Fat infiltration in muscle: new evidence for familial clustering and associations with diabetes

Fully-automated segmentation of muscle and inter-/intra-muscular fat from magnetic resonance images of calves and thighs: an open-source workflow in Python

BACKGROUND

INTER- and INTRAmuscular fat (IMF) is elevated in high metabolic states and can promote inflammation. While magnetic resonance imaging (MRI) excels in depicting IMF, the lack of reproducible tools prevents the ability to measure change and track intervention success.

METHODS

We detail an open-source fully-automated iterative threshold-seeking algorithm (ITSA) for segmenting IMF from T1-weighted MRI of the calf and thigh within three cohorts (CaMos Hamilton (N = 54), AMBERS (N = 280), OAI (N = 105)) selecting adults 45-85 years of age. Within the CaMos Hamilton cohort, same-day and 1-year repeated images (N = 38) were used to evaluate short- and long-term precision error with root mean square coefficients of variation; and to validate against semi-automated segmentation methods using linear regression. The effect of algorithmic improvements to fat ascertainment using 3D connectivity and partial volume correction rules on analytical precision was investigated. Robustness and versatility of the algorithm was demonstrated by application to different MR sequences/magnetic strength and to calf versus thigh scans.

RESULTS

Among 439 adults (319 female(89%), age: 71.6 ± 7.6 yrs, BMI: 28.06 ± 4.87 kg/m2, IMF%: 10.91 ± 4.57%), fully-automated ITSA performed well across MR sequences and anatomies from three cohorts. Applying both 3D connectivity and partial volume fat correction improved precision from 4.99% to 2.21% test-retest error. Validation against semi-automated methods showed R2 from 0.92 to 0.98 with fully-automated ITSA routinely yielding more conservative computations of IMF volumes. Quality control shows 7% of cases requiring manual correction, primarily due to IMF merging with subcutaneous fat. A full workflow described methods to export tags for manual correction.

CONCLUSIONS

The greatest challenge in segmenting IMF from MRI is in selecting a dynamic threshold that consistently performs across repeated imaging. Fully-automated ITSA achieved this, demonstrated low short- and long-term precision error, conducive of use within RCTs.

GSK3 regulation Wnt/β-catenin signaling affects adipogenesis in bovine skeletal muscle fibro/adipogenic progenitors

Clarifying the cellular origin and regulatory mechanisms of intramuscular fat (IMF) deposition is crucial for improving beef quality. Here, we used single-nucleus RNA sequencing to analyze the structure and heterogeneity of skeletal muscle cell populations in different developmental stages of Yanbian cattle and identified eight cell types in two developmental stages of calves and adults. Among them, fibro/adipogenic progenitors (FAPs) expressing CD29 (ITGA7)pos and CD56 (NCAM1)neg surface markers were committed to IMF deposition in beef cattle and expressed major Wnt ligands and receptors. LY2090314/XAV-939 was used to activate/inhibit Wnt/β-catenin signal. The results showed that the blockade of Glycogen Synthase Kinase 3 (GSK3) by LY2090314 promoted the stabilization of β-catenin and reduced the expression of genes related adipogenic differentiation (e.g., PPARγ and C/EBPα) in bovine FAPs, confirming the anti-adipogenic effect of GSK3. XAV-939 inhibition of the Wnt/β-catenin pathway promoted the lipid accumulation capacity of FAPs. Furthermore, we found that blocking GSK3 enhanced the paracrine effects of FAPs-MuSCs and increased myotube formation in muscle satellite cells (MuSCs). Overall, our results outline a single-cell atlas of skeletal muscle development in Yanbian cattle, revealed the role of Wnt/GSK3/β-catenin signaling in FAPs adipogenesis, and provide a theoretical basis for further regulation of bovine IMF deposition.

Thigh muscle fat fraction is independently associated with impaired glucose metabolism in individuals with obesity

Background

The aim of this study was to address the intramuscular adipose tissue (IMAT) accumulation in the lower extremities and further detect the relationship between adipose tissue (AT) distribution in the muscle and glucose metabolism in subjects with obesity.

Methods

We conducted a cross-sectional study in 120 Chinese obese adults (80 male and 40 female) with BMI ≥ 28 kg/m2. MRI was applied to access the IMAT content in lower extremities. The oral glucose tolerance test was used to evaluate the glucose metabolism and insulin secretion in all individuals. The correlations between glucose metabolism and the fat content of the lower extremities were further assessed.

Results

Among 120 included subjects, 54 were classified as subjects with normal glucose tolerance (NGT) and 66 with impaired glucose regulation (IGR). We presented that those with IGR had higher fat accumulation in semitendinosus, adductor magnus, gracilis and sartorius than those with NGT (all P < 0.05). In sex-specific analyses, females have higher IMAT in adductor magnus than males (P < 0.001). Males with IGR had higher fat fraction of semitendinosus and sartorius than those with NGT (P = 0.020, P = 0.014, respectively). Logistic regression analyses revealed that IMAT content in semitendinosus was the independent factor of IGR in individuals with obesity after adjustment for age, gender, triglycerides, creatinine and albumin (odds ratio: 1.13, 95% CI: 1.02-1.26, P = 0.024).

Conclusions

Increased adipose tissue accumulation in thigh muscles was associated with glucose dysregulation in patients with obesity. IMAT content in semitendinosus may serve as a possible risk factor for impaired glucose metabolism.

The sonographic quantitative assessment of the deltoid muscle to detect type 2 diabetes mellitus: a potential noninvasive and sensitive screening method?

BACKGROUND

In our previous published study, we demonstrated that a qualitatively assessed elevation in deltoid muscle echogenicity on ultrasound was both sensitive for and a strong predictor of a type 2 diabetes (T2DM) diagnosis. This study aims to evaluate if a sonographic quantitative assessment of the deltoid muscle can be used to detect T2DM.

