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With Muscle Mass and Muscle Strength in Elderly Men and Women
a Institute for Research in Extramural Medicine, VU University Medical Center, Amsterdam, The Netherlands
b National Institute on Aging, Laboratory of Epidemiology, Demography and Biometry, Bethesda, Maryland
c Sticht Center on Aging, Department of Internal Medicine, Wake Forest University School of Medicine, Winston-Salem, North Carolina
d School of Human Movement Studies, University of Queensland, Brisbane, Australia
e Department of Medicine, University of Pittsburgh, Pennsylvania
f Division of Geriatric Medicine, University of Pittsburgh, Pennsylvania
g Prevention Sciences Group, University of California, San Francisco
Marjolein Visser, Institute for Research in Extramural Medicine, VU University Medical Center, Van der Boechorststraat 7, 1081 BT Amsterdam, The Netherlands E-mail: m.visser.emgo{at}med.vu.nl.
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Abstract |
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Methods. We used baseline data (1997–1998) of the Health, Aging, and Body Composition (Health ABC) Study on 3075 black and white men and women aged 70–79 years. Midthigh muscle cross-sectional area (computed tomography), appendicular muscle mass (dual-energy x-ray absorptiometry), isokinetic knee extensor strength (KinCom), and isometric grip strength were measured. Plasma levels of interleukin-6 (IL-6) and tumor necrosis factor-
(TNF-) were assessed by enzyme-linked immunosorbent assay (ELISA).
Results. Higher cytokine levels were generally associated with lower muscle mass and lower muscle strength. The most consistent relationship across the gender and race groups was observed for IL-6 and grip strength: per SD increase in IL-6, grip strength was 1.1 to 2.4 kg lower (p < .05) after adjustment for age, clinic site, health status, medications, physical activity, smoking, height, and body fat. An overall measure of elevated cytokine level was created by combining the levels of IL-6 and TNF-. With the exception of white men, elderly persons having high levels of IL-6 (>1.80 pg/ml) as well as high levels of TNF- (>3.20 pg/ml) had a smaller muscle area, less appendicular muscle mass, a lower knee extensor strength, and a lower grip strength compared to those with low levels of both cytokines.
Conclusions. Higher plasma concentrations of IL-6 and TNF- are associated with lower muscle mass and lower muscle strength in well-functioning older men and women. Higher cytokine levels, as often observed in healthy older persons, may contribute to the loss of muscle mass and strength that accompanies aging.
HIGH levels of inflammatory markers have been associated with increased morbidity and mortality in older persons (1)(2)(3). In addition, higher interleukin-6 (IL-6) levels in community-dwelling elderly persons have been associated with physical disability (4). This association was recently confirmed by a longitudinal study reporting that older, nondisabled persons with higher plasma IL-6 levels were more likely to develop disability in the next 4 years (5). These data suggest an important role for inflammation in the development of disability in old age.
Whether elevated proinflammatory cytokine levels are on the causal pathway to the development of disability or a marker of underlying disease associated with disability is still unclear (6). Medical conditions such as diabetes mellitus (7), cancer, (subclinical) atherosclerosis (8), congestive heart failure and rheumatoid arthritis (9), and lifestyle factors such as smoking (10) all have been associated with higher levels of proinflammatory cytokines and may contribute to future disability. However, a direct role for these cytokines on the development of disability can also be hypothesized. Proinflammatory cytokines may have a catabolic effect on muscle mass and muscle strength, both important determinants of disability and functional performance (11)(12).
