Introduction
Menopause is a sensitive and challenging period for women, which leads to various problems such as osteoporosis, mood swings, decreased libido, low back pain, and short stature over time [1]. Osteoporosis is an age-related skeletal disease characterized by decreased bone density and loss of bone tissue, leading to increased bone fragility and fracture [3]. In general, osteoporosis occurs when new bones do not form and do not fit together [4]. Among the group with the highest risk and vulnerability is the group of older and postmenopausal women [4]. Various factors are effective in changing the markers of bone metabolism: internal factors such as gender and hormones and external factors such as lifestyle [5]. Calcium, phosphorus, and alkaline phosphate are indicators of bone metabolism, the increase or decrease of which has many effects on bone density. Any disorder in bone metabolism and minerals may lead to osteoporosis and other irreversible bone damage [6]. Among the markers of bone metabolism, alkaline phosphate is an essential enzyme. This enzyme induces the ossification process, which breaks down inorganic phosphate from organic phosphates and increases calcium-phosphate production [7]. The ability to mineralize has also led to the breakdown of inorganic pyrophosphate, an inhibitory factor in mineralization [8]. Regular physical activity has been shown to prevent falling and bone fractures and improve balance in people with osteoporosis [20]. 
Numerous studies have been performed on the effect of exercise on serum markers of bone metabolism, but the results are contradictory [26, 27]. Although the exact mechanisms of exercise activity in preventing osteoporosis or improving bone markers are still unclear, it is clear that increased mechanical pressure on bone tissue causes physiological adaptations that prevent skeletal diseases such as osteoporosis [28]. Prevention of skeletal diseases such as osteoporosis, especially in postmenopausal women, is a good way to treat this type of disease [34]. Medium-intensity aerobic exercise also affects the expression of the alkaline phosphatase gene and serum bone markers in sedentary postmenopausal women [29]. This study investigated the relationship between serum calcium, phosphorus, and alkaline phosphatase with lumbar bone mineral density in active and inactive postmenopausal women.
Materials and Methods
The number of subjects was 55 active postmenopausal women and 60 inactive postmenopausal women aged 45 to 85 years. Patients included postmenopausal women who had reached menopause two years earlier and received a bone densitometry scan at least six months earlier. Also, they had a file containing laboratory information in the computer archives of Ayatollah Kashani Hospital. Informed consent was obtained from all participants to use information related to blood tests and bone mineral density tests (dual-energy x-ray absorptiometry [DEXA]). Serum indices of calcium, phosphorus, and alkaline phosphatase and anthropometrics (age, weight, height, body mass index) were measured. Body mass index was obtained by dividing a person’s weight in kilograms by the power of height in meters. After contacting them and completing a questionnaire containing personal information, osteoporosis, and physical activity, 80 active postmenopausal women volunteered, 20 of whom were excluded due to using anti-osteoporosis drugs or having chronic disease. Finally, 55 active women and 60 inactive women were selected. In addition to descriptive statistics, the Pearson correlation coefficient test was used to analyze the research data. SPSS software v. 26 was used to analyze the data.
Results
The correlation coefficients between osteoporosis indices and lumbar bone mineral density of active postmenopausal women are presented in Table 1.


The values related to the correlation coefficient between body mass index and lumbar mineral density of inactive postmenopausal women are presented in Table 2. 


Discussion
The present study showed a significant relationship between serum calcium, phosphorus, and alkaline phosphatase with bone minerals among active postmenopausal women. Calcium-based phosphate is associated with a reduced risk of osteoporosis in hemodialysis patients. The researchers reported a significant relationship between phosphorus and serum calcium levels and osteoporosis [30]. Inconsistent with the above study, Tariq et al. (2019), in a study entitled “ Alkaline phosphatase is a predictor of Bone Mineral Density in postmenopausal females” reported that alkaline phosphatase and calcium do not predict bone mineral density in the elderly, while serum alkaline phosphatase and calcium are strong predictors of healthy elderly [34]. Plasma and calcium-phosphorus changes are mostly related to hormonal factors that regulate calcium levels. Parathyroid hormone is an important factor in regulating bone metabolism. On the other hand, increasing the presence of phosphate ions also increases the amount of parathyroid hormone. Maintaining inorganic calcium/phosphate ion hostage through protein-binding receptors in the kidney, bone, and intestine is the most important physiological function.
The implementation of exercise programs both in treatment and prevention has a positive effect on bone mass density [35]. Exercise improves the absorption of calcium from the digestive system and therefore reduces bone mineral absorption. On the other hand, sufficient amounts of calcium reduce the production of parathyroid hormone, which affects the positive changes in bone mass [32]. Fathi et al. investigated the effect of eight weeks of aerobic exercise on serum levels of parathyroid hormone, estrogen, and alkaline phosphatase in obese women with premature menopause [39]. Their research showed that aerobic exercise significantly increases serum calcium, phosphorus, and parathyroid hormone levels in postmenopausal women. Al-Ghadir et al. examined the effect of 12 weeks of aerobic exercise on 65 patients (36 males and 29 females) [40]. At the end of the period, there was a significant increase in bone metabolic parameters such as free calcium, alkaline phosphatase, and bone mineral density. Considering this issue, one of the applications of the present study is to use the amount of these indicators in adulthood and old age in predicting bone mineral density. It may also help to use these blood and anthropometric variables in identifying people at risk for osteoporosis in adulthood and old age (people 55 to 85 years old).

Ethical Considerations
Compliance with ethical guidelines
This study has been approved by the Ethics Committee of Allameh Tabatabai University with ID IR.ATU.REC.1399.038.

Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Authors' contributions
All authors contributed equally in preparing all parts of the research.

Conflicts of interest
The authors declared no conflict of interest.

Acknowledgements
The authors thank Ayatollah Kashani hospital for granting access to the subjects and data support and thank the participants in this study.


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