METHODS

Deltoid muscle ultrasound images from 124 patients were stored: 31 obese T2DM, 31 non-obese T2DM, 31 obese non-T2DM and 31 non-obese non-T2DM. Images were independently reviewed by 3 musculoskeletal radiologists, blinded to the patient's category. Each measured the grayscale pixel intensity of the deltoid muscle and humeral cortex to calculate a muscle/bone ratio for each patient. Following a 3-week delay, the 3 radiologists independently repeated measurements on a randomly selected 40 subjects. Ratios, age, gender, race, body mass index, insulin usage and hemoglobin A_1c were analyzed. The difference among the 4 groups was compared using analysis of variance or chi-square tests. Both univariate and multivariate linear mixed models were performed. Multivariate mixed-effects regression models were used, adjusting for demographic and clinical variables. Post hoc comparisons were done with Bonferroni adjustments to identify any differences between groups. The sample size achieved 90% power. Sensitivity and specificity were calculated based on set threshold ratios. Both intra- and inter-radiologist variability or agreement were assessed.

RESULTS

A statistically significant difference in muscle/bone ratios between the groups was identified with the average ratios as follows: obese T2DM, 0.54 (P < 0.001); non-obese T2DM, 0.48 (P < 0.001); obese non-T2DM, 0.42 (P = 0.03); and non-obese non-T2DM, 0.35. There was excellent inter-observer agreement (intraclass correlation coefficient 0.87) and excellent intra-observer agreements (intraclass correlation coefficient 0.92, 0.95 and 0.94). Using threshold ratios, the sensitivity for detecting T2DM was 80% (95% CI 67% to 88%) with a specificity of 63% (95% CI 50% to 75%).

CONCLUSIONS

The sonographic quantitative assessment of the deltoid muscle by ultrasound is sensitive and accurate for the detection of T2DM. Following further studies, this process could translate into a dedicated, simple and noninvasive screening method to detect T2DM with the prospects of identifying even a fraction of the undiagnosed persons worldwide. This could prove especially beneficial in screening of underserved and underrepresented communities.

Genetics of Obesity: What We Have Learned Over Decades of Research

There is a genetic component to human obesity that accounts for 40% to 50% of the variability in body weight status but that is lower among normal weight individuals (about 30%) and substantially higher in the subpopulation of individuals with obesity and severe obesity (about 60%-80%). The appreciation that heritability varies across classes of BMI represents an important advance. After controlling for BMI, ectopic fat and fat distribution traits are characterized by heritability levels ranging from 30% to 55%. Defects in at least 15 genes are the cause of monogenic obesity cases, resulting mostly from deficiencies in the leptin-melanocortin signaling pathway. Approximately two-thirds of the BMI heritability can be imputed to common DNA variants, whereas low-frequency and rare variants explain the remaining fraction. Diminishing allele effect size is observed as the number of obesity-associated variants expands, with most BMI-increasing or -decreasing alleles contributing only a few grams or less to body weight. Obesity-promoting alleles exert minimal effects in normal weight individuals but have larger effects in individuals with a proneness to obesity, suggesting a higher penetrance; however, it is not known whether these larger effect sizes precede obesity or are caused by an obese state. The obesity genetic risk is conditioned by thousands of DNA variants that make genetically based obesity prevention and treatment a major challenge.

Single-cell RNA sequencing and lipidomics reveal cell and lipid dynamics of fat infiltration in skeletal muscle

BACKGROUND

Ageing is accompanied by sarcopenia and intramuscular fat (IMAT) infiltration. In skeletal muscle, fat infiltration is a common feature in several myopathies and is associated with muscular dysfunction and insulin resistance. However, the cellular origin and lipidomic and transcriptomic changes during fat infiltration in skeletal muscle remain unclear.

METHODS

In the current study, we generated a high IMAT-infiltrated skeletal muscle model by glycerol (GLY) injection. Single-cell RNA sequencing and lineage tracing were performed on GLY-injured skeletal muscle at 5 days post-injection (DPI) to identify the cell origins and dynamics. Lipidomics and RNA sequencing were performed on IMAT-infiltrated skeletal muscle at 14 DPI (or 17 DPI for the cold treatment) to analyse alterations of lipid compositions and gene expression levels.

RESULTS

We identified nine distinct major clusters including myeloid-derived cells (52.13%), fibroblast/fibro/adipogenic progenitors (FAPs) (23.24%), and skeletal muscle stem cells (2.02%) in GLY-injured skeletal muscle. Clustering and pseudotemporal trajectories revealed six subpopulations in fibroblast/FAPs and 10 subclusters in myeloid-derived cells. A subpopulation of myeloid-derived cells expressing adipocyte-enriched genes and Pdgfra- /Cd68+ cells displayed lipid droplets upon adipogenic induction, indicating their adipogenic potential. Lipidomic analysis revealed the changes of overall lipid classes composition (e.g. triglycerides (TAGs) increased by 19.3 times, P = 0.0098; sulfoquinovosyl diacylglycerol decreased by 83%, P = 0.0056) and in the distribution of lipids [e.g. TAGs (18:2/18:2/22:6) increased by 181.6 times, P = 0.021] between GLY-group and saline control. RNA-seq revealed 1847 up-regulated genes and 321 down-regulated genes and significant changes in lipid metabolism-related pathways (e.g. glycerolipid pathway and glycerophospholipid pathway) in our model of GLY-injured skeletal muscle. Notably, short-term cold exposure altered fatty acid composition (e.g. saturated fatty acid decreased by 6.4%, P = 0.058) in fat-infiltrated muscles through directly affecting lipid metabolism pathways including PI3K-AKT and MAPK signalling pathway.