Several studies in humans have shown a relationship between proinflammatory cytokine levels and muscle mass. In rheumatoid arthritis patients, lean body mass, a crude indicator of skeletal muscle mass, was inversely related to tumor necrosis factor- (TNF-) production by circulating mononuclear cells (13). Among patients with chronic heart failure or chronic obstructive pulmonary disease, IL-6 and TNF- levels were correlated with reduced lean tissue mass (14)(15). Moreover, treatment of patients with human immunodeficiency virus (HIV) with thalidomide, a potent anticytokine agent, resulted in an increase of lean body mass (16). The presence of a direct link between proinflammatory cytokines and muscle mass is also supported by the results from experimental studies. Administration of IL-6 or TNF- in rats increases skeletal muscle protein breakdown, decreases the rate of protein synthesis, reduces the total skeletal muscle amino acid concentration, and causes muscle wasting (17)(18)(19)(20)(21)(22).
Whether the association of proinflammatory cytokine levels with muscle mass or muscle strength is also present in the general population of older adults remains unclear. The aim of the study was to investigate whether higher plasma levels of the proinflammatory cytokines IL-6 and TNF- are associated with lower muscle mass and lower muscle strength in well-functioning black and white men and women aged 70–79 years.
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Methods |
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)], and were more likely to have heart disease (27.7% vs 22.0%) and current symptomatic osteoarthritis (24.7% vs 13.2%) (all p values <.05).
Proinflammatory Cytokines
The plasma concentrations of the proinflammatory cytokines IL-6 and TNF- were determined. Blood samples were collected at the clinic in the morning following an overnight fast of at least 8 hours. Specimens were processed according to standardized protocols by the Laboratory of Clinical Biochemistry at the University of Vermont. Plasma IL-6 concentration was measured in duplicate by means of a commercial ELISA High Sensitivity HS600 Quantikine kit (R&D Systems Inc., Minneapolis, MN). The lower detection limit was 0.10 pg/ml, the detection range was 0.156–17.0 pg/ml, and the interassay coefficient of variation was 7%. This method has a high degree of reliability and reproducibility, and a single IL-6 measurement has been shown in elderly persons to be representative of an individual's IL-6 level over an extended period of time (23). Plasma TNF- concentration was measured in duplicate using the ELISA High Sensitivity HSTA50 kit (R&D Systems Inc., Minneapolis, MN). The lower detection limit was 0.18 pg/ml, and the detection range was 0.5–32 pg/ml.
Muscle Mass
Two indicators of muscle mass were used in the study: appendicular skeletal muscle mass by dual-energy x-ray absorptiometry (DXA) and midthigh cross-sectional muscle area by computed tomography (CT).
Appendicular skeletal muscle mass was measured using fan beam DXA (Hologic QDR4500A, software version 8.21, Hologic, Waltham, MA) and was calculated as the sum of nonfat, nonbone tissue of both arms and legs.
The cross-sectional area of muscle in both thighs was measured by using CT (Memphis clinic site: Siemens Somatom Plus 4 [Siemens, Erlangen, Germany] and Picker PQ 2000S [Marconi Medical Systems, Cleveland, OH], at Pittsburgh clinic site: GE 9800 Advantage [General Electric, Milwaukee, WI]). To locate the midthigh scan position, an AP scout including the entire femur was obtained. The femoral length was measured in cranial-caudal dimension, and the scan position was determined as the midpoint of the distance between the medial edge of the greater trochanter and the intercondyloid fossa of the right leg. A single, 10-mm thick axial image (120 kVp, 200–250 mAs) of both thighs was obtained. All CT scans were transferred to a reading center and were analyzed by a single observer on a SUN Workstation (SPARCstation II, Sun Microsystems, Mountain View, CA) using IDL development software (RSI Systems, Boulder, CO). Areas were calculated by multiplying the number of pixels of a given tissue type by the pixel area. Density values were determined by averaging the CT number (pixel density) values of the regions outlined on the images. CT numbers were defined on a Hounsfield Unit (HU) scale where 0 equals the HU of water and -1000 equals the HU of air. The external contours of the thigh were determined using a threshold of -224 HU, and the external bone contours were derived at 150 HU. For each participant, the determination of soft tissue type was made using the bimodal image distribution histogram resulting from the distribution numbers in adipose tissue and muscle tissue (24). Intermuscular and visible intramuscular adipose tissue was separated from subcutaneous adipose tissue by drawing a line along the deep fascial plane surrounding the thigh muscles. The total area of nonadipose, nonbone tissue within the deep fascial plane was used as a measure of muscle area. Reproducibility of muscle area was assessed by reanalyzing a 5% convenience sample of the study cohort and showed a coefficient of variation <5%.
