Sarcopenia is a common disease among the older adults. As the world’s population is ageing, there will be higher prevalence of sarcopenia in older adults. In this study, it was discovered that the severity of sarcopenia aggravates as people get older. The PhA and MNA-SF scores were different in different stages of sarcopenia. PhA and MNA-SF were positively correlated with skeletal muscle mass index, grip strength, walking speed, SPPB scores, and body function. Both PhA and MNA-SF could effectively identify sarcopenia among the older adults in the community. The screening accuracy of PhA was higher than that of MNA-SF. At present, there is no study comparing the value of MNA-SF and PhA in identifying different sarcopenia stages, such as pre-sarcopenia and possible sarcopenia. By analyzing different stages of sarcopenia, it was found that the PhA has the value of screening possible sarcopenia and sarcopenia (common type), while MNA-SF has a great screening value in pre-sarcopenia. The PhA and MNA-SF both played a role in the screening of severe sarcopenia. This provided reference for the timely screening of patients with sarcopenia in different stages in the community.
The PhA is directly related to the volume of intracellular fluid and is closely related to muscle tissue, which may explain that individuals with higher muscle mass may also have higher PhA . Phase angle is also significantly correlated with grip strength and other motion indexes. High phase angle is significantly positively correlated with grip strength, knee extension force, and high activity level of masters . Similar to the results obtained in this study, the reason behind is mainly muscle mass and decrease in muscle strength of people with low phase angle, which further leads to the decrease of body function performance . Although the possible sarcopenia patients have not yet developed to the sarcopenia, the reduce grip strength and body function can also cause decreased exercise ability among the older adults, affecting the quality of life. This study showed that the PhA had certain significance in the screening of possible sarcopenia, but the screening ability was not high, which was mainly due to the fact that the possible sarcopenia patients only experience a decrease in grip strength and body function without a loss of skeletal muscle mass. It indicated that the nutritional status of the possible sarcopenia has not yet changed significantly, and the decline function may be related to fewer physical activities. Health education for the patients with possible sarcopenia should be prioritized in the community work, so as to prevent them from developing sarcopenia. The diagnostic criteria of possible sarcopenia according to AWGS2019, included grip strength or/and walking speed. While grip strength was only one of the indicators reflecting muscle strength, it was reported that back muscle strength was also an important indicator of muscle strength . In the present study, only grip strength was detected, which does not fully reflect the muscle strength level of the older adults. It was necessary to comprehensively consider various factors and indicators that affect muscle strength in the future study.
Mainly charactered by decreased muscle mass, pre-sarcopenia may be related to insufficient protein synthesis caused by the poor nutritional status of patients. As the largest protein pool in human body, skeletal muscle accounts for about 60% of the total protein and 20% of the protein weight of muscle. With the increasing age, protein synthesis reaction decreases, resulting in the decline of muscle mass . As a tool for screening nutritional status of the older adults, MNA-SF delivered the same results of malnutrition with that obtained according to the laboratory standards. Therefore, malnutrition in patients with chronic diseases can be better identified . In addition, MNA-SF also has good application value due to its role in reducing the rate of missed diagnosis while screening protein energy consumption . This study also found that MNA-SF scores was mainly positively correlated with SMI, and that MNA-SF had a high value in the screening of pre-sarcopenia. This finding could provide a certain reference for the community to carry out large-scale screening of pre-sarcopenia. In the screening of pre-sarcopenia, it was found that the screening ability of PhA combined with MNA-SF was higher than that of PhA alone, with a sensitivity of 75.00%, indicating that the combination of PhA and MNA-SF could identify 75.00% of the patients with pre-sarcopenia, while 25.00% of them might be negative (a false negative rate of 25.00%). Furthermore, PhA combined with MNA-SF could also identify 85.00% (specificity) of the patients without pre-sarcopenia and 15.00% (a false positive rate) of older adults were classified as pre-sarcopenic patients. This study aimed to identify patients with pre-sarcopenia as early as possible to take measures. Therefore, PhA combined with MNA-SF was preferred in screening the patients with pre-sarcopenia.