CONCLUSIONS

Our results showed that a subpopulation of myeloid-derived cells may contribute to IMAT infiltration. GLY-induced IMAT infiltration changed the lipid composition and gene expression profiles. Short-term cold exposure might regulate lipid metabolism and its related signalling pathways in fat-infiltrated muscle. Our study provides a comprehensive resource describing the molecular signature of fat infiltration in skeletal muscle.

The Hyperechoic Appearance of the Deltoid Muscle on Shoulder Ultrasound Imaging as a Predictor of Diabetes and Prediabetes

OBJECTIVES

To evaluate whether the ultrasound appearance of the deltoid muscle in diabetic patients differs from that in obese nondiabetic patients.

METHODS

Ultrasound images of the deltoid muscle from 137 type 2 diabetic patients (including 13 prediabetic patients) and 49 obese nondiabetic patients were blindly reviewed by 2 musculoskeletal radiologists, and by a third when arbitration was needed, to determine whether the appearance was "normal," "suspected diabetes," or "definite diabetes." Age, sex, race, body mass index (BMI), insulin use, and hemoglobin A_1c were analyzed. This retrospective study included patients presenting between October 2005 and November 2017. Statistical analyses included a 2-sided sample t test or Wilcoxon rank sum test and a χ² or Fisher exact test. Statistical significance was defined as P < .05.

RESULTS

The type 2 diabetic patients included 98 women and 39 men aged 29 to 92 years, and the nondiabetic patients included 19 women and 30 men aged 18 to 75 years. A consensus diagnosis of definite diabetes by the musculoskeletal radiologists based on a hyperechoic deltoid was a powerful predictor of diabetes, with a positive predictive value of 89%. A hyperechoic deltoid was also a powerful predictor of prediabetes. Of the 13 prediabetic patients, all had the same hyperechoic appearance of the diabetic deltoid, regardless of BMI. Although obese diabetic patients more often had a diagnosis of definite diabetes, the BMI alone could not explain the increased echogenicity, as obese nondiabetic patients' deltoid muscles did not appear as hyperechoic and were correctly categorized as not having definite diabetes with 82% specificity.

CONCLUSIONS

The characteristic hyperechoic deltoid appearance is a strong predictor of both diabetes and prediabetes and differs from that of obese nondiabetic patients.

Falls and Fractures in Diabetes-More than Bone Fragility

PURPOSE OF REVIEW

Based on a systematic literature search, we performed a comprehensive review of risk factors for falls and fractures in patients with diabetes.

RECENT FINDINGS

Patients with diabetes have an increased risk of fractures partly explained by increased bone fragility. Several risk factors as altered body composition including sarcopenia and obesity, impaired postural control, gait deficits, neuropathy, cardiovascular disease, and other co-morbidities are considered to increase the risk of falling. Diabetes and bone fragility is well studied, but new thresholds for fracture assessment should be considered. In general, the risk factors for falls in patients with diabetes are well documented in several studies. However, the fall mechanisms among diabetic patients have only been assessed in few studies. Thus, a gab of knowledge exits and may influence the current understanding and treatment, in order to reduce the risk of falling and thereby prevent fractures.

The association of medical and demographic characteristics with sarcopenia and low muscle radiodensity in patients with nonmetastatic colorectal cancer

BACKGROUND

Sarcopenia and low skeletal muscle radiodensity (SMD) have been associated with adverse outcomes in patients with colorectal cancer (CRC); however, factors contributing to these 2 muscle abnormalities are unclear.

OBJECTIVES

The aim of this study was to investigate the association of medical and demographic characteristics with muscle abnormalities among patients with nonmetastatic CRC.

METHODS

Patients with stage I-III invasive CRC (2006-11) who had diagnostic computed tomography (CT) available from Kaiser Permanente Northern California electronic medical records were included. CT-assessed sarcopenia and low SMD were defined according to optimal stratification. Logistic regressions including age, stage, site, total adipose tissue (TAT), race/ethnicity, neutrophil-lymphocyte ratio, smoking history, alcohol use, and Charlson Comorbidity Score were performed to identify characteristics associated with muscle abnormalities.

RESULTS

The study included 3262 patients (49.9% females) with a mean ± SD age of 62.6 ± 11.4 y. Sarcopenia and low SMD were highly prevalent (42.4% and 29.6%, respectively). Age and sex interactions were noted for muscle mass, but not SMD. Age was associated with higher odds of muscle abnormalities in a dose-response manner. Compared with those aged ≤50 y, patients aged 70-80 y had considerably higher odds (OR: 6.19; 95% CI: 4.72, 8.11) of sarcopenia, and low SMD (OR: 17.81; 95% CI: 11.73, 27.03). High TAT was related to a higher odds of low SMD (OR: 9.62; 95% CI: 7.37, 12.56), but lower odds of sarcopenia (OR: 0.59; 95% CI: 0.48, 0.71). Compared with Caucasians, African Americans had lower odds of sarcopenia and low SMD. Patients with a higher neutrophil-lymphocyte ratio had higher odds of having both muscle abnormalities. Patients who were smokers or had any comorbidity had higher odds of low SMD, but not sarcopenia.

CONCLUSIONS

Muscle abnormalities were common in patients with nonmetastatic CRC, with great variability in muscle mass and SMD across age, TAT, and race/ethnicity. Factors associated with muscle abnormalities may be used to facilitate risk stratification and the guidance of targeted strategies to counteract these abnormalities.

Does Muscle Atrophy and Fatty Infiltration Plateau or Persist in Chronic Spinal Cord Injury?

