Creatinine originates from skeletal muscle and is affected by renal function, while cystatin C, another serum marker of renal function, is a cysteine proteinase inhibitor that functions as a housekeeping gene excreted by all nucleated cells [18–20]. Therefore, the ratio of Cre/CysC corrected for renal function can reflect the total muscle volume in the body. However, muscle function, another key diagnostic element of sarcopenia, is correlated with muscle density rather than muscle size. Moreover, patients with diabetes mainly present reduced muscle performance and strength. To date, no study has evaluated the predictive value of Cre/CysC for muscle density.
This study first reported that Cre/CysC is an independent predictor of both the SMI and MMA. The predictive function of Cre/CysC in MMA may be one of the main principles of Cre/CysC predicting sarcopenia in patients with type 2 diabetes. This result is also consistent with a previous study showing that Cre/CysC predicts muscle strength in patients with non-dialysis-requiring chronic kidney disease [11]. Moreover, both the SMI and MMA are independent predictors of the outcome in critically ill patients [21]. Therefore, Cre/CysC has also been proven to be a predictor of outcome in patients treated in the ICU [22, 23]. Based on the above evidence, Cre/CysC could be regarded as a more comprehensive indicator of skeletal muscle composition and thereby predict sarcopenia, muscle strength and the outcome of patients with critical illness.
In addition to Cre/CysC, anthropometric characteristics, including sex, age and BMI, are independent predictors of the SMI and MMA. It is easy to understand that SMI and MMA values are higher in male patients than in female patients and decrease gradually with increasing age. Since overnutrition and lack of physical exercise are major inducers of type 2 diabetes, obesity, dyslipidaemia and abnormal fat distribution are common comorbidities in these patients. This study indicated that BMI and WHR are positively correlated with the SMI but negatively correlated with MMA. Furthermore, diabetes duration and the FMI were significantly negatively associated with MMA but not with the SMI. Therefore, the decrease in muscle density might be a major pathogenic mechanism of sarcopenia in type 2 diabetes.
Considering that skeletal muscle is the main consumer of glucose and the main target of insulin activity [24], this study further evaluated the correlation between Cre/CysC and the 100 g steamed bun test. Our research showed that Cre/CysC is negatively correlated with postprandial blood glucose but has no correlation with c-peptide release. Interestingly, there was no correlation between Cre/CysC and HOMA2 indexes. These results indicate that patients with high levels of Cre/CysC may have better postprandial glucose disposal ability, even though they showed no differences in pancreatic function and insulin sensitivity. The present study did show a correlation between Cre/CysC and HbA1c, which is consistent with previous epidemiological studies showing that HbA1c is not associated with sarcopenia in type 2 diabetes individuals [3]. However, a relationship between glucose control and sarcopenia cannot be completely excluded. Our study indicates that postprandial glucose disposal ability may decrease in type 2 diabetes patients with sarcopenia.
There are some indications that low muscle mass and reduced function could be associated with diabetic complications [3]. The present study has shown that Cre/CysC is negatively correlated with macrovascular diseases, including cardiovascular disease and lower extremity arterial disease, and this might be attributed to the impact of cystatin C. This study also showed that cystatin C is positively correlated with cardiovascular disease and lower extremity arterial disease (r = 0.211, P = 0.003 and r = 0.164, P = 0.023, respectively; data not shown). This is consistent with previous studies showing that cystatin C, rather than creatinine, is independently associated with heart and peripheral arterial disease [25–29].
In adults, the normal range of cystatin C is 0.51 to 0.98 mg/L independent of sex and age [30]. The normal range of creatinine for males is 0.6 to 1.2 mg/dl, and for females, it is 0.5 to 1.1 mg/dl [31]. Therefore, the ratio of Cre/CysC should be near 10. Some studies provided Cre/CysC values near 1 or 100 because they calculated the Cre/CysC ratio with different formulas: Cre/CysC ratio = [(serum creatinine {mg/dl}/serum cystatin C {mg/l})×100] or (serum creatinine {mg/dl}/serum cystatin C {mg/l}) [9–11, 32–34]. With more attention given to the ratio of Cre/CysC, a recognized and accepted formula should be proposed by an official institute.
Our study has some limitations. First, this is a single-centre study with a small sample population, and the findings in the present study should be further validated by further large-sample studies. Second, we did not assess muscle function (e.g., handgrip strength and gait speed), which has recently been advised by the European Working Group on Sarcopenia in Older People (EWGSOP) as one of the referred criteria for severe sarcopenia [14]. Third, as this was a retrospective study, we did not repeatedly assess the muscle composition and Cre/CysC; therefore, the predictive value of changes in Cre/CysC for muscle composition over time is unknown. Finally, further large sample studies are needed to better understand the prognostic value of Cre/CysC for diabetic complications.