Accelerated Loss of Skeletal Muscle Strength in Older Adults With Type 2 Diabetes

RESEARCH DESIGN AND METHODS—

The Health ABC Study included well-functioning, community-dwelling older adults aged 70–79 years. The Health ABC Study was described in detail elsewhere (12). Among 2,618 participants who had completed baseline assessments for skeletal muscle mass and strength, 1,840 (70.3%) were reexamined 3 years later. The reasons for not having follow-up data were death (n = 146), development of disability and/or institutionalization (n = 302), missed contact (n = 77), withdrawal of the participants (n = 11), inability to perform a knee strength test (n = 191), and missing data on body composition (n = 51). All participants provided informed consent before participating in the study. The consent forms and study protocols were approved by the institutional review boards at each field center.

Diabetes assessment

Participants were considered to have type 2 diabetes if they had 1) a report of having the diagnosis of diabetes with onset after age 25 and/or 2) current use of oral hypoglycemic medications or insulin, or 3) a fasting plasma glucose concentration ≥7.0 mmol/l at baseline. Plasma glucose was measured using an automated glucose oxidase reaction (Vitros 950 analyzer; Johnson & Johnson, Rochester, NY), and A1C was measured by an enzymatic method (Bio-Rad, Hercules, CA).

Body composition

Lean masses of the upper and lower extremities and the total body were assessed using dual-energy X-ray absorptiometry (QDR 4500, software version 8.21; Hologic, Bedford, MA). The validity and reproducibility of the body composition data in the Health ABC Study may be found elsewhere (13,14). Quality assurance measures included the use of a body composition phantom for calibration and annual assessment for potential site differences or drift over time.

Strength assessments

Strength was measured using an isokinetic dynamometer (125 AP; Kin-Com, Chattanooga, TN) for knee extension and isometric dynamometer (Jaymar; JLW Instruments, Chicago, IL) for hand grip strength. For knee extension, maximal voluntary concentric isokinetic torque was assessed in Newton meters at an angular velocity of 60°/s. At least three, but no more than six, maximal efforts were allowed to produce three overlying curves, and the mean maximal torque was recorded and used for the analysis. The right leg was used unless contraindicated by pain or history of joint replacement. For validation of the knee strength assessments, we performed a reliability study in 63 participants. The interexaminer coefficient of variation (CV) was 4.85% with no significant differences between examiners. The intraparticipant CV was 10.68%, and the CV for combined effect of examiner and participant was 11.73%.

Isometric grip strength was assessed for each hand. Participants with severe hand pain or recent surgery were excluded. The vast majority of participants (96%) who had leg strength testing also had grip strength testing. For these analyses, we used the maximum of the force from two trials for the right upper extremity. A measure of muscle quality (leg-specific torque [Newton meters per kilogram] and arm-specific force [kilograms per kilogram]) was created by taking the ratio of strength to the entire corresponding leg or arm muscle mass in kilograms measured by dual-energy X-ray absorptiometry.

Other covariates

Sociodemographic characteristics included age, sex, race, and education. Combined chronic diseases such as coronary heart disease, congestive heart failure, stroke, peripheral artery disease, knee osteoarthritis, depression, and cancer were identified by self-report and confirmed by treatment and medication use. Self-reported poor eyesight was considered as impaired vision. Renal insufficiency was defined by serum creatinine level >1.5 mg/dl in men and 1.2 mg/dl in women (15). The ankle-arm index was calculated, and subclinical peripheral artery disease was defined by ankle-arm index <0.9. Health-related behaviors including smoking, alcohol drinking, and level of physical activity (kilocalories per week) were determined by using a standardized questionnaire (16). Interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) were measured in duplicate using an ultrasensitive enzyme-linked immunosorbent assay (R&D Systems, Minneapolis, MN). The lower limit of detection was <0.10 pg/ml for IL-6 and 0.18 pg/ml for TNF-α, with CVs of 6.3 and 16.0%, respectively.

Statistical analyses

Baseline characteristics of the cohort are presented separately for those with and without diabetes. χ² tests were calculated for categorical variables, and a Student's t test was used for continuous variables to test for any statistical differences between the two groups. Longitudinal changes of muscle strength and quality were calculated in both absolute terms and relative terms (percent change from baseline). Differences between older adults with and without diabetes were assessed by general linear models controlling for sex, race, age, and clinic site (model 1). Additional adjustments were made for BMI, baseline strength or quality, changes in muscle mass, and physical activity (model 2), plus combined chronic diseases and diabetes-related complications (model 3) and inflammatory cytokines (log-transformed IL-6 and TNF-α in model 4). P < 0.05 was accepted as statistically significant. All of the analyses were performed using SPSS software (version 12.0.0; SPSS, Chicago, IL).

