With the increase in human life expectancy and the aging of the population, osteoporosis has become a major global health concern. The prevalence of osteoporosis is particularly high among postmenopausal elderly women [20]. After menopause, bone density decreases by 2.5% per year, while premenopausal bone density decreases by approximately 0.13% per year [21]; thus, the premenopausal period is key for preventing and delaying the development of osteoporosis. Postmenopausal osteoporosis is affected by a variety of risk factors, and early symptoms are not obvious. The early identification of relevant risk factors and screening of high-risk groups should be prioritized in the clinic so that efforts can be made to control the loss of bone mass by adjusting lifestyle habits, dietary structure, and other measures, thus reducing the incidence of osteoporosis.

In addition to the effects of age gain and sex hormone reduction, factors such as gastrointestinal health, nutritional status, and bone conversion indices should not be ignored. In this study, we included various factors such as age, BMI, education level, dietary habits, exercise frequency, gastrointestinal symptoms, 25(OH)D, total protein, albumin, and prealbumin of postmenopausal women in our regression analysis. Our aim was to determine the impact of these factors on bone mineral density, which served as the dependent variable. The results of our analysis revealed that gastrointestinal health status and serum 25(OH)D concentration were significant predictors of osteoporosis risk in postmenopausal women. Additionally, we found that age, education level, BMI, dietary habits, and exercise frequency also had associations with bone mineral density, serving as important controlling factors.

Previous studies have shown that BMI can be used as an independent protective factor for BMD [22], but the relationship between BMI and osteoporosis has not been consistent among existing studies [23,24,25,26,27].In this study, the risk of osteoporosis increased with decreasing BMI. Although the BMI of the normal group was significantly greater than that of the osteopenia group and osteoporosis group, the BMI of the patients in each group was still within the normal range or was slightly overweight and did not reach the obesity level. The reason for the low risk of osteoporosis in the normal group under these conditions may be that adipose tissue can produce aromatase, which synthesizes estrogen outside the gonads and protects the bones [28]. Therefore, it is recommended that postmenopausal women try to gain weight within the normal BMI range.

In addition, dietary habits and exercise can also affect bone health. Studies have shown that supplementation with soy products, dairy products, nutritional supplements (calcium and vitamin D, etc.), and adherence to exercise have positive effects on BMD and bone metabolism [29,30,31]. Protein malnutrition reduces bone mass and changes muscle strength, leading to the development of osteoporotic fractures [32]. The results from the First National Health and Nutrition Examination Survey (NHANES I) also showed that hip fractures were more common in patients with low energy intake and low serum ALB levels [33]. In this study, the concentrations of total protein, albumin and prealbumin in the osteoporosis group were lower than those in the osteopenia group and much lower than those in the normal group. The reduction in protein led to a reduction in BMD, which was confirmed by the correlation analysis results. At the same time, among the participants included in this study, compared with those in the low-BMD population, those in the high-BMD population had relatively higher education levels and may have been more familiar with health care, which may have led to better lifestyle habits, such as the habit of consuming soy products, dairy products, nutritional supplements (calcium and vitamin D, etc.), and exercising regularly, which would result in healthier bones. Therefore, it is necessary to strengthen health education for middle-aged and elderly women with low educational attainment, promote reasonable supplementation with soy and dairy products and other nutritional supplements such as calcium and vitamin D, and increase the amount of exercise appropriately to prevent the occurrence of osteoporosis.

Dietary habits are also associated with gastrointestinal symptoms. The presence of gastrointestinal symptoms indicated that a subject had gastrointestinal disease or was in the predisease stage. The high gastrointestinal symptom scores in the osteopenia and osteoporosis groups in this study may indicate that gastrointestinal disorders lead to impaired bone health. Intestinal inflammation can adversely affect the accumulation of bone minerals [34,35,36], and patients with celiac disease have many gastrointestinal symptoms and are prone to osteoporosis or osteopenia [37]. Decreased bone density is a common consequence of gastrointestinal disease and in turn leads to a decreased quality of life in postmenopausal osteoporosis patients. Osteoporosis in postmenopausal women can lead to pain, fracture, and spinal deformation and may also be accompanied by sleep disorders, hot flashes, and night sweats, which can trigger anxiety, fear, depression, and other adverse psychological states. In this study, the PCS score of the normal group was greater than that of the osteopenia group and osteoporosis group. These findings indicate that elderly postmenopausal women with osteopenia or osteoporosis were more likely to suffer pain and fracture than were those with normal bone mass, which could lead to a decrease in the self-care ability and mobility of postmenopausal women. In this study, the normal group had the highest MCS, the osteopenia group had the second highest score, and the osteoporosis group had the lowest score. Bone pain, sleep disorders, and night sweats caused by reduced bone density can increase the susceptibility of postmenopausal osteoporosis patients to depression and anxiety, which can seriously affect their mental health.

In addition to the factors mentioned above, it is important to consider the relationship between bone turnover markers and the risk of osteoporosis. TPIN is a specific marker of type I collagen deposition that is formed by shearing off the amino-terminal prepeptide of type I procollagen under the action of protease during the formation of bone organic type I collagen; moreover, TPIN is a specific marker of type I collagen deposition. It can be used as a metabolite to directly assess the activity of osteoblasts after entry into the bloodstream and can sensitively reflect the state of bone formation in the whole body [38]. Alkaline phosphatase, a widely distributed membrane-bound glycoprotein, is also a marker of bone formation [39]. β-CTX is a known marker of bone resorption and reflects the degree of bone matrix degradation. Mature type I collagen in the bone matrix degrades into β-CTX and is released into the blood during bone metabolism [40].

In this study, all three groups of subjects were postmenopausal women, which may be the reason why there was no statistical difference in the above indicators. However, it can be seen from the data that the TP1N and CROSSL of the three groups were higher than the normal reference range, the patients all exhibited high bone turnover; the bone absorption indices (β-CTX) of patients in the osteoporosis group and osteopenia group were lower than those of patients in the normal group; and the bone formation indices (TP1NP) were greater than those of patients in the normal group. These findings may be related to the low bone mass of postmenopausal women, which further activated the high bone turnover state and resulted in more active osteoblasts and osteoclasts. Although TP1NP and β-CTX do not predict osteoporosis risk in postmenopausal women, Vasikaran et al. collated evidence from prospective PubMed studies published between 2001 and 2010 and concluded that bone turnover markers can predict fracture risk independently of other risk factors in postmenopausal women [41]. Therefore, it is meaningful to use these two indicators to evaluate bone health in postmenopausal women.

25(OH)D is the main source of vitamin D in the body, it is involved in regulating the metabolism of calcium and phosphorus, and is one of the essential components for intestinal calcium and phosphorus absorption and bone mineralization [42]; in addition, it can promote the activity of calcium and phosphorus, osteoblast and osteoclast proliferation [43].In this study, since all three groups of subjects were postmenopausal women, the serum 25(OH)D level remained below normal in the normal group, although it was significantly greater than that in the osteopenia and osteoporosis groups. Lieben et al. reported that the levels of calcium and 25(OH)D in the body are related to the quality and content of bone [44]. A low serum 25(OH)D concentration is considered an important risk factor for low BMD, and the serum 25(OH)D concentration in the lumbar spine and femoral neck of premenopausal women is positively correlated with BMD [45]. Therefore, increasing the serum 25(OH)D concentration is relevant for the prevention of osteoporosis in postmenopausal women.