From July 2016 to November 2016, in the case of informed consent, a random cluster sampling method was used to extract middle-aged and old people from Debao, Tiandeng, Daxin, Wuming County, and Yizhou City of southwest China in the Guangxi Zhuang Autonomous Region. A total of 837 people were selected, excluded according to the exclusion criteria, eventually 772 people including 300 males and 472 females.

Inclusion criteria: The study subjects selected people 40 to 90 years of age who were Zhuang and had no blood relationship with the three generations of grandparents in Guangxi province, China.

Exclusion criteria: Patients with chronic heart disease, liver and lung disease, patients with endocrine diseases, and those who have used drugs that affect bone metabolism within one year.

Since the age of 40, 10 years is an age group, 70 years and older are classified into the same age group, and men and women are divided into 4 age groups, a total of 8 groups.

The Medical Ethics Committee of Youjiang Medical University for Nationalities approved this study (2015030101). All procedures are carried out with the full understanding and written consent of all participants.

For OP, the diagnostic criteria established by the World Health Organization are used and the T score (T score is the value obtained by dividing the difference between the BMD of the measured person and the maximum bone mass of the healthy young person by the standard deviation) is judged. T > −1 SD is normal bone mass, −2.5 SD ≤ T < −1 SD for bone loss, and T < −2.5 SD for OP. Body mass index (BMI) = body weight (kg)/height (m²), diagnostic criteria: BMI < 18.5 for underweight, 18.5 ≤ BMI < 24 for normal weight, 24 BMI < 28 for overweight, BMI 28 is obesity. Grouping of BFP: male BFP < 12%, female body fat rate < 22%, low BFP group; male BFP 12% to 18%, female BFP 22% to 30%, standard BFP group; male BFP > 18%, female BFP > 30%, high BFP group.

The measured data was exported by body composition analysis software by Health Fitness Central Station Management System V9 (ACMEWAY Health Technology Co., Ltd., Beijing, CHINA). The data was analyzed using IBM SPSS Statistics version 18 (IBM SPSS Inc., Chicago, USA) software. Statistical methods such as the chi-square test, single-factor analysis, analysis of variance, and Pearson’s correlation analysis were used. P < 0.05 was considered significant. Classification variables were analyzed using logistic regression analysis for possible influencing factors. The difference was highly significant at P < 0.01.

WC, WtHR, and VFG in the male OP group were lower than in the normal bone mass group (P < 0.05), while the difference in BMI was statistically significant (P < 0.01). The BMI, WC and BFP of the female OP group were lower than those in the normal bone group (P < 0.05), but there was no significant difference in WtHR and VFG (P > 0.05) (Table 1).

The detection rates of OP in men and women were 29.3% and 64.2%, respectively, the detection rates of bone loss were 27.7% and 14.4%. The female OP detection rate is 2.19 times that of males, and bone loss reduction and OP are combined into the abnormal bone mass group. The abnormal bone mass rate for female bone mass is 78.6% higher than that of male 57%.

Male BMD was positively correlated with BMI, WC, WtHR and VFG (P < 0.05), and negatively correlated with age (P < 0.05) (Table 2). Female BMD was positively correlated with BMI and BFP (P < 0.01), negatively correlated with age (P < 0.01) and did not have a significant correlation with WtHR and VFG (P > 0.05).

Note: Compared to the normal group **P < 0.01 *P < 0.05.

Note: **P < 0.01 *P < 0.05.

The difference in bone mineral density and OP rate between the high body fat group and the low body fat group was statistically significant only in women (P < 0.05) (Table 3).

T value (T ≥ −1 is normal bone mass. T < −1 is bone mass loss) as a dependent variable. Age, height, weight, waist circumference, waist-to-height ratio, body fat percentage, visceral fat grade, visceral fat grade group, waist-to-hip ratio grouping, and body fat percentage grouping were used as covariates, and forward step (condition) of logistic regression was used for the analysis.

Regardless of whether male and female, BMI was a protective factor for bone mass reduction in model a (P < 0.01). In model b, BMI was still a protective factor for bone mass reduction (P < 0.001), while visceral fat grade was a risk factor for reduced bone mass (P < 0.05) (Table 4).

In the same BMI group, the BMD of males in Zhuang was lower than that of the Jing nationality. There was significant statistical significance between the normal BMI group and the obesity group (P < 0.01). The difference between the other two groups was not statistically significant (P < 0.01); the same BMI group of Zhuang female and Jing female had statistical significance (P < 0.01), and the bone density of each group of Zhuang female was lower than that of Jing (Table 5) and (Figure 1).

