BMD is an important parameter of bone strength [12]. Bone strength in good condition may reduce the risk of fracture [13]. It is essential to strengthen the BMD monitoring and management of infant and young child, which can help medical workers to take targeted early measures to solve the problem of abnormal BMD. In our research, QUS was used to detect the BMD of the middle tibia on the left side of the subject, and the proportion of insufficient BMD in infant and young child aged 6~36 months in the urban area of Chongqing was 14.4% based on the Z-score of the SOS value of Asian infants and young children. And the report result is similar to other provinces and cities in China [14-16].
The study found that the BMD deficit detection rate for boys was lower than that in girls among infant and young child aged 6~36 months in the urban area of Chongqing. This conclusion is nicely in agreement with the reported literature [16, 17]. Previous studies have reported that there is no difference between the male and female in BMD and bone mineral content in the prepubertal [18]. Research has indicated that sex hormones and growth hormone are key regulator of skeletal development during the period of bone development, however, no sex differences in the sex hormones and growth hormones levels were noted in prepubertal infant and young child and thus, contributions to BMD difference were not observed [19-24]. Studies have found that sporting activities has a performance enhancing effect for BMD [25, 26]. The positive effect of muscle activity can be due to an increase in BMD in response to a mechanical load on the bone and enhancement of osteoblast cell [27]. The higher BMD observed in infant and young child that was engaged in moderate exercise training for 10~15 min per day compared with those of children who do not exercise. In general, the amount of exercise of male infant and young child in the same age group were higher than those of the female, especially in infants and young children>1 year of age. Accordingly, we consider that different exercise volume may be responsible for part of the differences BMD between gender in our study.
Here, we observed that, as age increased, the incidence of insufficiency BMD was decreased. The result was agreement with the studies of Li et al. [28] and Klimu et al. [29]. Therein, the BMD deficit detection rate was highest at the 6 to 12 months old at 32.6%. This may be due to the fact that infancy is indeed the most characterized by rapid growth and development, had a greater demand for various nutrients [30]. And when Vitamin D and calcium preparations are not timely complementary, retention of calcium and phosphorus is decreased, resulting decreased bone mineral deposition [31]. Irregular diets are an important factor responsible for inadequate calcium intake and undernutrition in infants under 6 months of age, which may cause BMD insufficiency [32]. Vegetables, fruits and meat/fish were also increasingly introduced as the infants grew older, reflecting an increasing variety of complementary foods with age [33]. Moreover, the regular meal pattern formed and the digestive system also gradually matured occur with aging [34, 35]. It is mentioned above that may account for BMD increase with age. At the same time, with the acquisition of the ability to sit, crawl, stand with support and stand independently during age of 6 to 36 months, the posture of infant and young child changes from supine to upright position, which can enhance lower limb voluntary movement intention and load, contributing to the improvement of BMD [36].
In our study, it was found that overweight infant and young child had a greater prevalence of insufficient BMD, which is consistent with the results of a study of infant and young child aged 0 to 5 years in Jiangsu province [15]. Some investigators believed that obesity can protect bone due to greater mechanical loading on muscles and bones in obese individuals leads to increased BMD and decreased fracture risk [37-41]. Whereas some studies have demonstrated that overweight and obesity are more likely to sequestrate of vitamin D into a larger pool of adipose tissues in the body, leading to less bioavailability and disruption of hormone levels, which eventually leads to abnormal bone turnover [42, 43]. Dong et al. [44] pointed out that the relationship between obesity and BMD was not unidirectional. The relative size of two stimuli determines the BMD of our body. It is noteworthy that body weight, compose of body weight, fat mass, and lean body mass, is a heterogeneous phenotype [45]. Studies have reported that the effects of lean body mass on bone are mainly the result of mechanical loading signals. while fat tissue may also involve other non-weight-bearing effects, because of its metabolically active, such as hormone metabolism as proposed above [46, 47]. However, body composition analysis was not carried out in this study. We hope to expand the sample size and collect the information of body composition analysis to determine the contribution of fat mass or lean mass to BMD in future studies.
The effect of body weight on BMD has been clarified previously. We also find there was no significant difference in BMD between full-term infants with different birth weight, and it was speculated that BMD of infants and young children was related to calcium and phosphorus reserves in the late-pregnancy [48]. The third trimester thus makeup about 80% of fetal bone mineral accumulation and infants who are premature will be bereaved of this mineral accumulation [49]. Multiple studies have indicated that the BMD levels in term babies were significantly higher than preterm infants [50, 51]. In this study, the subjects included were singleton at term infant and young child. In addition to calcium and phosphorus accrual, many factors such as genetics, lifestyle and feeding pattern may affect BMD, so that there is no significant difference in BMD among infants with different birth weight.
In conclusion, this study we fulfilled the preliminary investigation of BMD of infant and young child aged 6-36 months in the main urban area of Chongqing, and it is clear that the rate of BMD insufficiency in girls is higher than that in boys, and BMD insufficiency rate is inversely associated with age and weight, but not related to birth weight. Limitations of this study lies in the fact as follows (1) Body composition analysis was not carried out; We can determine the contribution of fat mass or lean mass to BMD via body composition analysis in future studies. (2) The changes of bone calcium content with age were not analyzed in infant and young child. We will enlarged the sample size in subsequent research and collect more information to lay the groundwork for future studies
Some researchers [15, 52] proposed that the propaganda and education of knowledge is conducive to the improvement of children's BMD. Inadequate knowledge on child health care information likely leads to a high BMD deficiency prevalence in this region. Reinforcing the valued of parents to child health care and optimizing the living environment of children play a key role in promoting bone health and reducing the incidence of bone diseases in children.