Atrophy and fatty infiltration of lower extremity muscle after spinal cord injury (SCI) predisposes individuals to metabolic syndrome and related diabetes and cardiovascular disease. The objective of this study was to prospectively measure changes in muscle atrophy and fat content of distal lower extremity muscles and explore related factors in a cohort of adults with chronic SCI and diverse impairments. Muscle cross-sectional area and density were calculated from peripheral quantitative computed tomography scans of the 66% site of the calf from 70 participants with chronic SCI (50 male, mean age 49 years, C2-T12, American Spinal Injury Association Impairment Scale A-D) at study enrollment and annually for 2 years. Mixed-model repeated measures analysis of variance (rANOVA) examined longitudinal changes in muscle area and density, and regression analyses explored factors related to muscle changes using 16 potential correlates selected a priori. A high degree of individual variation in muscle area and density change was observed over 2 years (range: 8.5 to  -22.6 cm²; 6.4 to -8.6 mg/cm³). Repeated measures analysis of variance revealed significant reductions in muscle area (estimated mean difference [95% confidence intervals] -1.76 [-3.29 to -0.23]) cm², p = 0.025) and density (-1.04 [-1.94 to -0.14] mg/cm³, p < 0.024); however, changes in area were not significant with outliers removed. Regression analyses explained a small proportion of the variability in muscle density change; however, none of the preselected variables were significantly related to changes in muscle density after post hoc sensitivity analyses. Lower extremity muscle size and fat content may not reach a "steady-state" after chronic SCI. Progressive atrophy and fatty infiltration of lower extremity muscle may have adverse implications for metabolic syndrome and cardiovascular disease risk and related mortality after chronic SCI.

Lower insulin sensitivity is related to lower relative muscle cross-sectional area, lower muscle density and lower handgrip force in young and middle aged non-diabetic men

OBJECTIVES

This study investigated whether an association between insulin resistance (IR) and muscle parameters is appreciable in young healthy men, independent of obesity. Furthermore, markers of muscle metabolism and hormones/possible determinants, were explored.

METHODS

358 healthy young men were divided into a less and more insulin sensitive (LIS [age=33.2±5.4, BMI=23.4±2.3] and MIS [age=35.5±5.3, BMI=28.1±3.7]) group based on upper and lower quartile of HOMA-IR. Muscle cross-sectional area (CSA), -density, handgrip force, serum testosterone, estradiol, SHBG, Vitamin 25(OH)D, creatinine, IGF-1, IGFBP-3 and leptin levels were compared between these groups, correcting for differences in age, physical activity and fat mass. Correlations between HOMA-IR and these parameters, and between muscle measures and biochemical parameters, were calculated.

RESULTS

LIS is related to lower relative muscle CSA, muscle density, muscle/fat CSA ratio, relative handgrip force and level of physical activity. Furthermore, lower levels in SHBG, testosterone, Vitamin 25(OH)D and higher leptin, IGF-1 and IGFBP-3 levels were observed in LIS. Bio available T, FT, TE2, FE2, bioavailable E2, serum and urinary creatinine levels did not differ between groups.

CONCLUSION

Differences in muscle performance are already present in healthy men with lower insulin sensitivity and could be possibly modifiable risk factors for the development of type 2 diabetes.

Measurement of muscle and fat in postmenopausal women: precision of previously reported pQCT imaging methods

Peripheral quantitative computed tomography (pQCT) imaging has been used to quantify muscle area and density as well as intermuscular adipose tissue (IMAT) and subcutaneous adipose tissue (SAT) area in the lower and upper limb. Numerous protocols have been reported to derive these soft-tissue outcomes, but their precision has not been assessed in community-dwelling postmenopausal women. The objective of this study was to compare the precision of previously reported analysis protocols for quantifying muscle area and density, as well as IMAT and SAT area in postmenopausal women. Six image analysis protocols using two available software suites (Stratec XCT, BoneJ) were identified from the pQCT literature. Analysis protocols were applied to a sample of 35 older female adults (mean age 73.7; SD 7.2 years), randomly selected from a population based-cohort and scanned twice within an average of 9.7 (SD 3.6) days. Relative precision was calculated as absolute values and as a percentage of the sample mean (root mean square coefficient of variation; CV%RMS). Soft-tissue outcomes across protocols were compared on their log-transformed coefficients of variation using multilevel linear models and Tukey contrasts. For most protocols, CV%RMS for muscle area, density, and SAT area ranged between 2.1 and 3.7%, 0.7 and 1.9%, and 2.4 and 6.4%, respectively. Precision for IMAT area varied considerably, from 3 to 42%. Consideration of these study results will aid in the selection of appropriate image analysis protocols for pQCT-derived soft-tissue outcomes in postmenopausal women.

Development of a Skeletal Muscle Mimic Phantom Compatible with QCT and MR Imaging

OBJECTIVE

The purpose of this study was to develop a skeletal muscle mimic phantom compatible with quantitative computed tomography (QCT) and magnetic resonance imaging, yielding physiologically appropriate values.

METHODS

Agar-based phantoms contained varying concentrations of CuCl₂ and EDTA to adjust T2 relaxation time and phantom density concurrently. T2 relaxation times were quantified using a 4-mm single-slice fast spin echo sequence repeated for six serial echo times at 937-μm resolution. T2 relaxation maps were generated using the Levenberg-Marquardt equation. A peripheral QCT scanner measured linear attenuation coefficients of phantoms, which were converted to density (mg/cm3) values. Five 2.3 ± 0.5 mm thick slices were acquired at 15 mm/s scan speed and 500-μm resolution. Logarithmic or linear regression models were fitted to EDTA or CuCl₂ versus density and T2 relaxation data.

RESULTS

Density (D) was linearly dependent on CuCl₂ (D = 0.27 [CuCl₂] + 63.92, R² = 0.84, P = 0.01) and invariant to EDTA. T2 relaxation time was related negatively to CuCl₂ (T2 = -10.13 ln [CuCl₂] + 66.70, R² = 0.91, P < .01) and positively to EDTA (T2 = 5.72 ln [EDTA] + 54.47, R² = 0.86, P < .01). Reproducibility within and between phantoms of the same compositions was acceptable (<5% error). Long-term stability was achieved for density but poorer for T2 relaxation time.

