key: cord-318355-38x3f3ee
authors: Yang, Yang; Ding, Lin; Zou, Xianlun; Shen, Yaqi; Hu, Daoyu; Hu, Xuemei; Li, Zhen; Kamel, Ihab R.
title: Visceral Adiposity and High Intramuscular Fat Deposition Independently Predict Critical Illness in Patients with Sars‐COV‐2
date: 2020-07-17
journal: Obesity (Silver Spring)
DOI: 10.1002/oby.22971
sha: 
doc_id: 318355
cord_uid: 38x3f3ee

OBJECTS: To assess the association between adipose tissue distribution and severity of clinical course in patients with severe acute respiratory syndrome coronavirus 2 (SARS­CoV­2). METHODS: For this retrospective study, 143 hospitalized patients with confirmed coronavirus disease 2019(COVID‐19) who underwent un‐enhanced abdominal computed tomography (CT) scan between January 1(th), 2020 and March 30(th), 2020 were included. Univariate and multivariate logistic regression analyses were performed to identify the risk factors associated with the severity of COVID‐19 infection. RESULTS: There were 45 patients who were identified as critically ill. High visceral to subcutaneous adipose tissue area ratio (VSR, called visceral adiposity) (OR: 2.47, 95CI: 1.05 to 5.98, p=0.040) and low mean attenuation of skeletal muscle (SMD, called high intramuscular fat [IMF] deposition) (OR: 11.90, 95CI: 4.50 to 36.14, p<0.001) were independent risk factors for critical illness. Furthermore, visceral adiposity or high IMF deposition increased the risk of mechanical ventilation (p=0.013, p<0.001; respectively). High IMF deposition increased the risk of death (p=0.012). CONCLUSION: COVID‐19 patients with visceral adiposity or high IMF deposition have higher risk for critical illness. Hence, patients with abdominal obesity should be monitored more carefully when hospitalized.

An outbreak of a novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that emerged in Wuhan has rapidly spread throughout China and became a global health threat (1) (2) (3) (4) . As of May 28 th , SARS-CoV-2 had affected over 5,593,631 individuals，resulted in more than 353,334 deaths worldwide, and the cases continue to rise (4) . The mortality rate of coronavirus disease 2019(COVID- 19) so far is about 2.3-7.2%, lower than that of severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS) coronavirus diseases (1) (2) (3) .

In prior studies, approximately 5-10% of COVID-19 patients rapidly developed critical illness in the form of acute respiratory distress syndrome (ARDS) or sepsis with acute organ dysfunction (1, 5, 6) . Among those, 71-75% of patients required mechanical ventilation and roughly half ended in death (7, 8) . Early identification of patients with high risk for critical illness may help prevent worsening at early stage of the disease and may reduce mortality (9) . Advanced age and comorbidity are the most recognized risk factors for adverse outcome of COVID-19 (1, 2) .

The rising prevalence of obesity has been described as a global pandemic, and became a major public health concern (10) . Over-nutrition, as well as under-nutrition, has been considered an important factor in the body's response to infection for centuries (11) . For patients infected with influenza A (H1N1) virus, obesity was identified as a risk factor for hospitalization and mechanical ventilation in several different populations (12, 13) . The distribution of adipose tissue has major influence on body's immune system (14) . The elevated inflammatory cytokines levels observed in patients with high visceral fat may be associated with increased obesity-associated morbidity in H1N1 infections (15) . Furthermore, lipids can be stored within skeletal muscle when the capacity for fat storage by subcutaneous and visceral are exceeded, known as intramuscular fat (IMF) tissue (14) . IMF deposition not only reflects the quality of muscle and poor physical function, it is also an independent risk factor for prognosis after cardiac and oncologic surgery (16) (17) (18) (19) .

In the present study, we explored the relationship between abdominal adipose tissue distribution, skeletal muscle area and IMF deposition and severity of COVID-19, in a retrospective cohort of 143 patients who had an un-enhanced abdominal computed tomography (CT).

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The study was approved by the Ethics Commission of our institution, and the requirements for informed consent were waived because of its retrospective nature.

