Estrogen is produced by the ovaries, adrenal glands, and adipose tissue. “Estradiol” is specifically produced by the ovaries while “Estrone” is by the adipose tissues. With menopause there is a decrease in Estradiol. This negates the osteoblastic activity and stimulates osteoclast activity which targets the bone receptors. The prolonged duration of menopause depletes the action of Estradiol and its task is replaced by estrone. Estrogen produced by adipose tissue correlates well with the fat tissues present in a woman (13). Another theory proposes that obesity releases hormones like preptin and amylin from the beta cells of the pancreas along with insulin that stimulates bone formation (14). Thus, implying that an obese woman has a protective role against osteoporosis which was seconded in our study. Our findings reported low BMI in women with a longer duration of menopause and also in osteoporotic women as compared to non-osteoporotic women. This finding was supported by Morin et al. (15). There are also some studies that refute this finding.
Women undergo two phases of bone loss. In the initial years of menopause, there is a sudden suppression of Estrogen that accelerates the rate of bone resorption coupled with formation, but comparatively the rate of resorption is more than formation. The first phase affects mainly the trabecular bone. However, the second phase, which occurs in late menopause reflects a slower rate of bone formation and accelerated resorption activity. Thus, explaining the decrease in osteocalcin and increase in urinary hydroxyproline in the longer duration of menopause. This is also called age-related bone loss and affects both cortical and trabecular bone. Park et al (16) also found that women with more than 10 years of menopause had significantly lower OC levels than lesser years of menopause. Atalay et al (17) seconded our findings with the highest OC levels in the early menopausal phase. Duration of menopause thus correlates well with BMI, OC, and urinary hydroxyproline. This finding was also supported by Montazerifar et al (18). In our study, the duration of menopause was divided into quartiles for analysing the in-depth effect of the duration of menopause on bone turnover. The majority of the osteoporotic women in our study had > 15 years duration of menopause.
Bone is a dynamic tissue that undergoes transformation throughout life with the coordinated action of osteoblasts and osteoclasts. During the embryonic stage, the cartilage is replaced by skeleton bone with the combined effect of these two osteocytes. Infancy to puberty thickness of bone is achieved by osteoblasts and length is achieved by the proliferation of growth plate chondrocytes. Osteoclasts provide shape to the bone during this phase of life and in the third decade, they attain maximum bone mass. After which there is a decline in the rate of bone formation due to Estrogen deficiency leading to negative bone balance thus contributing to osteoporotic fractures (19). Osteocalcin is an osteoblastic marker that is released by the bone matrix and is carboxylated on three glutamine acid residues. It is bone-specific and now a trending marker for detecting osteoporosis (20). In premenopausal women, the median serum OC concentration was 14.4 ng/mL and in postmenopausal women, the median serum OC concentration was 18.6 ng/ml (21). Similar finding was noted in our study with OC level being higher in postmenopausal women without fractures. There are a number of studies describing the same with osteocalcin levels ascending by 50–150% after menopause. However, studies on bone formation at different stages or years of menopause were sparse, OC level in our study slashed significantly between quartiles in all subjects with low OC values in women with more > 15 years and highest in less than 7 years of menopause. Among those without fractures (non-osteoporotic women), a significant reduction in serum osteocalcin occurred in the second and third quartile compared to the first. However, there was no difference in serum osteocalcin levels between the second and third quartiles suggesting that the osteoblastic activity plateaus after 8 years of menopause. This may indicate serum osteocalcin to be used as a screening tool for osteoporosis in post-menopausal women. However, in the case of osteoporotic women, there was no significant difference in osteocalcin levels between quartiles, thus low osteocalcin levels demonstrate osteoporosis. Osteocalcin monitoring is irrelevant once the fractures have occurred.
