Loss of muscle mass is a prerequisite for the diagnosis of sarcopenia. DM has been linked with an increasing risk of sarcopenia. However, most studies have been conducted on diabetic versus non-diabetic controls in the general population. The majority of study participants were hospitalized in the endocrinology department because of poor glycemic control. Even though some of them had comorbidities and other diseases, we excluded hospitalized patients with other disease comorbidities to minimize the potential impact of these diseases on muscle mass.
The present study demonstrated the adverse effects cast by elevated glycated hemoglobin levels on muscle mass, as well as the gender disparity. The association between muscle mass loss and elevated glycated hemoglobin was more pronounced among male patients, even after adjusting for age, BMI, diabetic comorbidities, hypertension, TyG index, and serological markers. In both between-group and stratified analyses, the risk of "low muscle mass" was increased for HbA1c ≤ 7%. In addition, not only was the risk of "low muscle mass" increased for HbA1c ≥ 8.0% but there was also a significantly lower muscle mass in this cohort compared with male patients with HbA1c < 6.5% and 6.5%≤ HbA1c < 7.0%. However, a similar phenomenon was not observed in women.
Previous studies have shown that elevated HbA1c in diabetic patients may be linearly associated with the development of sarcopenia, in which chronically hyperglycemic patients were predisposed to reduced muscle mass compared to those with normal HbA1c < 6.5%[15]. A longitudinal cohort study followed 588 patients with T2DM for one year and found that patients with a reduction in HbA1c levels of more than one percent restored their muscle mass during the follow-up period[10]. It can be hypothesized that high levels of HbA1c negatively affect muscle mass. However, their hyperglycemic state appeared to normalize after a certain period when the patient's muscle mass improved, thus suggesting a potential bi-directional relationship between DM and sarcopenia. Although the detailed mechanism of this relationship has not been fully elucidated, skeletal muscle could likely have increase blood glucose levels as its mass decreases as it is one of the target organs for material conversion and energy metabolism in response to glucose[16, 17]. Suboptimal blood glucose control can render our body in a state of "glycation", subsequently causing an accumulation of advanced glycosylation end products (AGEs). Proteins that are present in the extracellular matrix of human skeletal muscles are highly susceptible to chemical modification. For instance, the reduction of glyoxal groups to form AGEs can cause extensive damage to the corresponding tissues and exacerbate the negative effects on skeletal muscle mass through an up-regulation of inflammatory responses[18, 19].
On the other hand, insulin resistance has also been postulated as the link of the relationship between sarcopenia and DM. Insulin, the only hypoglycemic hormone secreted synthetically by the human body, occurs as a direct result of disruption in normal blood glucose levels[20]. In addition, insulin induces the synthesis of several proteins via the IGF1-Akt-FoxO pathway, resulting in skeletal muscle hypertrophy and inhibition of protein degradation in skeletal muscle cells. Subsequently, the down-regulation of this molecular pathway in insulin-resistant cells may lead to muscle wasting[21, 22].
Glycated hemoglobin levels in this study were considered high for both men (8.00 ± 2.02) and women (7.77 ± 1.78). Therefore, its effect on muscle mass appeared to be more pronounced. Age and BMI of diabetic patients had been established as potential risk factors of sarcopenia[23], consistent with our findings (Supplementary Table 1, Supplementary Table 2). With regard to gender disparity in the effect of blood glucose levels on the muscle mass of diabetic patients, several studies reported similar findings as our research. Xiulin Shi et al. recruited 1084 participants and reported an association between low muscle mass and higher blood glucose fluctuations in male patients, even if they were on the same insulin regime[6]. In another study conducted in Korea, HbA1c ≥ 8.5% was a risk factor for low muscle mass in men[24]. However, the definition of low muscle mass in this study was limited to the lowest quartile of the sample rather than following the recommended guidelines. The study also did not include female patient controls, thus compromising the generalizability of the study results and comparability with other studies.
As far as we are concerned, our study is one of the first that reported gender differences in the effect of HbA1c on muscle mass. Separate analyses of male and female patients hospitalized during the same period revealed a linear relationship between HbA1c and muscle mass in males. Furthermore, HbA1c levels of more than 7% could be a risk factor for low muscle mass. Despite the unclear mechanism underlying the gender difference, we postulated that it could be influenced by certain underlying factors and biological mechanisms. To begin with, the prevalence of diabetes is higher in men than in women due to lifestyles, behaviors, and other risk factors[25]. Biologically, women are protected against insulin resistance triggered by non-esterified fatty acids (NEFA)[26] and therefore show a greater capacity to combat lipotoxicity in skeletal muscle than men[27]. In addition, sex hormones play different roles in regulating skeletal muscle homeostasis. For males, testosterone promotes protein synthesis, muscle regeneration, and maintenance of muscle mass. Older men with declining testosterone levels are at an increased risk of sarcopenia[28]. In contrast, estrogens in females may protect skeletal muscles by reducing inflammatory responses[29].
There are certain limitations and strengths to this study. One of the strengths is that the effects of thyroid function on muscles were taken into account. Previous research on the association between thyroid hormones and muscle mass in T2DM patients showed that FT3 levels were protective against loss of muscle mass[30, 31]. To better assess the association between HbA1c and muscle mass, we further adjusted for FT3. After adjusting for these associations, the association was still highly significant in men. In terms of study limitations, the cross-sectional design means that a causal relationship cannot be established between HbA1c levels and muscle mass. Therefore, future prospective studies are needed to validate the results. Secondly, although diabetes complications were accounted for, we did not assess the glucose-lowering medications used and the duration of diabetes among the patients. Some medications (e.g. insulin, metformin) may affect muscle growth. For example, a multicenter study of older men with diabetes showed reduced muscle loss among those taking metformin compared to those not taking it[32, 33]. As these medications are commonly prescribed for T2DM patients, their effects on muscle mass should be further investigated. Thirdly, our results suggested that high HbA1c in men might predispose to low muscle mass. However, this result should be interpreted with caution due to the small sample size, especially with the low number of female patients with low muscle mass. Further expansion of the sample size is needed to better elucidate gender differences between high HbA1c and low muscle mass. Despite these limitations, our research provides new strategies and recommendations for reducing the risk of muscle mass loss among older T2DM hospitalized patients.