Muscle Strength
Two indicators of muscle strength were obtained: isometric grip strength and isokinetic knee extensor strength.
Grip strength was measured using a hand-held dynamometer (Jamar, TEC, Clifton, NJ). The dynamometer was individually adjusted for hand size, and two trials were performed for each hand. The sum of maximum strength of each hand was used as a measure of grip strength.
The maximal isokinetic strength of the knee extensors was assessed by a Kin-Com 125 AP Dynamometer (Chattanooga, TN) at 60° per second and was calculated from the average of three reproducible and acceptable trials out of a maximum of six trials. Participants with a systolic blood pressure 
200 mm Hg, diastolic blood pressure 110 mm Hg, or who reported a history of cerebral aneurysm or cerebral bleeding, bilateral total knee replacement, or severe bilateral knee pain were excluded from the test. Due to these stringent exclusion criteria, data from 2456 participants were available for the statistical analyses of knee extensor strength (79.9% of original cohort).
Potential Confounders
Covariates included clinic site, age, body height, total body fat, physical activity, health status, use of antiinflammatory drugs, and smoking. Body height was measured to the nearest mm using a wall-mounted stadiometer. Because taller persons have more muscle and greater strength, height was included in the regression models. Total body fat was assessed using fan beam DXA (Hologic QDR4500, software version 8.21). Physical activity of the past 7 days was assessed by questionnaire during a home interview. The time spent on climbing stairs, walking for exercise, walking for other purposes, aerobics, weight or circuit training, high- intensity exercise activities, and moderate-intensity exercise activities was obtained as well as information on the intensity level at which each activity was carried out. A metabolic equivalent value was assigned to each activity/intensity combination and was used to calculate the number of kilocalories per week per kilogram of body weight spent on that activity (25). For each participant, the scores of all performed activities were summed and multiplied by body weight to create an overall physical activity score in kilocalories per week. Previous or current presence of chronic conditions was determined using self-reported physician-diagnosed disease information, clinic data, and medication use. The inflammation-related diseases included in this study were heart disease, lung disease, current symptomatic osteoarthritis, and diabetes mellitus. Daily use of antiinflammatory drugs (nonsteroidal antiinflammatory drugs [NSAIDs], salicylates, and other antiinflammatory agents) was determined from drug data coded using the Iowa Drug Information System (IDIS) ingredient codes (26). Smoking was defined as a continuous variable assessing lifetime cigarette use in pack-years.
Statistical Analyses
Analyses were performed stratified by gender and race using SAS software (SAS Institute, Inc., Cary, NC). Reported correlations were Pearson's product-moment correlations. Multiple linear regression analysis was used to test the association of the proinflammatory cytokine levels with muscle mass or muscle strength. Muscle mass and muscle strength were used as continuous dependent variables in the regression models. IL-6 and TNF- concentrations were log transformed to derive a near-normal distribution and were used as continuous variables in the regression models (Fig. 1). To facilitate interpretation of the results, regression coefficients (with standard error) were expressed per population standard deviation of log(IL-6) (=0.6 pg/ml) or log(TNF-) (=0.4 pg/ml). Potential racial differences in the relationship of cytokine levels with muscle mass and muscle strength were assessed in stratified analyses and were tested by using product-terms in additional analyses stratified by gender only. We also combined the levels of both cytokines to create an overall measure of cytokine status with higher specificity. High proinflammatory cytokine status was defined as IL-6 and TNF- concentrations above the population median (IL-6 > 1.80 pg/ml and TNF- > 3.20 pg/ml). Low proinflammatory cytokine status was defined as IL-6 < 1.80 pg/ml and TNF- < 2.40 pg/ml. The intermediate group included persons with all other possible combinations of cytokine levels. To test for trend, the three categories were entered in the model as an ordinal variable. All analyses were adjusted for clinic site, height, age, heart disease, lung disease, diabetes mellitus, current symptomatic osteoarthritis, use of antiinflammatory drugs, physical activity, and smoking. When investigating the relationship between proinflammatory cytokine levels and muscle strength, we additionally adjusted for muscle mass to study the independent effect of cytokine levels on muscle strength. Previous studies from our group and other groups have shown that high body fat or high BMI is associated with greater muscle area, greater strength, and higher inflammation levels (2)(27)(28)(29). Body fat was therefore considered as an important confounder of the investigated associations and was adjusted for in all analyses.
