Soy foods and phytoestrogens can also provide bone benefits

Mounting evidence suggests that the inclusion of soy foods in the diet may be one key strategy for good bone health and the prevention of bone fractures in older women. The evidence is sourced from various levels, including the impact of soy and/or its isoflavones on biochemical markers of the activity of bone cells, changes in the strength and quality of bone and risk of actual bone fractures.

Several recent studies using urinary and serum surrogate markers of bone cell activity show that taking soy foods and/or isoflavones increases markers of bone formation and decreases the levels of bone resorption markers in postmenopausal women ^{18 19} . In one 3 month study of post-menopausal women in the US, those who consumed soy drink or soy nuts each day experienced a drop in NTx - a chemical marker showing that bone-dissolving cells were less active on the soy diet. The level of a protein in the blood called osteocalcin also increased indicating that bone-making cells were more active with the soy intake ²⁰ . A meta-analysis of 9 randomised controlled studies investigating the effects on urinary deoxypyridinoline (a bone breakdown marker) and serum alkaline phosphatase (a bone formation marker) concluded that isoflavones taken as part of soy protein significantly inhibit bone resorption (breakdown) and stimulate bone formation in postmenopausal women, even if the dose provided is less than 90 mg per day and the intervention is shorter than 12 weeks ²¹ .

Earlier short-term studies in post-menopausal women and several more recent studies, 1-3 years in duration and in Asian and Caucasian women, have found an improvement in bone mineral density (BMD) and/or bone mineral content (BMC) with the consumption of soy and/or isoflavones. The first human trial published by Potter et al., showed that postmenopausal women who consumed 40 grams of soy protein containing naturally occurring isoflavones, daily for six months, significantly increased the BMD and BMC of their spines, in a study at the Department of Food Science and Human Nutrition, University of Illinois ²² . In peri-menopausal women, supplementing the diet with soy protein (with naturally occurring isoflavones) for six months also suppressed bone loss from their spine ²³ . Incorporating 45 grams of soy grits into the daily diets of Australian women resulted in a 5% increase in BMC, according to a short term study from the Royal Women's Hospital, Melbourne. ²⁴

Chen and colleagues from the Chinese University of Hong Kong conducted a one year study where they supplemented postmenopausal Chinese women with mid (40 mg) and high (80 mg) doses of isoflavones compared to placebo. They found that only women with lower initial BMC levels in the hip who received the daily high dose experienced significant but mild increases in their BMC but BMD did not increase with supplementation. ²⁵ Another research group from the same university followed 438 women over a period of 30 months in The Hong Kong Perimenopausal Women Osteoporosis Study (where 40 per cent of participants transitioned to menopause during this time) and found that soy intake was protective against BMC loss from trabecular bone. ²⁶

In a two-year randomised controlled trial of 89 postmenopausal women, Lydeking-Olsen and colleagues from the Institute of Optimum Nutrition in Denmark found that supplementing a nutritionally adequate diet with two cups of soy milk daily containing its naturally occurring isoflavones (76 mg aglycones) prevents lumbar spine bone loss as does the use of transdermal progesterone. ²⁷ But there is a negative interaction that occurs if both treatments are combined resulting in greater bone loss than either treatment alone.

A study of 389 osteopenic postmenopausal women living in Italy given a supplement of 54 mg isolated genistein daily for two years, found a significant increase in BMD in the lumbar and femoral neck region. ²⁸ Approximately half of the subjects continued on with the trial for an additional year and Marini et al., recently reported that after a total of three years of taking this dose of genistein each day, BMD increased by 8 per cent and 9 per cent at the spine and femoral neck, respectively, whereas taking the placebo resulted in a decrease in BMD of 12 per cent and 8 per cent at these sites ²⁹ , despite both groups receiving calcium and vitamin D supplements. These are the most impressive results of the effect of soy isoflavones on BMD to date.

However, not all researchers have reported a benefit of soy in preventing bone loss, ^{30 31} although positive effects were seen on bone turnover in some of these studies, ^{32 33} and the meta-analysis by Liu et al., found that the larger (at least 80 mg per day), but not smaller, dose of isoflavones taken for at least one year, tended to have a weak beneficial effect on spine BMD. ³⁴

An increased risk of bone fracture is the main outcome of osteoporosis. Although more long-term research is required, the data available from two population studies suggests that soy food consumption protects against bone fracture among postmenopausal women. In a study of 24 403 Chinese women (aged 40-70) from the Shanghai Women's Healthy Study followed for 4.5 years, soy intake was linked with a significantly lower fracture risk particularly among early postmenopausal women. Women within 10 years of menopause and in the highest soy food intake category (averaging at least 13 grams of soy protein or 60 mg isoflavones per day) compared to the lowest (less than 5 grams soy protein and 21 mg isoflavones per day) had a 48 per cent lower risk of bone fracture whereas late menopausal women with the same higher soy intake enjoyed a 29 per cent lower risk. ³⁵ These findings support the thinking that soy is most beneficial in the early period after the menopause and that habitual dietary levels may be adequate to protect bones. ³⁶

Interim findings from the Singapore Chinese Health Study presented by Koh et al. at the 8th International Soy Symposium held in Tokyo in 2008 support these results. Data available up until 2006 on this study of 63 257 middle aged and elderly subjects recruited between 1993 and 1998 indicates that while there is no relationship between isoflavone intake and hip fracture risk among men, the risk is reduced by about one third in women with the highest dietary soy intake. ³⁷ Interestingly, risk reduction was observed to take place in a dose-dependent manner when comparing the highest quartile with the lowest quartile of soy intake, meaning that soy is most beneficial for bone health when it is included more frequently in the diet compared to less often, even among a soy consuming population group.