CONCLUSIONS

This phantom optimization method provides a means for altering a soft tissue phantom suited for calibrating magnetic resonance imaging and QCT signals within values representative of muscle. Phantoms can be used during scans for calibrating magnetic resonance signals between and within individuals over time and can cross-calibrate different scanners.

Lower-extremity muscle atrophy and fat infiltration after chronic spinal cord injury

BACKGROUND

Atrophy and fatty-infiltration of lower-extremity muscle after spinal cord injury (SCI) predisposes individuals to metabolic disease and related mortality.

OBJECTIVES

To determine the magnitude of atrophy and fatty-infiltration of lower-extremity muscles and related factors in a group of individuals with chronic SCI and diverse impairment.

METHODS

Muscle cross-sectional area and density were calculated from peripheral quantitative computed tomography scans of the 66% site of the calf of 70 participants with chronic SCI [50 male, mean age 49 (standard deviation 12) years, C2-T12, AIS A-D] and matched controls. Regression models for muscle area and density were formed using 16 potential correlates selected a priori.

RESULTS

Participants with motor-complete SCI had ≈ 32% lower muscle area, and ≈ 43% lower muscle density values relative to controls. Participants with motor-incomplete SCI had muscle area and density values that were both ≈ 14% lower than controls. Body mass (+), tetraplegia (+), motor function (+), spasticity (+), vigorous physical activity (+), wheelchair use (-), age (-), and waist circumference (-) were associated with muscle size and/or density in best-fit regression models.

CONCLUSIONS

There are modifiable factors related to muscle size, body composition, and activity level that may offer therapeutic targets for preserving metabolic health after chronic SCI.

Improving reliability of pQCT-derived muscle area and density measures using a watershed algorithm for muscle and fat segmentation

In peripheral quantitative computed tomography scans of the calf muscles, segmentation of muscles from subcutaneous fat is challenged by muscle fat infiltration. Threshold-based edge detection segmentation by manufacturer software fails when muscle boundaries are not smooth. This study compared the test-retest precision error for muscle-fat segmentation using the threshold-based edge detection method vs manual segmentation guided by the watershed algorithm. Three clinical populations were investigated: younger adults, older adults, and adults with spinal cord injury (SCI). The watershed segmentation method yielded lower precision error (1.18%-2.01%) and higher (p<0.001) muscle density values (70.2±9.2 mg/cm3) compared with threshold-based edge detection segmentation (1.77%-4.06% error, 67.4±10.3 mg/cm3). This was particularly true for adults with SCI (precision error improved by 1.56% and 2.64% for muscle area and density, respectively). However, both methods still provided acceptable precision with error well under 5%. Bland-Altman analyses showed that the major discrepancies between the segmentation methods were found mostly among participants with SCI where more muscle fat infiltration was present. When examining a population where fatty infiltration into muscle is expected, the watershed algorithm is recommended for muscle density and area measurement to enable the detection of smaller change effect sizes.

Muscle activity, cross-sectional area, and density following passive standing and whole body vibration: A case series

OBJECTIVE

To investigate the effects of intermittent passive standing (PS) and whole body vibration (WBV) on the electromyography (EMG) activity, cross-sectional area, and density of lower extremity muscles in individuals with chronic motor complete spinal cord injury (SCI).

DESIGN

Case series.

METHODS

Seven adult men with chronic (≥2 years), thoracic motor complete (AIS A-B) SCI completed a 40-week course of thrice-weekly intermittent PS-WBV therapy, in a flexed knee posture (160°), for 45 minutes per session at a frequency of 45 Hz and 0.6-0.7 mm displacement using the WAVE(®) Pro Plate, with an integrated EasyStand™ standing frame. EMG was measured in major lower extremity muscles to represent muscle activity during PS-WBV. The cross-sectional area and density of the calf muscles were measured using peripheral quantitative computed tomography at the widest calf cross-section (66% of the tibia length) at pre- and post-intervention. All measured variables were compared between the pre- and post-intervention measurements to assess change after the PS-WBV intervention.

RESULTS

PS-WBV acutely induced EMG activity in lower extremity muscles of SCI subjects. No significant changes in lower extremity EMG activity, muscle cross-sectional area, or density were observed following the 40-week intervention.

CONCLUSIONS

Although acute exposure to PS-WBV can induce electrophysiological activity of lower extremity muscles during PS in men with motor complete SCI, the PS-WBV intervention for 40 weeks was not sufficient to result in enhanced muscle activity, or to increase calf muscle cross-sectional area or density.

Links between ectopic fat and vascular disease in humans

The average of overweight individual can have differential fat depots in target organs or specific compartments of the body. This ectopic fat distribution may be more of a predictive factor for cardiovascular risk than obesity. Abdominal visceral obesity, a representative ectopic fat, is robustly associated with insulin resistance and cardiovascular risk. Fat depots in the liver and muscle tissue cause adverse cardiometabolic risk by affecting glucose and lipid metabolism. Pericardial fat and perivascular fat affect coronary atherosclerosis, cardiac function, and hemodynamics. Fat around the neck is associated with systemic vascular resistance. Fat around the kidney may increase blood pressure and induce albuminuria. Fat accumulation in or around the pancreas alters glucose metabolism, conferring cardiovascular risk. Ectopic fat may act as an active endocrine and paracrine organ that releases various bioactive mediators that influence insulin resistance, glucose and lipid metabolism, coagulation, and inflammation, which all contribute to cardiovascular risk. Because both obese and apparently lean individuals can have ectopic fat, regional fat distribution may play an important role in the development of cardiovascular diseases in both nonobese and obese people.