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Results |
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levels compared to whites. The racial difference in TNF- levels remained after differences in body fat, physical activity, smoking, medication use, and health status were taken into account. Of the total population, 95% had IL-6 levels below 6.8 pg/ml and TNF- levels below 6.1 pg/ml (Fig. 1).
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) as continuous variables with muscle mass and muscle strength (Table 2 ). To facilitate the interpretation of the results, we expressed muscle area and muscle strength per standard deviation increase in IL-6 or TNF-. Consistent across race and gender, a standard deviation increase in IL-6 was associated with a 1.1 to 2.4 kg decrease in grip strength. Similarly, in women, a standard deviation increase in TNF- was associated with a decrease in grip strength of 1.2–1.3 kg. In black men, muscle mass and muscle strength were associated with both proinflammatory cytokines, while for black women, muscle mass was decreased in those with higher TNF-.
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and knee extensor strength in women (p = .04). Per standard deviation increase in TNF-, black women had a greater decrement in knee extensor strength than white women.
By combining the levels of IL-6 and TNF-, we created an overall measure of proinflammatory cytokine status with high specificity. The prevalence of a high cytokine status according to our definition was 31.2% and 28.5% in white and black men, respectively, and 24.1% and 22.4% in white and black women, respectively. We investigated the association of this overall marker of cytokine status with muscle mass and muscle strength (Table 3 ). With the exception of white men, elderly persons having high levels of IL-6 (>1.80 pg/ml) as well as high levels of TNF- (>3.20 pg/ml) had less appendicular muscle mass, a smaller muscle area, a lower grip strength, and a lower knee extensor strength compared with those with low levels of both cytokines. For example, those with high cytokine status had a 3.3% to 6.5% smaller muscle area and a 5.5% to 8.8% lower knee extensor strength compared with those with low cytokine status. No statistical interaction was observed between cytokine status and antiinflammatory drug use (p > .2) in all gender and race groups. The results for grip strength were similar after additional adjustment for appendicular muscle mass (p value for trend .0007 and .2 for black and white men, and .0008 and .0006 for black and white women) or additional adjustment for muscle area (p value for trend .002 and .2 for black and white men, and .0008 and .001 for black and white women). However, the results for knee extensor strength were attenuated after additional adjustment for muscle mass. For example, after adjustment for appendicular muscle mass, the p values for trend were .2 in black and white men, and .07 and .08 in black and white women.
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Discussion |
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levels are associated with lower muscle mass and lower muscle strength in well-functioning elderly persons. It is the first large-scale study investigating this association using accurate methodology to assess muscle mass and muscle strength.
The combined measure of IL-6 and TNF- showed the strongest association with muscle mass and muscle strength. Persons with IL-6 levels above the median as well as TNF- levels above the median had lower muscle mass and lower muscle strength compared to those who had cytokine levels below the median. The log transformed concentrations of IL-6 and TNF- were positively correlated (r = +0.28, p = .0001). The use of a combined measure is likely to increase the specificity for proinflammatory cytokine status, that is the probability that persons with low levels of both IL-6 and TNF- have no ongoing subclinical inflammation, and will reduce nondifferential misclassification. This may explain the stronger associations observed for the combined measure as compared to the associations for each individual cytokine.