RESULTS—

Among the 1,840 older adults with complete assessments of baseline and follow-up skeletal muscle mass and strength tests, 305 (16.6%) had type 2 diabetes at baseline. Older adults with type 2 diabetes were more likely to be men and black and to have a lower level of education (Table 1). Those with diabetes had greater body weight, BMI, and total fat mass as well as higher total lean mass than their nondiabetic counterparts. As expected, combined chronic conditions such as coronary heart disease, peripheral artery disease, impaired vision, and renal insufficiency were more prevalent in those with type 2 diabetes. IL-6 and TNF-α levels were significantly higher in older adults with diabetes (Table 1).

Both diabetic and nondiabetic older adults lost significant amounts of initial muscle strength in 3 years. However, older adults with type 2 diabetes lost their knee extensor strength more rapidly than those without diabetes (P = 0.001) (Table 2). Older adults with type 2 diabetes also lost greater amounts of leg lean mass than those without diabetes (P < 0.05). Furthermore, muscle quality (maximal strength per unit of muscle mass in Newton meters per kilogram) declined more rapidly in older adults with type 2 diabetes (P < 0.05). When expressed in relative changes, older adults with type 2 diabetes showed ∼50% more rapid declines in knee extensor strength (−9.0 vs. −13.5%, P = 0.002) and muscle quality (−6.2 vs. −10.0%, P = 0.01) in 3 years than those without diabetes. However, the changes in hand grip strength and arm muscle quality were not different between those with and without diabetes although older adults with diabetes lost greater amounts of arm muscle mass (Table 2). There was no indication of an interaction effect (P < 0.10) of sex or race with diabetes on the changes in muscle strength or muscle quality.

Table 3 presents the changes in knee extensor strength and muscle quality with controls for potential confounders. A greater decline of knee extensor strength in older adults with type 2 diabetes was not changed by adjustments for sex, race, age, clinic site, BMI, baseline strength, changes in leg muscle mass, and physical activity (models 1 and 2). The association of diabetes and loss of knee extensor strength was slightly attenuated by additional adjustments for combined chronic diseases and inflammatory cytokines (models 3 and 4).

A greater decline of leg muscle quality in older adults with type 2 diabetes was also evident, even after adjustments for demographics, BMI, baseline muscle quality, changes in leg lean mass, and physical activity (P = 0.001, model 2). Further adjustments for combined chronic diseases and inflammatory cytokines attenuated the association of diabetes and declines in muscle quality. However, the greater declines in muscle strength and quality in older adults with type 2 diabetes remained significant throughout the adjustment models (Table 3).

CONCLUSIONS—

In this study, older adults with type 2 diabetes lost 13.5% of their knee extensor strength, whereas those without diabetes lost 9.0% of initial strength in 3 years. An ∼50% more rapid decline in the knee extensor strength in older adults with diabetes was not accounted for by a greater loss of leg muscle mass. Muscle quality also declined more rapidly in older adults with type 2 diabetes, suggesting that diabetes may result in functional impairments in muscular function of the lower extremities, not necessarily accompanied by loss of muscle mass.

Sarcopenia, a status of decreased skeletal muscle mass, is commonly observed in older adults as a result of age-related loss of muscle mass (17–21). In general, it is frequently accompanied by lower skeletal muscle strength. However, determinants or risk factors for sarcopenia and low muscle strength in older adults have not been well identified (22). This is the first epidemiological study showing that type 2 diabetes is associated with rapid loss of skeletal muscle mass and strength in older adults. It confirms the previous cross-sectional finding of lower muscle strength in individuals with diabetes (10–12). It is also consistent with the finding of Andreassen et al. (23) who showed a rapid decline in ankle strength in patients with symptomatic diabetic neuropathy. The findings of this longitudinal study strongly suggest that low muscle strength in adults with type 2 diabetes is a consequence of rather than just a coincidence with type 2 diabetes.

