Effect of low skeletal muscle mass and sarcopenic obesity on chronic kidney disease in patients with type 2 diabetes

This study aimed to investigate the association between low muscle mass or sarcopenic obesity and the risk of incident chronic kidney disease (CKD) in patients with type 2 diabetes mellitus (T2DM).


INTRODUCTION
Sarcopenia is a progressive and generalized skeletal muscle disorder involving loss of muscle mass and decline in physical function with advancing age [1]. Sarcopenia is associated with functional disability, risk of falls and fractures, poor quality of life, and even death, especially in elderly individuals [2,3]. Sarcopenia has been recently recognized to be associated with cardiometabolic disorders, including type 2 diabetes mellitus (T2DM) [4]. Moreover, another phenomenon during the aging process, obesity, particularly visceral obesity in combination with sarcopenia, may exert additive deleterious effects on metabolic and cardiovascular health compared with obesity or sarcopenia alone [5,6].
There has been growing evidence suggesting a link between sarcopenia and chronic kidney disease (CKD) [7,8]. Accumulation of uremic toxins, chronic inflammation, insulin resistance, hormonal imbalance, malnutrition, vitamin D deficiency, and oxidative stress contribute to the pathogenesis of sarcopenia in patients with CKD [9]. Several crosssectional studies have shown that both sarcopenia and obesity are associated with a higher prevalence of CKD in the general population [8,10].
Limited studies involving patients with diabetes have also indicated that sarcopenia or sarcopenic obesity is associated with CKD [11][12][13]. To date, only a single longitudinal study, to our knowledge, has examined the causal relationship between sarcopenic obesity and renal function decline exclusively in patients with T2DM [12]. Furthermore, no studies to our knowledge have assessed the association between low muscle mass and the risk of incident CKD in a large cohort of patients with T2DM and preserved renal function. Therefore, this longitudinal study aimed to evaluate whether low skeletal muscle mass (SMM) is independently associated with the development of incident CKD in a large cohort of patients with T2DM and preserved renal function. Moreover, we further explored the combined effect of sarcopenia and obesity on the risk of incident CKD.

Study population
Patients from the Seoul Metabolic Syndrome cohort were enrolled between January 2000 and December 2016 at the Huh Diabetes Center in Seoul, Korea, as previously described [14]. We included 3123 patients with baseline bioelectrical impedance analysis for measurements of muscle mass and more than three annual follow-up visits with renal function assessment. The exclusion criteria were age < 19years, diagnosis of type 1 diabetes, and baseline estimated glomerular filtration rate (eGFR) ≤ 60 mL/min/1.73 m 2 . The interval between visits varied among patients, and the last follow-up was conducted in December 2019. All participants provided written informed consent, and the study was approved by the Institutional Review Board of Inha University Hospital (Institutional Review Board No. 2020-06-031).

Measurements of clinical and laboratory indices
Anthropometric indices, including weight, height, and waist circumference (WC), were measured in all patients by a well-trained nurse who was blinded to the patients' clinical and laboratory data. WC was measured at the midpoint between the lower ribs and the iliac crest at the end of the expiratory phase. All patients underwent renal function tests and blood tests for metabolic parameters, including fasting plasma glucose, glycated hemoglobin (HbA 1c ), C-peptide, insulin, total cholesterol, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), triglyceride (TG), and high-sensitivity C-reactive protein (hs-CRP). Each patient's social and medical histories were collected using a self-administered questionnaire.

Short insulin tolerance test
A short insulin tolerance test was performed to assess the insulin sensitivity. The rate constant for plasma glucose disappearance (KITT: percentage per minute) was used as a marker of insulin sensitivity [16]. As previously described [14], the test was performed at 8:00 AM after an 8-hour fast. Venous blood samples were collected at 0, 3, 6, 9, 12, and 15 minutes after regular insulin (Humulin; Eli Lilly and Company, Indianapolis, Indiana) intravenous bolus injection at a dose of 0.1 U/kg. The plasma glucose disappearance rate (KITT) value was calculated from the slope of the fall in log-transformed plasma glucose from 3 to 15 minutes, which was used to calculate the time taken for the basal level of blood glucose concentration to decrease by half (t 1/2 ). The formula used was KITT = 0.693/t 1/2 Â 100 (percentage per minute). Higher KITT values indicated higher insulin sensitivity.

Measurement of body composition using bioelectrical impedance
The patients' body composition was assessed using a segmental multifrequency bioelectrical impedance analysis (BIA) system (InBody version 4.0, Biospace, Korea). In this study, the appendicular skeletal mass (ASM) was calculated as the sum of the lean muscle mass in the bilateral upper and lower limbs. Skeletal muscle mass index (SMI) was

Study Importance
What is already known?
• Previous studies have shown that sarcopenia is associated with renal function, especially with albuminuria, in patients with type 2 diabetes mellitus (T2DM).
• However, the causal relationship between low muscle mass and development of chronic kidney disease (CKD) is uncertain, particularly in patients with T2DM.