A positive correlation of serum osteocalcin was seen with BMI and calcium in our study whereas a negative correlation was observed with a duration of menopause and age which is in accordance with many studies (22, 23). With the increase in the duration of menopause the serum calcium level decreased. Menopause induces the synthesis of cytokines, monocytes, and T-cells that stimulates osteoclastic activity thereby modifying the calcium and phosphate homeostasis. Calcium and phosphorus are the two cardinal macro minerals required for bone building and therefore these correlations are predictable. No significant variation was observed in these parameters among groups with various years since menopause (24). However, in parallel to our findings higher phosphorus, calcium, and ALP activity have been demonstrated in early menopause (25). Heterogeneity, variabilities in body structure, and varied rate of aging may be responsible for the disparity in these results.
Gurban CV et al (26) propounded those osteoporotic women with longer periods of menopause, presented with higher values of resorption markers. Demirtas O et al (27) emphasized the positive correlation of bone loss and duration of menopause which was supported by the findings in the present study where a significant increase in urinary hydroxyproline across the quartiles, suggesting increased osteoclastic activity causing fragile bones in the elderly population. In those without fractures, a significant decrease in urinary hydroxyproline was observed in the second and third quartiles in comparison to the first as osteoblastic and osteoclastic activity is at equilibrium thus both values dip and plateaus. High urinary hydroxyproline levels were observed in osteoporotic women. Hydroxylation of proline and lysine are required for collagen formation which constitutes bone matrix and hydroxyproline is excreted in the urine during collagen degradation. There are various factors that increase the excretion of hydroxyproline such as vitamin D and calcium deficiency that explains its better correlation with calcium, phosphorus and alkaline phosphatase. Osteocalcin has a negative correlation with hydroxyproline. Therefore, low osteocalcin and high urinary hydroxyproline levels in the later phase of menopause are afflicted with osteoporotic fractures. However, variability in their measurements is obtained by a gelatine rich diet and is non-specific to bony lesions (28). Thus, making osteocalcin a better predictor to detect early osteoporosis.
The maximum strength to the bone is achieved by the combined effect of the inferior cortex that resists compressive loads along with the superior cortex that resists tensile loads. The cortical porosity of the bone is also an important independent factor in providing strength. Failure of this combined effort causes femoral neck fractures. Increased osteoclast activity in menopausal women opens up the Haversian canal forming large macro-pores that are dispersed thus making the cortical layer fragile, incapable to bear weight. The close proximity of these macro-pores reduces the impact on the bone during a trauma (29). The percentages of hip fractures in our study were higher in the third and fourth quartile demonstrating a higher risk of hip fracture in women with > 15 years duration of menopause, followed by wrist and spine fractures. Studies have revealed a two-fold increase in the risk of fractures in the highest quartile (30, 31). Trabecular bone mass reduces by 2.3% and 1.4% in the vertebral column and pelvic bones in the initial years of menopause. However, after 5 years of menopause, the bone mass starts further descending by 10% and 7% in these regions respectively (32). Osteoporotic fractures especially hip fractures are associated with excruciating pain, decreased mobility, and high dependence hence to find a bone turnover marker that could delineate the risk of hip fractures is very essential. There was a significant difference in osteocalcin levels between wrist and spine fractures and spine and hip fractures with the lowest osteocalcin levels observed in the spine fracture group. Urinary hydroxyproline showed no significant difference among the various fracture types. As hip fractures are more prone in subjects with a longer duration of menopause, the osteocalcin level accordingly should recede and urinary hydroxyproline should peak in subjects with hip fracture. However, the disparity in the findings in our study lacked the discovery of a marker in predicting different fractures. This was seconded by many studies where neither bone formation nor bone resorption markers were significantly associated with hip fracture risk in the elderly age group (33, 34). Few studies CTX and NTX to be useful to assess the risk of hip fractures but not osteocalcin (35). The type of fracture sustained cannot be gauged, as it may be due to the grade and impact of fall, along with the applied load to the bones.
Limitations
Other resorption markers should have been investigated and correlated to detect the most potent biomarker for primary osteoporosis. Vitamin D levels should have been assessed, as that affects bone formation. Decreased sample size in osteoporotic fracture group limited the use of BTMS in detecting fracture risk