We found discordance between the upper and lower extremities for diabetes and changes in muscle strength. A relative preservation of upper extremity strength has been observed in the process of aging (21,24). Our findings are, in fact, consistent with previous cross-sectional studies showing decreased skeletal muscle strength at the ankle and knee but not at the wrist and elbow in patients with type 2 diabetes (10). Andersen et al. (11) reported that upper extremity strength was preserved even in long-standing type 1 diabetic patients. They also found that muscle strength was related to the presence and severity of peripheral neuropathy in both type 1 and type 2 diabetic patients (10,11). It is well known that the lower extremities are predominantly involved in diabetic neuropathy presumably because of a length-dependent degeneration of nerve fibers (25,26). If neuropathy is a factor, skeletal muscle function is more likely to be affected in the lower extremities than in the upper extremities.

The exclusion of many participants for follow-up knee extensor strength test may have potentially biased the results to the null because proportionally more subjects with diabetes were excluded because of high mortality and other reasons (see Table 1 of the online appendix, available at http://dx.doi.org/10.2337/dc06-2537). We identified 47 participants with diabetes and 181 without diabetes who were excluded from the knee strength test but had the grip strength test at the follow-up examination. Among them, declines in hand grip strength were greater in older adults with diabetes than in those without diabetes (−3.3 ± 6.7 vs. −1.1 ± 6.2 kg, P < 0.05), suggesting that strict criteria for knee strength testing might select stronger individuals and actually obscure the true declines in muscle strength, particularly in those with diabetes (see Table 2 of the online appendix).

Lower extremity strength is essential for maintaining basic physical function, especially mobility such as walking and climbing stairs. It is well known that lower knee extensor strength is associated with an increased risk of incident mobility limitations (27–29). Although it is unclear whether there is a certain threshold level of leg strength to maintain physical function, lower muscle strength is definitely a risk factor for physical disability, independent of lower muscle mass itself (29).

Several studies suggested that the combination of sarcopenia and obesity (“sarcopenic obesity”) was more strongly associated with disability than either body composition type alone (30,31). It is possible that the rapid decline in muscle strength in older adults with type 2 diabetes may be associated with sarcopenic obesity. However, to our knowledge there is no study examining the changes in muscle strength in relation to sarcopenic obesity.

The mechanisms for the rapid loss of skeletal muscle strength, in older adults with diabetes are not known. Neuropathic processes involving motor neurons could affect muscle strength, as evidenced by the close association of muscle strength and severity of diabetic neuropathy in the previous cross-sectional observations (10,11). Electrophysiological studies showed that muscle strength in diabetic patients correlated with fiber density and amplitude of the macromotor unit potential, suggesting incomplete reinnervation after axonal loss (32). Longitudinal studies suggest an average loss of compound muscle action potential amplitude at a rate of ∼3%/year in patients with type 2 diabetes over a 10-year period (33). Further research should identify the role of the decrease in motor amplitudes on skeletal muscle strength and quality in subjects with diabetes.

In our study, adjustments for comorbid conditions such as cardiovascular disease, stroke, congestive heart failure, peripheral arterial disease, depression, impaired vision, and renal insufficiency slightly attenuated the declines in muscle strength. These results suggest that chronic complications of diabetes have a limited role in declines in skeletal muscle strength in older adults with diabetes. However, we had no reliable assessment of nerve function in our study at baseline. It is possible that declines in muscle function may indeed be the result of diabetic neuropathy (23).

Another potential mechanism is increased levels of inflammatory cytokines in subjects with diabetes. It has been reported that systemic inflammatory cytokines such as TNF-α and IL-6 have detrimental effects on muscle mass, strength, and physical performance in older adults (34,35). In our study, rapid declines in muscle strength and quality in older adults with diabetes are attenuated, albeit still significant, after adjusting for IL-6 and TNF-α. These findings may suggest a potential role of inflammatory cytokines on the muscular function in diabetes.

Our study has several limitations. The study participants were well-functioning, community-dwelling older adults who were relatively healthier than other individuals in the typical population of the same age. There were many dropouts, and only ∼70% of participants completed follow-up assessments. However, we believe that the loss to follow-up may produce an underestimation of the true decline in muscle function in those with diabetes (see Tables 1 and 2 of the online appendix). The questionnaire to assess physical activity in the Health ABC Study has not been validated in older diabetic subjects. It may not be sensitive enough to detect the influence of different physical activity level on the changes in muscle strength and quality in our study. We also lacked information about neuropathy at baseline, which would be related with muscular function in those with type 2 diabetes.

In summary, the present study demonstrates an accelerated loss of knee extensor strength and quality in older adults with type 2 diabetes.