The PhA and MNA-SF indicated that nutritional status was closely related to sarcopenia components. In this study, it was found that the PhA was positively correlated with SMI, grip strength, 6 m gait speed, and SPPB scores, indicating that the change of phase angle could lead to changes in skeletal muscle mass, strength, and function. The PhA reflects the quantity and quality of soft tissue, nutritional status, and body function. When the nutrient intake is insufficient or protein deficiency is caused, the phase angle will be reduced. Our results also indicated that the PhA had a good application value in the screening of sarcopenia in the community, which is consistent with the results of Chen Xinyu et al. . The MNA-SF scale is widely used in the survey of nutritional status of patients with various chronic diseases because it is inexpensive and easy to use. Tan VMH et al.  found that MNA-SF can identify micronutrient deficiency caused by unbalanced diet, which further lead to decreased muscle mass and strength . It is also found in this research that the lower MNA-SF scores indicated malnutrition, which leads to the decline in muscle mass and strength and gradually develops into sarcopenia. Therefore, the PhA and MNA-SF both played a role in the screening of older adults with sarcopenia in the community.
Without timely treatment for the sarcopenia (common type), patients will suffer severe sarcopenia, which is mainly manifested by the simultaneous reduction in muscle mass, muscle strength and body function. Our study also found that patients with severe sarcopenia had lower PhA and MNA-SF scores than those in other stages, indicating a continuous decline in nutritional status. Both PhA and MNA-SF can effectively identify severe sarcopenia. Therefore, patients with severe sarcopenia should be should be provided with nutritional and medical treatment to alleviate the disease.
There were several limitations in our study. Firstly, the number of patients with pre-sarcopenia and sarcopenia (common type) and severe sarcopenia is small, so the sample size should be expanded for future study. Secondly, as a single-center, cross-sectional study, the results obtained only reflect the situation at the certain time and cannot determine the chronological sequence between malnutrition and sarcopenia. Thirdly, it is known that BIA is a technique for tests by introducing alternating current signals into the human body. The results of a test are susceptible to the unevenly distributed current and the different postures during the test. The subjects included in this study are older adults who cannot maintain a fixed posture for a while during the test. In addition, the results are also affected by the hydration status of the body. As a questionnaire, MNA-SF mainly asks subjects about their situation in the past month, which is highly influenced by the subjective feelings and recall bias of the older adults, and thus the results might also be affected . Fourthly, Hormones, protein supplements, and vitamin D were not significantly different among the groups at baseline, but other medications which may affect muscle mass were not fully included. In addition, we found differences in age, sex, and comorbidities among the groups at baseline, which were the potential influencing factors.
In conclusion, PhA and MNA-SF were correlated with community sarcopenia and its components, and both the PhA and MNA-SF showed a certain value in the screening of sarcopenia in different stages. Moreover, the cutoff value for possible sarcopenia and pre-sarcopenia was put forward. The screening ability of PhA alone or in combination was higher than that of MNA-SF in the screening of possible sarcopenia. The combination of PhA and MNA-SF performed better than the PhA or MNA-SF alone in the screening of pre-sarcopenia. The combination of PhA and MNA-SF or PhA alone all performed better value in the screening of sarcopenia (common type). Compared to MNA-SF, the PhA performed better in the screening of severe sarcopenia. BIA and MNA-SF are two indicators adopted in this paper to identify patients with sarcopenia in different stages. As a quantitative evaluation method, BIA was cheap and portable, which was more easily accepted by the older adults. MNA-SF was a questionnaire with six questions, which required less time to conduct surveys among the older adults. Both of the two methods were highly practical in the community. The test of the PhA and MNA-SF can help community workers identify the stage of sarcopenia and provide scientific prevention and control measures, which have important public health significance. As the results mentioned above are obtained from our research on the older adults in the community of China, the relevant researches targeting different groups in other countries or regions are needed to verify the results.