The relationship between cytokine levels and muscle mass/strength was observed after adjustment for several inflammation-relation diseases including heart disease, lung disease, current symptomatic osteoarthritis, and diabetes mellitus. These findings support the presence of a direct relationship between cytokine levels and muscle mass. This relationship has already been suggested by the results of several (experimental) studies in humans (13)(14)(15)(16) and animals (17)(18)(19)(20)(21)(22). Several smaller studies suggest that IL-6 levels may increase with aging, even in healthy persons (30)(31). Recently, higher muscle TNF- protein levels were observed among frail elderly compared to young healthy individuals (32). The higher cytokine levels in older persons may predispose to sarcopenia, thereby increasing the risk for functional decline and disability. Future studies should focus on proinflammatory cytokine levels in relation to loss of muscle mass and muscle strength.
Cytokine levels were also inversely related to muscle strength. The associations with grip strength were generally stronger than those with knee extensor strength. Due to the stringent exclusion criteria of the KinCom test, 290 persons were excluded from that test. These persons were older, had a lower grip strength, and had a smaller thigh muscle area compared with those who were not excluded from this test. These frailer persons were, however, included in the grip strength test. This difference may explain the stronger associations for grip strength compared to knee extensor strength. After additional adjustment for muscle mass, the relationship between cytokine levels and knee extensor strength was reduced. This suggests that proinflammatory cytokine levels may partly influence muscle strength through their effect on muscle mass.
Except for white men, participants with a high proinflammatory cytokine status had a 1.9% to 4.1% smaller appendicular muscle mass, a 3.3% to 6.5% smaller muscle area, a 7.5% to 10.8% lower grip strength, and a 5.5% to 8.8% lower knee extensor strength compared with those with a low cytokine status. These results suggest that a high cytokine status may be critical for those persons at the thresholds for maintaining physical function and independence. However, our results were based on cross-sectional data and should be confirmed in prospective studies. The associations were observed in a well-functioning population that excluded the oldest old. Persons with low muscle mass and low muscle strength were more likely to be excluded from the Health ABC study. The cohort may therefore have limited our ability to observe associations between higher proinflammatory cytokine levels and low muscle mass/strength. Another limitation of the study may be that information was used on the presence of chronic disease but not on disease severity. The severity of the disease may be related to the proinflammatory cytokine levels, and residual confounding due to disease severity cannot be excluded. In addition, potential confounding due to other factors, including type of physical activity and sex steroids, factors that also may be different between gender and race groups, cannot be excluded.
Total body fat was included as a potential confounder of the relationship of IL-6 and TNF- with muscle mass and muscle strength. As previously stated, high body fat and high body mass index have been associated with greater muscle area, greater strength, and higher levels of inflammation (2)(27)(28)(29). Indeed, in the present study, body fat was positively correlated with cytokine levels, especially in women who have relatively more body fat (IL-6: r = +0.23–0.26, TNF-: r = +0.09–0.20). In addition, body fat was positively correlated with muscle area (r = +0.42–0.50) and appendicular muscle mass (r = +0.50–0.71), justifying our decision. However, if body fat is considered the main source of cytokine production (33)(34), the following pathway could be hypothesized: body fat 
cytokines muscle mass. In this case, body fat should not be considered a confounder of the relation between cytokines and muscle mass. Because increases in body fat are paralleled by increases in lean body mass (35), indicating a direct relationship between body fat and muscle mass that can be explained by the training effect of weight bearing and support of a larger body mass, we still consider adjustment for body fat appropriate.
In conclusion, higher plasma concentrations of IL-6 and TNF- are associated with lower muscle mass and lower muscle strength in well-functioning older men and women. Higher cytokine levels may contribute to the loss of muscle mass and strength with aging, also called sarcopenia.
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Acknowledgments |
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Received October 3, 2001
Accepted January 14, 2002
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