Discussion

The results demonstrated soy protein consumption to have caused increase in TALP and reduction in DPD in menopausal women with osteopenia, while other parameters were not significantly difference. Comprehensive human studies on the effect of soy on TALP have not been carried out. In Arjmandi et al studies on rats a slight but insignificant increase in TALP was seen [18-20]. The crucial role of gut microflora in the metabolism of isoflavones in human beings has also been previously explored [21]. In vivo studies proved that bacteria in the gastrointestinal tract play an important role in determining the magnitude and pattern of isoflavone bioavailability [22]. However, only 30 to 40% of the population can produce equol from daidzein and interindividual differences in the bacteria responsible for equol production [23,24]. Register et al observed a significant fall in TALP in monkeys after 12 weeks [25]. Animal models such as monkey may convert daidzein into equol more readily than 30 to 50% of humans. It has been shown that equol possesses more estrogen-like properties than daidzein. This is why isoflavone efficacy has been less pronounced in monkeys and the results have been reported as reduced TALP levels [26,27].

TALP is an insensitive marker for bone formation compared to bone-specific alkaline phosphatase (BAP) or osteocalcin (OC). In this study, serum osteocalcin as a sensitive marker for bone formation has not changed during interventional period, indicating that soy protein may not enhance bone formation.

With regard to bone resorption, our results showed reduced urinary DPD levels following soy consumption which agrees with the finding of other investigators [28-30]. The effect of isoflavones on this indicator is so strong that Uesagi et al [31] observed consuming 61.8 mg of isoflavone for 4 weeks results in a significant reduction in urinary DPD. It can be said that DPD acts as a bridge between collagen fibrils which enter urine with collagen breakdown. As this is a very specific marker for bone resorption, its significant reduction in our study suggests soy consumption may prevent degradation of collagen the major protein in bone matrix [17].

Other serum indicators of bone metabolism were not affected in our study. In most studies on the effects of isoflavones in rats these phytochemicals, have been reported to have caused rises [32], no change [19,20] and even reduction [14] in bone formation as well as reduction or no change [33,34] on bone resorption. The changes observed in this study, therefore, are not contradictory to other studies and slight differences observed may be attributed to sample size, isoflavone dosing, period of intervention and dissimilarity of the studied groups.

Conclusion

In conclusion it seems soy protein can be effective in protecting bone mass through curbing bone resorption specially in high risk groups as was demonstrated in our osteopenic subjects but not enhance bone formation. Different studies have reported intake of 70–90 milligrams of isoflavones per day to be effective. Soy protein in our study provided 98 mg of isoflavones which is in accordance with other studies [35,36]. Some have reported lesser amounts can be effective in longer periods of time [37]. Soy protein consumption, thus, is a valuable plant estrogen which can be recommended for osteoporosis prevention.