We consecutively identified 170 hospitalized patients with confirmed COVID-19 who underwent at least once abdominal CT scan (including thoracoabdominal CT) through a review of clinical and imaging database in Tongji hospital in Wuhan, China, between January 1 th , 2020 and March 30 th , 2020( Figure.1) . The end date for follow-up was April 17 th , 2020. Exclusion criteria were as follows: (a) patients with abdominal CT scan 2 weeks prior to the onset of symptoms (n=8); (b) patients with contrast-enhanced CT of the abdomen(n=7); (c) patients with suboptimal image quality for analysis due to artifacts or ascites (n=5); (d)patients with insufficient scanning coverage for imaging evaluation for subcutaneous adipose tissue (SAT)(n=3); (e)patients died of diseases other than COVID-19(n=4). Indication for CT of the abdomen included abdominal symptoms such as abdominal pain, diarrhea, vomiting and/or abnormal laboratory tests. Finally, a total of 143 patients with un-enhanced abdominal CT were included in this study. For patients with multiple abdominal CT examinations, CT scan with the shortest time interval between imaging and the onset of symptom was used for this analysis.

Demographic and clinical parameters were obtained from clinical electronic medical records as follows: sex, age, height, weight, underlying comorbidities, history of surgery, treatment measures and clinical outcome. Results of laboratory examinations that were closest to the abdominal CT scan were also collected. The durations from onset of symptom to requiring mechanical ventilation, death or discharge were recorded. Critically ill patients are defined as patients with ARDS or sepsis with acute organ dysfunction (6) . Body mass index (BMI) was calculated as weight/height 2 .

All abdominal CT examinations were performed on designated CT scanners for COVID-19 patients ([uCT 780, United Imaging, China], or [Somatom Force/Somatom Definition AS+, Siemens Healthineers, Germany]). After patients were placed in the supine and feet first position

This article is protected by copyright. All rights reserved on the couch, images were acquired at breath-holding following inspiration, using tube voltage 120 kVp, automatic tube current modulation and slice thickness 10 mm. All images were then reconstructed with a slice thickness of 5 mm.

Detailed spatial maps of attenuation coefficients determined by CT provide a reliable and convenient way to quantify tissue areas and mean attenuation within tissue-specific thresholds of attenuation values. The attenuation values were defined on a Hounsfield unit (HU) scale using calibration points of air (-1000 HU) and water (0 HU) (20, 21) . Abdominal adipose tissue and skeletal muscle variables were measured on a single 5mm cross-sectional CT image at the level of third lumbar vertebra (L3) using Advance workstation (GE Healthcare, Milwaukee, USA), software ADW server 4.6 (22) . Two radiologists (L.D. and X.L.Z., with 7 and 3 years of experience, respectively) independently analyzed adipose tissue and skeletal muscle variables, and were blinded to the patient's medical record other than the images belonged to patients with . First, the outer and inner perimeters of SAT were delineated and only the pixels between the inner and outer perimeters were retained. Then, tissue threshold was set to a minimum value of -190 HU and a maximum value of -30 HU. The applied threshold isolated the area of subcutaneous adipose tissue. The skeletal muscle was obtained using the similar method with tissue-specific thresholds ranging between -29 and 150 HU. Skeletal muscle area (SMA) and mean attenuation of the entire skeletal muscle (SMD) were measured. Visceral adipose tissue area (VAT) was obtained after hand tracing the outer perimeter of VAT and removing all pixels outside the contoured area as well as the pixels of intestines, kidney and other organs in the abdominal cavity. Tissue-specific thresholds of -150 to -50 HU were applied and resulted in total VAT area.

Visceral to subcutaneous adipose tissue area ratio (VSR) was calculated to indicate abdominal adipose tissue distributions and high VSR was termed as visceral adiposity (23) . Low-density muscle reflects increased muscle lipid content, termed as high IMF deposition (22, 23) .

The cut-off value of 100cm 2 was used for VAT and SAT, irrespective of sex and age. This value is widely used as a cut-off point to evaluate visceral fat obesity in Asian populations (24) . There are no previously recognized threshold values defining abnormal ranges with regard to VSR, SMA and

This article is protected by copyright. All rights reserved SMD for healthy subjects or patients with virus infection. Hence, the sex-specific median values were used as cut-off points.

The interobserver agreements of skeletal muscle area, the corresponding muscle mean CT attenuation, and subcutaneous and visceral adipose tissue area of the 143 patients measured by the two radiologists independently were analyzed with intraclass correlation coefficient (ICC). The ICC of <0.2 is considered poor agreement, fair (0.21-0.40), moderate (0.41-0.60), good (0.61-0.80), or excellent (0.81-1) to assess inter-observer reproducibility (25) . The mean value of the aforementioned parameters measured by the two radiologists was calculated for further statistical analysis.