What does this study add?
• We have demonstrated that combination of sarcopenic obesity, but not obesity or sarcopenia alone, was independently associated with an increased risk of incident CKD.
How might these results change the direction of research?
• Future prospective trials are warranted to better understand the natural course of CKD associated with longitudinal dynamic changes in skeletal muscle mass and waist circumference in patients with T2DM. calculated by dividing ASM by body weight (kilograms) and expressed as a percentage (SMI = ASM/body weight Â 100%) [17] Definition of sarcopenia, obesity, and sarcopenic obesity Sarcopenia was defined as an SMI (percentage) < 2 SDs below the gender-specific mean for healthy young adults in the Korean population: SMI (percentage) < 29.0 in men and <22.9 in women was considered sarcopenia [18]. Obesity was defined as WC ≥ 90 cm in men and ≥85 cm in women [19]. Sarcopenic obesity was defined as the coexistence of sarcopenia and obesity. Individuals without sarcopenic obesity were classified as follows: reference (non-sarcopenic and nonobese), sarcopenic (sarcopenia without obesity), and obese (non-sarcopenic with obesity). Sensitivity analysis was performed using obesity defined by a BMI ≥ 25 kg/m 2 [19].

Statistical analysis
Continuous variables are reported as mean AE SD or median with interquartile range, whereas categorical variables are reported as numbers and percentages. To evaluate the differences in demographic T A B L E 1 Baseline characteristics of study population according to sex-specific SMI tertiles Cox proportional-hazards model was used to determine the independent association between four categories of body composition or SMI tertiles and incident CKD after adjustment for confounding variables,

Baseline characteristics of study population according to sex-specific SMI tertiles
A total of 3123 people were included in the study. The mean age was 56.9 AE 10.0 years, and the mean duration of diabetes was 7.4 AE 6.8 years. The characteristics of the study participants according to sex-specific SMI tertiles are summarized in Table 1

Association between sarcopenic obesity and incident CKD
Next, we investigated whether sarcopenia and obesity had a combined effect on the development of CKD in patients with T2DM.
F I G U R E 1 Cumulative CKD incidence according to sex-specific SMI tertiles (Kaplan-Meier analysis). CKD, chronic kidney disease; SMI, skeletal muscle mass index  (Figure 2; p < 0.001 by log-rank test).
To further examine the relationship between body composition and incident CKD, the Cox proportional-hazards model was used. Sarcopenia without obesity and obesity without sarcopenia were not T A B L E 2 Baseline characteristics of the study population stratified by incident CKD  Table 6).