Estimation of thigh muscle cross-sectional area by single- and multifrequency segmental bioelectrical impedance analysis in the elderly

Bioelectrical impedance analysis (BIA) has been used to estimate skeletal muscle mass, but its application in the elderly is not optimal. The accuracy of BIA may be influenced by the expansion of extracellular water (ECW) relative to muscle mass with aging. Multifrequency BIA (MFBIA) can evaluate the distribution between ECW and intracellular water (ICW), and thus may be superior to single-frequency BIA (SFBIA) to estimate muscle mass in the elderly. A total of 58 elderly participants aged 65–85 years were recruited. Muscle cross-sectional area (CSA) was obtained from computed tomography scans at the mid-thigh. Segmental SFBIA and MFBIA were measured for the upper legs. An index of the ratio of ECW and ICW was calculated using MFBIA. The correlation between muscle CSA and SFBIA was moderate ( r = 0.68), but strong between muscle CSA and MFBIA ( r = 0.85). ECW/ICW index was significantly and positively correlated with age ( P < 0.001). SFBIA tends to significantly overestimate muscle CSA in subjects who had relative expansion of ECW in the thigh segment ( P < 0.001). This trend was not observed for MFBIA ( P = 0.42). Relative expansion of ECW was observed in older participants. The relative expansion of ECW affects the validity of traditional SFBIA, which is lowered when estimating muscle CSA in the elderly. By contrast, MFBIA was not affected by water distribution in thigh segments, thus rendering the validity of MFBIA for estimating thigh muscle CSA higher than SFBIA in the elderly.

Physical disability in the elderly with diabetes: epidemiology and mechanisms

Diabetes mellitus is a common chronic condition worldwide, especially in the elderly population. Several epidemiologic studies in the last 2 years have consistently associated diabetes with physical disability, a condition that may profoundly affect the quality of life of older people. Although in older people with diabetes, the pathogenesis of functional limitation and disability has not been completely elucidated, it is certainly complex and involves multiple potential pathways. In this narrative review, we described the most recent epidemiologic and clinical evidence supporting the association between diabetes and impaired physical function in older persons focusing on emerging biological mechanisms explaining the excess risk of disability associated with diabetes.

Comparison of single- or multifrequency bioelectrical impedance analysis and spectroscopy for assessment of appendicular skeletal muscle in the elderly

Bioelectrical impedance analysis (BIA) is used to assess skeletal muscle mass, although its application in the elderly has not been fully established. Several BIA modalities are available: single-frequency BIA (SFBIA), multifrequency BIA (MFBIA), and bioelectrical impedance spectroscopy (BIS). The aim of this study was to examine the difference between SFBIA, MFBIA, and BIS for assessment of appendicular skeletal muscle strength in the elderly. A total of 405 elderly (74.2 ± 5.0 yr) individuals were recruited. Grip strength and isometric knee extension strength were measured. Segmental SFBIA, MFBIA, and BIS were measured for the arms and upper legs. Bioelectrical impedance indexes were calculated by squared segment length divided by impedance ( L2/Z). Impedance at 5 and 50 kHz (Z5 and Z50) was used for SFBIA. Impedance of the intracellular component was calculated from MFBIA (Z250-5) and BIS (RICW). Correlation coefficients between knee extension strength and L2/Z5, L2/Z50, L2/RICW, and L2/Z250-5 of the upper legs were 0.661, 0.705, 0.790, and 0.808, respectively ( P < 0.001). Correlation coefficients were significantly greater for MFBIA and BIS than SFBIA. Receiver operating characteristic curves showed that L2/Z250-5 and L2/RICW had significantly larger areas under the curve for the diagnosis of muscle weakness compared with L2/Z5 and L2/Z50. Very similar results were observed for grip strength. Our findings suggest that MFBIA and BIS are better methods than SFBIA for assessing skeletal muscle strength in the elderly.

Ectopic fat and cardiometabolic and vascular risk

Given that the variation in how regional adipose tissue handles and stores excess dietary energy has substantial cardiometabolic implications, ectopic fat distribution might be an important predictor of cardiometabolic and vascular risk, in addition to overall obesity itself. Conceptually, ectopic fat depots may be divided into systemically acting fat depots and locally acting fat depots. Systemically acting fat depots include visceral fat, fat in the liver, muscle, or neck, and subcutaneous fat. Accumulation in the abdominal visceral area, compared with overall obesity, has an equally or more important role in the development of cardiometabolic risk. Fat depots in liver/muscle tissue cause adverse cardiometabolic effects by affecting energy metabolism. Fat depots in lower-body subcutaneous areas may be protective regarding cardiometabolic risk, by trapping remnant energy. Fat accumulation in the neck is a unique type of fat depot that may increase cardiovascular risk by increasing insulin resistance. Locally acting fat depots include pericardial fat, perivascular fat, and renal sinus fat. These fat depots have effects primarily on adjacent anatomic organs, directly via lipotoxicity and indirectly via cytokine secretion. Pericardial fat is associated with coronary atherosclerosis. Perivascular fat may play an independent role in adverse vascular biology, including arterial stiffness. Renal sinus fat is a unique fat depot that may confer additional cardiometabolic risk. Thus, ectopic fat depots may contribute to the understanding of the link between body composition and cardiometabolic risk. In this review, we focus on the role and clinical implications of ectopic fat depots in cardiometabolic and vascular risk.

RhoA mediates defective stem cell function and heterotopic ossification in dystrophic muscle of mice

Heterotopic ossification (HO) and fatty infiltration (FI) often occur in diseased skeletal muscle and have been previously described in various animal models of Duchenne muscular dystrophy (DMD); however, the pathological mechanisms remain largely unknown. Dystrophin-deficient mdx mice and dystrophin/utrophin double-knockout (dKO) mice are mouse models of DMD; however, mdx mice display a strong muscle regeneration capacity, while dKO mice exhibit a much more severe phenotype, which is similar to patients with DMD. Our results revealed that more extensive HO, but not FI, occurred in the skeletal muscle of dKO mice versus mdx mice, and RhoA activation specifically occurred at the sites of HO. Moreover, the gene expression of RhoA, BMPs, and several inflammatory factors were significantly up-regulated in muscle stem cells isolated from dKO mice; while inactivation of RhoA in the cells with RhoA/ROCK inhibitor Y-27632 led to reduced osteogenic potential and improved myogenic potential. Finally, inactivation of RhoA signaling in the dKO mice with Y-27632 improved muscle regeneration and reduced the expression of BMPs, inflammation, HO, and intramyocellular lipid accumulation in both skeletal and cardiac muscle. Our results revealed that RhoA represents a major molecular switch in the regulation of HO and muscle regeneration in dystrophic skeletal muscle of mice.