Categorical variables were presented as number (percentage) and continuous variables were presented as median (interquartile range [IQR]). To compare variables between different groups, the X 2 test or Fisher exact test were used for categorical variables, and the Student t test or

Mann-Whitney U test were used for continuous variables, when appropriate. Correlations between the continuous adipose tissue and skeletal muscle variables were assessed using Spearman rank correlation analysis. Univariate and multivariate logistic regression analyses were performed to identify the risk factors associated with the severity of COVID-19. Sex, age and variables with p < 0.05 at univariate logistic regression analysis were applied to multivariate analysis. The linear relationship between continuous predictor variables and the logit of the outcome were checked.

Variance inflation factor (VIF) was used to check the collinearity of variables included in the multivariate logistic regression equation. No significant interaction was found between variables included in multivariate analyses. Based on likelihood ratio test, we were allowed to keep sex, age, hypertension, cerebrovascular disease, visceral adiposity and high IMF deposition in the final model. Cumulative rates of mechanical ventilation and mortality were calculated using Kaplan-Meier methods, and differences between curves were evaluated using the log-rank test.

Statistical analysis was done with R software (version 3.6.1, R Project for Statistical Computing, www.r-project.org). A two-tailed p value less than 0.05 was considered statistically significant.

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Critically ill patients were defined as patients with ARDS or sepsis with acute organ dysfunction based on the clinical management of COVID-19 published by World Health Organization (WHO) (6) . There were 45 patients who met the criteria for critical illness while the remaining 98 patients did not (Figure1). The clinical and physical characteristics of the two groups were summarized in Table1. No significant difference in age or sex was found between the two groups (p=0.105; p=0.107; respectively). Critical illness group had significantly higher height and body weight (p= 0.034; p= 0.022; respectively), but no significant difference was found between the two groups in BMI values (p=0.148). Hypertension, diabetes, cardiovascular disease, cerebrovascular disease and malignancy were the most common comorbidities in patients with COVID-19, among these comorbidities hypertension and cerebrovascular disease were significantly more common in critical illness group (p=0.03; p=0.025; respectively). 

Interobserver agreement between the two radiologists' measurements of adipose tissue and skeletal muscle variables was excellent (ICC = 0.995 for SAT, ICC = 0.999 for VAT, ICC =0.909 for SMA and ICC= 0.993 for SMD).

Critically ill patients had significantly higher VAT and VSR (p= 0.003; p= 0.042; respectively).

Median value of SMD was 25.4HU and 35.7HU in critically ill and non-critically ill groups, respectively, and the difference was statistically significant (p <0.001)(Table1).

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An area of 100cm 2 was used as cut-off points to distinguish high and normal fat areas for VAT and SAT, irrespective of sex. The median values of VSR (1.33 in male and 0.71 in female) were used to distinguish high and normal VSR. Similarly, the cut-off values of SMA were 115.5 cm 2 in males and 84.7 cm 2 in females. The cut-off values of SMD were 32.7 HU in males and 28.9 HU in females.

The clinical characteristics of the patients with and without visceral adiposity or high IMF deposition were shown in Table 2 . Patients with visceral adiposity or high IMF deposition were significantly older (p=0.045; p<0.001; respectively). A relatively larger area of VAT and smaller area of SMA were found in patients with high IMF deposition (p=0.016; p=0.006; respectively).

There was no difference in sex and complication rate between patients with and without visceral adiposity or high IMF deposition.

The overall rate of critical illness in our study was 31.5%. The rates of critical illness in patients with visceral adiposity were almost twice as much as patients with no visceral adiposity (42.3% vs. 22.5%, p=0.035; Table3). Patients with high IMF deposition showed significantly higher rate of critical illness compared those without (52.1% vs. 11.1%, p<0.001; Table3).

Both univariate and multivariate logistic regression analyses were used to examine the association of adipose tissue and skeletal muscle variables with the risk of critical illness ( 

This article is protected by copyright. All rights reserved High IMF deposition increased the risk of death (p=0.012, Figure3d).

As adipose tissue distribution is usually associated with age, we studied the relationship between them. While SAT, VAT and VSR were not correlated with age (R= -0.162, p=0.054; R=-0.063, p=0.454; R=0.127, p=0.130; respectively), SMA and SMD were moderately correlated with age (R= -0.333, p<0.001; R=-0.459, p<0.001; respectively). Otherwise, SMD was not correlated with VSR(R=-0.025, p=0.760).

