occurring compounds in bone health regardless of the dietary protein source. For instance, two weeks of a genistin-rich treatment (1.0
mg/day) in lactating ovariectomized rats was effective in maintaining trabecular bone tissue in comparison with ovariectomized control animals [34
]. Furthermore, in the same report, genistin stimulated alkaline phosphatase activity of an osteoblast-like cell line, suggesting a positive effect on bone formation. In another study, Fanti et al. [35
] reported that genistein (5
mg/kg body weight) maintained both cortical and trabecular bones in ovariectomized rats, and the bone-sparing effect of genistein appeared to be biphasic.
Although it is believed that genistin is the most potent of all the soy isoflavones, study by Picherit et al. [36
] reported that daidzin, is more efficient than genistin in preventing the ovariectomy-induced increase in bone turnover and decrease in bone mineral density. Clearly, this demonstrates that there are uncertainties as to which isoflavone plays a more important role in skeletal health. Use of a single isoflavone may not necessarily be the approach to be taken and future studies should address whether the combination of isoflavones exerts a more pronounced effect on bone.
To study the relation between soy isoflavone intake and bone mineral density (BMD), Greendale et al.[37
] analyzed baseline data from the Study of Women's Health Across the Nation, a US community-based cohort study of women aged 42–52 years. Their 1996-1997 analysis included African-American (n
= 497), Caucasian (n
= 1,003), Chinese (n
= 200), and Japanese (n
= 227) participants. Genistein and daidzein intakes were highly correlated (r
= 0.98); therefore, analyses were conducted by using genistein. Median intakes of genistein (measured in micrograms/day) by African Americans and Caucasians were too low to pursue relational analyses further. For Chinese and Japanese women, median genistein intakes were 3,511 and 7,151μ
g/day, respectively. Ethnic-specific, linear models were used to predict BMD as a function of energy-adjusted tertile of intake, controlled for relevant covariates. For Chinese women, no association between genistein and BMD was found. Premenopausal, but not perimenopausal, Japanese women whose intakes were greater had higher spine and femoral neck BMD. Adjusted mean spinal BMD of those in the highest tertile of intake was 7.7% greater than that of women in the lowest tertile (P
= .02); femoral neck BMD was 12% greater in the highest versus the lowest tertile (P
Most of the studies suggest that Phytoestrogens are somewhat effective in maintaining bone mineral density in postmenopausal women [38
]. A double blind placebo controlled study of postmenopausal women showed significant increase in BMD at the femoral neck after 12 months of daily administration of 54
mg genistein, isolated from soy, although a significant increase in osteocalcin and bone specific alkaline phosphatase (BAP) was also observed [39
]. In contrast 17β
-estradiol 1 increased BMD with a significant decrease in osteocalcin and bone alkaline phospatase (BAP) levels [39
In a 24-week study comparing isoflavone rich soy protein (80.4
mg aglycone isoflavones/day) and isoflavone poor soy protein (4.4
mg aglycone isoflavones/day) in perimenopausal women, both BMD and bone mineral content (BMC) were significantly higher with the diet high in isoflavones [38
]. There was no significant change in the BMD or BMC of the lumbar spine over the 24 weeks for the women on either the high or low isoflavone soy protein diet; however, the woman on the control diet had a significant decrease of BMD and BMC during this time. The women who had more bone loss had higher BAP levels. Therefore BAP may be a better indicator of bone turnover than bone formation [38
The lumbar spine seems to benefit the most from consumption of soy phytoestrogens. A 24-week long study with postmenopausal women consuming soy protein with 90
mg isoflavones/day showed a significant increase in BMD of the lumbar spine, with no effect on the femoral neck or total body BMD [40
]. There were no BMD effects in woman consuming soy protein with 56 mg isoflavones/day [40
]. The only study to examine the effects of soy isoflavones in premenopausal women showed no effect on bone mineral density levels [41
], while significant effects were observed in postmenopausal women in this study.
Brink et al. [42
] carried out studies in healthy men (59.2 ± 17.6
y), who were assigned to consume 40
g of either SP or milk-based protein (MP) daily for 3
mo in a double-blind, randomized, controlled, parallel design. Serum insulin-like growth factor-I (IGF-I), which is associated with higher rates of bone formation, was greater (P
< .01) in men supplemented with SP than in those consuming MP. Serum alkaline phosphatase and bone-specific alkaline phosphatase activities, markers of bone formation, and urinary deoxypyridinoline excretion, a specific marker of bone resorption, were not different between the SP and MP groups. Furthermore, because substantial reductions in bone density occur in men at ~65
y of age, these authors analyzed separately data for men ≥65
y and those <65
y of age. The response to protein supplementation was consistent in the two age groups. The effects of SP on serum IGF-I levels suggest that SP may positively influence bone in men. It should however be mentioned that their study was short, so longer-duration studies examining the effects of SP or its isoflavones on bone turnover and bone mineral density and content in men are warranted.
There are studies showing that the consumption of isoflavones has no any benefits for women in early postmenopausal period. Brink et al. [42
] conducted a randomized, double-blind, placebo-controlled, parallel, multicenter trial in two hundred thirty-seven healthy early postmenopausal women (mean (±SD) age of 53 ± 3
y and time since last menses of 33 ± 15
mo), who consumed isoflavone-enriched foods providing a mean daily intake of 110
mg isoflavone aglycones or control products for 1
y while continuing their habitual diet and lifestyle. These authors found that Consumption of isoflavone-enriched products did not alter bone mineral density of the lumbar spine and total body or markers of bone formation and bone resorption. Furthermore, Hormone concentrations did not differ between the isoflavone and control groups. The intake of isoflavone-enriched products resulted in increased isoflavone concentrations in plasma and urine, whereas control products did not.
A recent meta-analysis by Liu et al. [43
] showed only very slight benefits of soy isoflavones in increasing BMD in women. These authors identified 10 eligible RCTs in 896 women. A mean dose of 87
mg soy isoflavones for at least one year did not significantly affect BMD changes. The mean (95% CI) differences in BMD changes (in mg/cm2
/year) were 4.1 (−1.6, 9.8) (0.4%) at the lumbar spine, −1.5 (−7.2, 4.3) (−0.3%) at the femoral neck under random-effects model, and 2.5 (−0.5, 5.4) (0.2%) at the total hip by fix-effects model, respectively. Similar results were obtained in subgroup analyses by isoflavone sources (soy protein versus isoflavone extract), ethnic differences (Asian versus Western). Larger dose (≥80
mg/d), but not lower dose (<80
mg/d), of isoflavone intervention tended to have a weak beneficial effect on spine BMD (P
.08 versus P
= .94). Liu et al. [43
] concluded that soy isoflavone supplementation is unlikely to have significant favorable on BMD at the lumbar spine and hip in women.