Age- and stroke-related skeletal muscle changes: a review for the geriatric clinician

Independently, aging and stroke each have a significant negative impact on skeletal muscle, but the potential cumulative effects of aging and stroke have not been explored. Optimal interventions for individuals post stroke may include those that specifically target skeletal muscle. Addressing changes in muscles may minimize activity limitations and enhance participation post stroke. This article reviews the impact of aging and stroke on muscle morphology and composition, including fiber atrophy, reductions in muscle cross-sectional area, changes in muscle fiber distributions, and increases in intramuscular fat. Relationships between changes in muscle structure, muscle function, and physical mobility are reviewed. Clinical recommendations that preserve and enhance skeletal muscle in the aging adult and individuals post stroke are discussed. Future research directions that include systematic comparison of the differences in skeletal muscle between younger and older adults who have sustained a stroke are suggested.

Fat and muscle indices assessed by pQCT: relationships with physical activity and type 2 diabetes risk

The aim of this study was to compare and determine the repeatability of foreleg and forearm muscle and fat indices evaluated by the peripheral quantitative computed tomography (pQCT). Effects of habitual physical activity and associated health risk of type 2 diabetes were examined within the interrelations of intermuscular adipose tissue (IMAT) and muscle density. Eighty-two premenopausal women (mean age ± standard deviation: 38.6 ± 4.7 yr) underwent dual-energy X-ray absorptiometry scans and pQCT of foreleg and forearm scans to assess muscle and fat parameters. Physical activity status was based on 4-d self-reported log and pedometer step counts. Fat and muscle distribution between the foreleg and forearm were similar and highly correlated to total body adiposity. The pQCT device reliably measured muscle density in the foreleg and forearm; coefficients of variation were 0.8% and 2.1%, which was therefore used to reflect IMAT status. Muscle density was positively related to physical activity and negatively associated with markers of fat distribution and risk for type 2 diabetes. The pQCT is a novel, noninvasive tool to assess IMAT and muscle density in the foreleg and forearm. Additional research is necessary to understand the biology of IMAT and its relations with physical activity and potentially, with risks for cardiometabolic disease.

Heritability and genome-wide association analysis of renal sinus fat accumulation in the Framingham Heart Study

Background

Ectopic fat accumulation in the renal sinus is associated with chronic kidney disease and hypertension. The genetic contributions to renal sinus fat accumulation in humans have not been well characterized.

Methods

The present analysis consists of participants from the Framingham Offspring and Third Generation who underwent computed tomography; renal sinus fat and visceral adipose tissue (VAT) were quantified. Renal sinus fat was natural log transformed and sex- and cohort-specific residuals were created, adjusted for (1) age, (2) age and body mass index (BMI), and (3) age and VAT. Residuals were pooled and used to calculate heritability using variance-components analysis in SOLAR. A genome-wide association study (GWAS) for renal sinus fat was performed using an additive model with approximately 2.5 million imputed single nucleotide polymorphisms (SNPs). Finally, we identified the associations of renal sinus fat in our GWAS results with validated SNPs for renal function (n = 16), BMI (n = 32), and waist-to-hip ratio (WHR, n = 14), and applied a multi-SNP genetic risk score method to determine if the SNPs for each renal and obesity trait were in aggregate associated with renal sinus fat.

Results

The heritability of renal sinus fat was 39% (p < 0.0001); results were not materially different after adjustment for BMI (39%) or VAT (40%). No SNPs reached genome-wide significance in our GWAS. In our candidate gene analysis, we observed nominal, direction consistent associations with renal sinus fat for one SNP associated with renal function (p = 0.01), two associated with BMI (p < 0.03), and two associated with WHR (p < 0.03); however, none remained significant after accounting for multiple testing. Finally, we observed that in aggregate, the 32 SNPs associated with BMI were nominally associated with renal sinus fat (multi-SNP genetic risk score p = 0.03).

Conclusions

Renal sinus fat is a heritable trait, even after accounting for generalized and abdominal adiposity. This provides support for further research into the genetic determinants of renal sinus fat. While our study was underpowered to detect genome-wide significant loci, our candidate gene BMI risk score results suggest that variability in renal sinus fat may be associated with SNPs previously known to be associated with generalized adiposity.

Lean mass predicts asthma better than fat mass among females

The obesity phenotype associated with asthma is not known. Our objective was to define the relative contribution of various distributions of fat and lean mass to asthma prevalence. Data were obtained from 2,525 participants (including 1,422 females) who underwent dual-energy X-ray absorptiometry (DEXA) at the year 20 examination in the Coronary Artery Risk Development in Young Adults (CARDIA) cohort. Total, truncal, arm and leg distributions of fat and lean mass were adjusted to the person's height. Self-reported asthma was the outcome. Asthma among females was associated with greater total fat mass, arm fat mass, total lean mass, truncal lean mass and arm lean mass. Among males, none of these mass measures were significantly associated with asthma. Among females, the association with asthma was stronger for total lean mass than for total fat mass. Further, among various regional distributions of lean and fat mass in females, truncal lean mass was the strongest predictor. Total lean mass is more strongly associated with asthma than total fat mass among females. These findings are contrary to the popular perception that excess physiological fat drives the obesity-asthma association. Rather, we hypothesise that ectopic fat within the "lean" tissues drives this association among females.