DISCUSSION
In this retrospective longitudinal cohort study of 3123 patients with T2DM, we demonstrated that sarcopenic obesity, defined by SMI and WC, was independently associated with an increased risk of incident CKD during a mean follow-up of 8.9 AE 3.5 years, even after adjustment for BMI. However, patients with obesity or sarcopenia alone did not exhibit a higher risk of CKD. Moreover, there was no significant association between the SMI and incident CKD in patients with T2DM.
Previous studies have demonstrated an association between sarcopenia and CKD in the general population. In a population study in the United States, there was a stepwise increase in the prevalence of sarcopenia with declining eGFR levels; however, the association between eGFR levels and sarcopenia became insignificant after adjusting for clinical risk factors, including age [7]. Similar findings were also noted in a Korean population study in which there was an increase in the prevalence of sarcopenia with increasing stages of CKD; however, these associations remained significant in men only after adjustment for clinical risk factors [8]. This discrepancy may be due to differences in the definitions of sarcopenia, assessment modalities, and ethnicities. Only a few studies have examined the association between sarcopenia and CKD exclusively in the diabetic population.
In a meta-analysis on the association between sarcopenia and renal function in patients with diabetes, the main outcome of interest was albuminuria, which was significantly associated with sarcopenia [11].
Regarding the relationship between sarcopenia and eGFR, the results were inconclusive because of a lack of evidence in patients with T2DM. In a Chinese cross-sectional study, there was no correlation between muscle mass and eGFR in both men and women with T2DM [13]. In contrast, in a Japanese study of T2DM patients, sarcopenia was associated with rapid renal function decline in men during 3.6 years of follow-up, and, in agreement with our study, the combination of sarcopenia and obesity resulted in the greatest risk of rapid renal function decline [12]. However, they included patients with reduced eGFR levels at baseline and their outcome of interest was >30% annual rate of eGFR decline, whereas we exclusively included those with preserved renal function with an outcome of interest of incident CKD, with two consecutive eGFR < 60 mL/min/1.73 m 2 .
Moreover, they used the android/gynoid fat mass ratio to define obesity, assessed by a whole-body dual-energy x-ray absorptiometry (DEXA) scan, which may have comparable accuracy with WC [20] but a potential error in measuring a particular region of interest due to overlapping regions in patients with obesity [21]. In the present study, we observed that the risk of incident CKD did not increase in patients with either sarcopenia or obesity but was restricted to patients with sarcopenic obesity and T2DM.
Generally, sarcopenia is considered a complication of CKD, contributing to an increased risk of major adverse cardiovascular events and mortality [22,23]. However, when we address the issue of CKD from the point of view of sarcopenia, decreased insulin sensitivity and endothelial dysfunction due to loss of SMM may be a potential mechanism linking CKD to sarcopenia [24]. As skeletal muscle is a major target organ for insulin, loss of muscle mass can result in decreased insulin sensitivity with subsequent hyperinsulinism [25], which is linked to low-grade inflammation, endothelial dysfunction, and imbalance in adipokines [24]. These findings have been well replicated in our study, in which hs-CRP levels were significantly higher and KITT values were significantly lower in patients with sarcopenia and sarcopenic obesity. However, the association between sarcopenic obesity and incident CKD remained significant even after adjusting for tradi- finding may imply that there may be other factors beyond chronic inflammation and insulin resistance underlying the relationship between sarcopenic obesity and incident CKD.
Numerous observational studies have documented a significant association between obesity and the development and progression of CKD in the general population [26,27]. Increased adiposity may have a direct impact on kidney function by exerting unfavorable renal hemodynamic effects such as glomerular hypertrophy and hyperfiltration along with activation of growth factors and alteration in adipocyte-driven hormones [28]. Similar findings have been reported in patients with T2DM [29]. These findings were further supported by the Look AHEAD (Action for Health in Diabetes) trial, which showed that 8% weight loss by intensive lifestyle intervention resulted in a 31% reduction in CKD [30]. Recently, particular attention has been directed to regional adiposity rather than general adiposity; in addition, visceral adiposity, measured using WC, has been shown to predict various obesity-related outcomes [31,32]. In accordance with these findings, few studies have reported that visceral adiposity, but not general adiposity, is associated with CKD in patients with T2DM [33,34]. In line with these observations, our study demonstrated that T A B L E 4 Baseline characteristics of study population according to sex-specific sarcopenic obesity status (according to WC and SMI) The combination of sarcopenia and obesity, especially visceral obesity, is an age-related change in body composition and is currently one of the concerns in a growing aged society [35]. The actual prevalence of sarcopenic obesity significantly differs among studies depending on the background of the studied population, methods used for muscle volume quantification, and cut-off values [36]. There is accumulating evidence that obesity combined with sarcopenia further deteriorates the risk of insulin resistance and/or metabolic syndrome compared with obesity or sarcopenia alone [5]. Although it is still uncertain whether the increased risk of incident CKD is restricted to patients with sarcopenic obesity or applies to all patients with either sarcopenia or obesity, our study demonstrated that the combination of sarcopenia and obesity has a synergistic effect on the development of CKD in patients with T2DM. Therefore, it is important to identify those with sarcopenic obesity in the diabetic population as such patients also have a higher risk of developing CKD, as reported in our study.
This study had several strengths. It was a longitudinal study that involved serial measurements of kidney function to accurately identify incident CKD during 8.9 AE 3.5 years of follow-up. Our cohort consisted of a large number of participants from a homogeneous population with more than 8 years of follow-up. Moreover, we also measured the degree of insulin resistance using a standard KITT method and included a broad spectrum of clinical variables as clinical risk factors in our analysis. To our knowledge, this is the first longitudinal study to report that sarcopenic obesity is associated with an increased risk of incident CKD in Korean patients with T2DM and preserved renal function.
However, this study has several limitations. First, our estimation of SMM did not involve highly accurate modalities such as DEXA or computed tomography. However, BIA is a non-invasive method for assessing SMM and is useful in large population-based studies. Previous studies have reported a good correlation between the results of BIA and DEXA for estimating SMM [37]. Second, we did not have longitudinal data on the changes in body composition. Future prospective studies will be necessary to investigate the association between the relative changes in SMM and reduction in the incidence of CKD in patients with T2DM. Third, there is a possibility of inaccurate estimation of eGFR as a decrease in muscle mass may have led to a decrease in serum creatinine level, leading to an inaccurate reflection of renal function [38]. Fourth, as the follow-up interval was not predetermined, this may have introduced bias in the timing of detection of incident CKD. However, the median interval between each follow-up was 8.0 (interquartile range: 5.5-10.3) months, of which, the interval did not differ significantly among groups. Last, we did not have data on albuminuria, which is an important marker of diabetic nephropathy.
However, it is valuable to assess the risk factors that can lead to a reduction in renal function in patients with diabetes, as reduced eGFR in the absence of albuminuria is more commonly recognized and is a stronger predictor of cardiovascular diseases, especially coronary events and mortality [39,40]. Therefore, our findings may have an additive role in identifying individuals at a higher risk of CKD.

CONCLUSION
In a large cohort of patients with T2DM, we demonstrated that sarcopenic obesity is associated with an increased risk of incident CKD, independent of clinical risk factors. However, low muscle mass alone was not an independent risk factor. In addition to its role as a nutri-