Compared with female body fat rate standard group**P < 0.01.

a. Variables entered in Model 1: Body mass index. Visceral fat grade (male); b. Variables entered in Model 2: Body mass index Visceral fat grade (female).

Compared with normal same BMI group *P < 0.05 **P < 0.01.

With the increasing incidence of osteoporosis and obesity, OP common complications, cardiovascular diseases and other chronic diseases caused by obesity seriously affect the health and quality of life of middle-aged and elderly people [7] . However, the effect of obesity on bone density remains controversial [3] [8] . Body weight, as a reflection of the body’s nutritional status, has been shown to be a protective factor for bone density, including fat free mass (FFM) and fat mass (FM), which is composed of muscle mass and bone mass. The amount of fat includes the amount of VFG and the amount of subcutaneous fat [9] . BMI can indirectly reflect the degree of fatness and thinness. In this study, the above indicators were used to evaluate the effect of obesity on BMD. Previous studies have suggested that the amount of fat has a protective effect on BMD [10] [11] . This study also showed that the BMD of the middle-aged and elderly people in Zhuang is positively correlated with BMI.

The BMI in the normal bone mass group is higher than that in the osteoporosis group, which is consistent with previous studies [12] . Multivariate conditional regression analysis showed that BMI was a protective factor for normal bone mass [13] . Small and middle-aged people are more likely to suffer from osteoporotic fractures than those with high height and high body weight [12] . However, studies found that obese people are more stressed on the vertebral body, and spinal fractures are more likely to occur when the bone density decreases [13] . This was also found in our research (data not show).

Comparing the research data of Zhuang and Jing and Han, high BMI also has a protective effect on BMD of Jing and Han people, which is consistent with the above conclusions. Under the same BMI group, BMD of male and female groups in Zhuang was lower than that of Jing and Han nationalities (Figure 1). The BMD of female in Zhuang was significantly different from that of Jing female. Because Zhuang and Jing belong to the same branch genetically, the possible reasons are presumed: Zhuang people live in hilly areas. Rice, corn and vegetables are their mainly food, while Jing people live on the coast for a long time with seafood such as fish and shrimp. This seafood is rich in protein and calcium ions, which are beneficial to increase the bone density of the Jing people and slow down their bone loss.

Recently, it has been found that obesity also has an adverse effect on bone. The bone density changes with the effects of fat on sugar, fat metabolism and endocrine in different parts [14] . When the local fat is excessively accumulated and the bone metabolism is increased, the bone loss of the relative parts is increased [15] . As the age increases, fat can be redistributed, the accumulation of visceral and abdominal fat increases, and abdominal obesity occurs [16] .

This study found that the effect of abdominal obesity on BMD in Zhuang female is not obvious, and the increase in female WtHR may lead to a decrease in bone density. The WC, WtHR and VFG of the Zhuang male normal bone group were higher than those of the osteoporosis group. The BMD was positively correlated with abdominal obesity index (WC, WtHR, VFG). However, after adjusting the age, body mass, height and other factors, using multivariate conditional logistic regression analysis showed that visceral fat grade was significantly associated with calcaneal bone mass loss and independent of bone mass loss of traditional risk factors. In the male population, the risk of bone mass loss was significantly higher in individual with visceral fat grade (over high), which was 9.286 times that of Visceral fat grade (normal). Visceral fat grade increases per unit in the female population, the individual’s risk of osteopenia increased by 38%.

Although fat has a certain protective effect on BMD, VFG over high is associated with bone mass loss, hyperglycemia, hyperlipemia, hypertension and coronary heart disease. Therefore, reducing VFG accumulation, proper physical exercise, and avoiding abnormal abdominal obesity can effectively delay bone mass loss and other chronic diseases [17] .

The effect of body fat percentage on the BMD of Zhuang female is larger than that of male. The small decrease of BFP can decrease the bone density, which may be related to the load of fat and bone tissue and the promotion of estrogen production [18] . The muscle mass ratio is consistent with the BFP. The higher the BFP, the higher the muscle mass, and the lower the incidence of osteoporosis, which is consistent with the study [19] [20] . Within a certain range, the greater the mechanical stimulation produced by exercise, the stronger the effect on bone promotion [21] [22] , which is related to the effect of osteoblasts and bone marrow stem cells on motor stimuli response and muscle strength [23] [24] . Therefore, high body fat ratio can cause many chronic diseases, so proper diet and proper physical activity are the main factors to maintain normal BFP and bone health [25] [26] [27] .