We found that women who consumed about 1.3 cups of soy milk daily reduced the odds of having a low T-score by 57% compared to non-users even after adjusting for other important covariates including dairy consumption. This is almost identical to the 62% reduction in the odds of a low T-score among those consuming at least one serving of dairy compared to those who used these products less than twice a week. Our findings agree with several animal and human studies showing that calcium fortified soy milk are beneficial to bone loss prevention to a similar degree as cows’ milk [24
The similarity in physiologic characteristics between ovariectomized rats and postmenopausal women has brought supportive evidence from animal studies indicating the protective effect of soy milk on bone loss [24
]. Compared to the control ovariectomized rats, soy fed ovariectomized rats have been found to have higher levels of trabecular bone volume, insulin-like growth factorI (IGF-I), serum estradiol levels, and serum osteocalcin [25
]. In addition, there were lower levels of serum parathyroid hormone, trabecular area of resorption surface and bone resorption marker urinary deoxypyridinoline (DPD) among the soy milk fed rats compared to the control ovariectomized rats [25
Few studies have directly compared soy milk to cow’s milk. However, the effect of soy protein versus milk-based protein on bone health has been studied [24
]. The bone formation biomarker - serum insulin like growth factor I (IGF-I), bone resorption biomarker - serum urinary deoxypiridinoline (DPD) and urinary calcium excretion were compared among postmenopausal women who were fed with either soy protein or milk-based protein. Soy protein was found to have more impact in increasing serum IGF-I, especially among women who were not on hormonal replacement therapy, compared to the milk-based protein group. The bone resorption marker DPD was significantly reduced among the soy protein group, but not so among the milk-based protein group. While there was a 33% increase in urinary calcium excretion among the milk-based protein group, this effect was not observed among the soy protein group [27
]. In another study, the bone sparing effects of soy milk was observed in a 2-year randomized, placebo-controlled trial comparing the changes in BMD and BMC among postmenopausal women who were either fed soy milk containing isoflavones or applying transdermal progesterone or having both treatments. In both the soy milk group and the transdermal progesterone application group, there was no significant loss in BMD or BMC from baseline measurements as compared to the control group without any treatment [28
Supporting these studies is evidence from the Shanghai Women’s Health Study which found a significant negative association between fracture incidence and quintiles of soy protein intake among a large prospective cohort study of 24,403 postmenopausal women [29
]. The mechanisms behind a possible protective effect of soy is unclear. There is some suggestive evidence indicating that a higher consumption of soy is related to the capacity for equol production, a metabolite of soy isoflavone, that may have a favorable effect on bone loss [30
]. Thus, women in our study, having on average higher soy consumption due to their often lifetime lacto-ovo-vegetarian diet, may have acquired the capacity to produce more equol. However, this needs further assessment.
The protective effect of soy milk observed in our study could also be enhanced due to our endpoint - calcaneal BUA. Calcaneal bone consists mainly of trabecular bone which has been found to be the first region to respond to any mechanical stimulus [32
]. In one animal study, high doses of isoflavones improved microstructural properties of trabecular bone independent of BMD among ovariectomized rats compared to the control group [33
]. Among postmenopausal women, the effect of physical activity has been found to be related to calcaneal BUA independently of BMD [34
]. Heel bone is superior to femur or spine in reflecting the effect of physical activity [36
]. It is possible that the BUA can better reflect the trabecular changes of the calcaneal bone due to dietary stimulus as well.
In our study, dairy products showed a similar protective effect on bone as did soy milk. However, of all the dairy products, only cheese showed a significant protective effect on low T-scores when evaluated separately. These findings are similar to earlier findings of a protective effect of cheese observed among peri- and postmenopausal women in our previous cohort of Adventists where intake of cheese 3 or more times per week reduced the risk of wrist fracture by 58% [37
]. This could be due to the higher calcium and protein content per serving of cheese compared to milk [38
The effects of age, BMI, hormonal replacement therapy, smoking and previous minor fractures are similar to that observed by others [39
]. No significant associations between low T-score and use of calcium and vitamin D supplements were observed. The lack of association with calcium or vitamin D could be due to the several factors. The effect of calcium can only be observed when the total calcium intake is below the threshold point [49
]. Also, serum 25 (OH)D is a better marker for vitamin D exposure [49
]. Our findings of a protective effect of a vegetarian diet on low T-scores warrants further studies.
The strengths of our investigation are its prospective design with a follow-up period of at least 2 years and the employment of our validated food frequency questionnaire [19
]. The food frequency questionnaire has shown a high correlation with dietary intake, including soy protein intake, estimated from 24-hour dietary recall and isoflavonoid urinary excretion [19
]. The follow-up period of at least 2 years is ideal to test the effect of any clinical intervention on bone since the normal bone remodeling cycle takes approximately 6 to 12 months [50
]. A limitation is that despite a high correlation between ultrasound measurement, bone mineral density and fracture risk, we cannot apply our results to overall bone health as changes observed in bone density after feeding trials do not occur uniformly at all sites but vary even in the same individual [50
]. Furthermore, subjects could have changed their dietary pattern during the 2-year period between the baseline questionnaire and the BUA measurement. However, we have found that subjects in this population rarely change their overall dietary pattern over time [51
In conclusion, both soy milk and dairy intake are beneficial to bone health showing an independent protective association with a low T-score when evaluated together. While the protective effect of dairy products ranges from 14% – 83%, the protective effect of soy milk is similar from 11%–79%. Our findings on soy milk support the belief that soy isoflavones tend to reduce bone loss among subjects with low baseline bone density or those with low serum estrogen levels such as among postmenopausal women [52
]. Based on our findings, postmenopausal women could benefit from drinking soy milk and/or having a regular intake of dairy products.