Skeletal muscle lipid content and insulin sensitivity in black versus white obese adolescents: is there a race differential?

OBJECTIVE

We tested the hypothesis that skeletal muscle lipid content (SMLC) is higher in obese black adolescents compared with their white peers and assessed the relationship between SMLC and insulin sensitivity (IS).

METHODS

Subjects were healthy obese black (n = 42) and white (n = 38) adolescents. Measurements included an oral glucose tolerance test, IS by a 3-h hyperinsulinemic-euglycemic clamp, abdominal adipose tissue (AT) by magnetic resonance imaging and midthigh SMLC by computed tomography.

RESULTS

All measures of SMLC including intermuscular AT (IMAT), low-density muscle, and thigh sc AT increased (P < 0.05) with increasing total adiposity independent of race. For a given total body adiposity or thigh circumference, SMLC did not differ between black and white adolescents; however, for a given visceral adipose tissue, IMAT was higher in blacks. Consistent with prior observations, IS did not differ between black and white obese adolescents despite lower visceral fat in blacks. In whites, all markers of SMLC were associated (P < 0.05) with IS, whereas in blacks, only IMAT correlated (P < 0.05) with IS. However, in both races, these relationships did not remain significant after accounting for total fat (kilograms).

CONCLUSIONS

SMLC is not different between black and white obese adolescents who have similar total body adiposity but lower visceral fat in blacks. The lack of association between IS and SMLC after adjusting for total adiposity suggest that muscle fat does not make a unique contribution to IS in this group of obese adolescents regardless of race.

Obesity-asthma association: is it explained by systemic oxidant stress?

BACKGROUND

The mechanism for the obesity-asthma association is unknown. This study evaluated the hypothesis that systemic oxidant stress explains this association.

METHODS

This cross-sectional study used year-20 follow-up evaluation data of 2,865 eligible participants in the Coronary Artery Risk Development in Young Adults (CARDIA) cohort. Current asthma was self-reported. Oxidant stress primarily was assessed by plasma F2-isoprostane concentrations. Obesity measures included categories of BMI and dual-energy x-ray absorptiometry-assessed fat mass index (FMI) and lean mass index (LMI). Logistic and linear regressions were used for analyses.

RESULTS

Asthma was associated with higher plasma F2-isoprostane concentrations (p = 0.049); however, this association was not significant when adjusted for either gender or BMI. The BMI-asthma association was seen only among women (p = 0.03; gender-specific interaction, p = 0.01), and this association was not explained by plasma F2-isoprostane levels. Similarly, both FMI and LMI were positively associated with asthma in women (p = 0.20 and 0.01, respectively). These associations also were not explained by plasma F2-isoprostane levels. Similar results were obtained when plasma levels of oxidized low-density lipoprotein were used instead of F2-isoprostane levels to study the BMI-asthma association at the year-15 evaluation.

CONCLUSIONS

Systemic oxidant stress, primarily assessed by plasma F2-isoprostane concentrations, was not independently associated with asthma and, therefore, may not explain the obesity-asthma association in women. The asthma-oxidant stress association is confounded by gender and obesity. This study is limited by the inability to measure airway oxidant stress. It is possible that another (as yet undetermined) measure of systemic oxidant stress may be more relevant in asthma.

Association of the CPT1B gene with skeletal muscle fat infiltration in Afro-Caribbean men

Skeletal muscle fat is greater in African ancestry individuals compared with whites, is associated with diabetes, and is a heritable polygenic trait. However, specific genetic factors contributing to skeletal muscle fat in humans remain to be defined. Muscle carnitine palmitoyltransferase-1B (CPT1B) is a key enzyme in the regulation of skeletal muscle mitochondrial beta-oxidation of long-chain fatty acids, and as such is a reasonable biological candidate gene for skeletal muscle fat accumulation. Therefore, we examined the association of three nonsynonymous coding variants in CPT1B (G531L, I66V, and S427C; a fourth, A320G, could not be genotyped) and quantitative computed tomography measured tibia skeletal muscle composition and BMI among 1,774 Afro-Caribbean men aged > or =40, participants of the population-based Tobago Health Study. For all variants, no significant differences were observed for BMI or total adipose tissue. Among individuals who were homozygous for the minor allele at G531L or I66V, intermuscular adipose tissue (IMAT) was 87% (P = 0.03) and 54% lower (P = 0.03), respectively. In contrast, subcutaneous adipose tissue (SAT) was 11% (P = 0.017) and 7% (P = 0.049) higher, respectively, than among individuals without these genotypes. These associations were independent of age, body size, and muscle area. Finally, no individuals with type 2 diabetes were found among those who were homozygous for the minor allele of either at G531L and I66V whereas 14-18% of men with the major alleles had type 2 diabetes (P = 0.03 and 0.007, respectively). Our results suggest a novel association between common nonsynonymous coding variants in CPT1B and ectopic skeletal muscle fat among middle-aged and older African ancestry men.

Assessment of material, structural, and functional properties of the human skeleton by pQCT systems

Peripheral quantitative computed tomography (pQCT) systems measure bone parameters noninvasively using low radiation doses. This limits image resolution but is practical for the diagnosis and quantitative monitoring of the properties of the peripheral human skeleton. pQCT determines volumetric bone mineral density separately in trabecular and cortical bone. It may combine densitometry determinations with geometric estimates and use strain-stress indexes, and it may be used to analyze muscle variables in some areas, allowing the study of regional fragility. Experimental and clinical ex vivo studies show that pQCT variables correlate with biomechanical predictors of fragility and/or fractures. Since pQCT was approved by the US Food and Drug Administration in 1997, new skeletal regions (human femur and mandible) have been considered in the development of the system. Basically, pQCT explores intraindividual and interindividual variations in greater detail and compares the impact of skeletal diseases, risk factors, and anabolic and catabolic treatments within a given bone cross section.