The Study On Skeletal Muscle,Fat And Their Associations With Metabolic Risk Factors

Skeletal muscle (SM) is the largest non-fat component on tissue-organ level, which is central to nutrition, physiology and metabolism. Magnetic resonance imaging (MRI), which is of high accuracy and no radiation, is considered as the golden standard. However, the high cost of MRI precludes its routine use in clinical and research studies. Recently, DXA, which is at low cost and acceptable radiation, is now believed as an alternative measure of skeletal muscle in vivo. So far, to the best of our knowledge, there is no applicable skeletal muscle predication for Chinese. Therefore, the establishment of an applicable SM prediction model in Chinese is of great significance in further instructing the measurement and evaluation of physique and improving national physical fitness. In addition, the establishment of SM prediction model contributes to further understand the characteristics of SM and explore the association between muscle loss and fat accumulation with metabolic risk factors.With the lifestyle changes and age increases, the occurrence of muscle loss is accompanied by fat accumulation, which is defined as sarcopenic obesity (SO). Given that muscle loss and obesity both associate with metabolic abnormalities, it is of great significance to explore whether muscle loss and fat accumulation would more closely associate with metabolic disturbances in Chinese adults.This thesis includes the following aspects: 1) To establish a DXA-based SM prediction model in Chinese adults by using MRI as the reference standard.2) To describe the characteristics of SM and investigate its association with other human body components in Chinese adults.3) To explore the prevalence of SO and its association with metabolic risk factors in Chinese adults.The main findings are summarized as follows:1. The establishment of SM prediction model:SM predicted according to previous equation was significantly less than that measured by MRI which was underestimated by2.3%and3.4%in men and women, respectively. There was a significant correlation between SM and appnedicular lean soft tissue (ALST)(r=0.97, P<0.001). In the model development, ALST (in kg) was the strongest predictor of SM (in kg), explaining94.3%of the between-subject variance with a SEE of1.11kg. The age of45years was identified as the cut-off point, after which an obvious negative association between age and SM was observed. The correlation between measured and predicted SM is high (r=0.975, P<0.001).2. There was gender difference for total body SM. On average, SM in men was47%greater than in women. The gender difference on SM for total body still remained after controlling body weight and height. There was a curvilinear relationship between SM and age, which SM tended to remain relatively stable before the age of50years, yet began to decrease afterwards.SM is positively associated with height, weight, bone mineral content and negatively with fat mass. With age increase, the percentage change in SM was found to be greater than that in fat-free mass in both genders, indicating that SM was proportionally smaller than non-skeletal muscle mass in older adults.3. The prevalence of sarcopenia and SO varies with different diagnostic methods. In men, except HDL-C, a strong associtation was observed between SO and TG, blood pressure and fasting plasma glucose. In addtion, SO group has a higher risk of having TG and blood pressure abnormalities. In women, except TG, SO group didn’t indicate a higher risk of other metabolic risk abnormalities. However, in men and women, SO group both have a higher risk of have metabolic syndrome.In conclusion, we, for the first time, developed and validated a DXA-SM prediction models applicable for Chinese adults. This prediction model obviously made up for the defects of underestimating SM in Chinese according to a previous equation. By applying this model, we found that the combination of muscle loss and fat accumulation are more closely associated with metabolic abnormalities, although this observation should be further confirmed in a larger sample-size study. From the perspective of public health, the consideration of muscle together with fat may be of more theoretical and practical significance in improving health and the quality of life in the elders.