We divided the patients into two subgroups based on age. In the group younger than 60 years old (n=44), patients with visceral adiposity (OR: 7.58, 95CI: 1.7 to 42.21, p=0.011) and high IMF deposition (OR: 18.67, 95CI: 3.64 to 147.42, p=0.001) had higher risks for critical illness. In the subgroup older than 60 years old (n=99), only high IMF deposition (OR: 6.21, 95CI: 2.39 to 18 .47, p<0.001) was significantly associated with the severity of COVID-19 infection.

In this study, we analyzed the impact of adipose tissue distribution on the severity of COVID-19 infection in a hospitalized cohort. Visceral adiposity and high IMF deposition were independent risk factors associated with critical illness, and those factors increased the risks of mechanical ventilation and death. Moreover, in the subgroup younger than 60 years old, patients with visceral adiposity or high intramuscular fat deposition had higher risk for critical illness, even though sample size of the subgroup was small.

Body composition not only reflects nutritional status, but has significant influence on metabolic activity and inflammatory response (14) . Several methods are available for the evaluation of those components and BMI is the most commonly used one. Previous study reported that high BMI was significantly associated with critical illness for patients with COVID-19 in a large prospective cohort study (26) . However, no significant difference in BMI values was found between critical illness and non-critical illness groups in our cohort. The inconsistent results may be due to the small sample size of our cohort. As BMI is an indirect measurement of adipose tissue, it cannot provide information on fat distribution. More recent data proposes that visceral adiposity and

This article is protected by copyright. All rights reserved ectopic fat accumulation contributes to various features of the metabolic syndrome, over and above the BMI (14) . A cross-sectional image of CT scans at the level of the third lumbar vertebra (L3) could quantify adipose tissues and skeletal muscle, and currently it is a widely used method to evaluate adipose tissue distribution (23) . In this study, visceral adiposity and high IMF deposition were significantly associated with critical illness for patients with COVID-19. This suggested that the fat distribution rather than BMI values could be better risk factor for critical illness of patients with COVID-19.

In this study, we found that visceral, rather than subcutaneous adipose deposition, was associated with the severity of COVID-19 infection. Previous studies showed that visceral adipose tissue was different from subcutaneous adipose tissue in the pro-inflammatory cytokines production (27) .

Visceral fat was supposed to be an endocrine organ and had pro-inflammatory characteristics (14) .

Different studies demonstrated that people with visceral adiposity have more concentration of circulating inflammatory cytokines compared with lean individuals (14, 28) . Tumor necrosis factor-α (TNF-α), interleukin (IL)-6, and IL-1b are the main inflammatory cytokines secreted by visceral adipose tissue from adipocytes and resident macrophages, while anti-inflammatory cytokines such as adiponectin was secreted from subcutaneous adipocytes (29) . In SARS-CoV-2 infected individuals, studies had reported that IL-6, IL-10 and TNFα surged during illness and declined during recovery (30, 31) . And a part of severe patients with COVID-19 had an elevated cytokine profile resembling cytokine storm in SARS and MERS (32) . The dysregulated and exuberant immune responses could exacerbate lung damage as well as lead to other fatal complications (33) .

On the other hand, excessive adipose tissue, especially visceral adipose, can also alter cell-mediated immune responses through the increase in leptin and the decrease in adiponectin (34) .

The immune dysfunction caused by visceral fat aggravated COVID-19 induced immune damage may be the reason why patients with visceral adiposity were more susceptible to developing a serious illness. In addition, abdominal obesity can profoundly alter pulmonary function by diminishing exercise capacity and augmenting airway resistance, resulting in an increased work of breathing (35) . For supine patients, ventilation may be more difficult with the decreased diaphragmatic excursion.

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The present study showed that high IMF deposition was significantly associated with risk of critical illness and requiring mechanical ventilation in patients with COVID-19. There was a moderate correlation between the value of SMD and age, and high IMF deposition was more common in older patients in our study, but high IMF deposition was still a significant risk factor for critical illness when age was adjusted statistically. In the subgroup under 60 years old, patients with high IMF deposition had higher risk for critical illness, however the 95% confidence of the odds ratios were large. This may be due partly to the small sample size of the subgroup. Previous studies have shown that high IMF deposition contributes to muscle weakness independent of the age-associated loss in muscle mass, which is consistent with our findings (19) . Previous study reported that IMF deposition delayed and blunted immune responses, especially natural killer lymphocytes involved in innate immunity (14) . Montano Loza et al demonstrated that the presence of skeletal muscle loss and intra-muscular adipose infiltration increased the risk of sepsis-related death in patients with cirrhosis, probably due to impaired immunity (36) . Hence, high IMF deposition would impair immune function of COVID-19 patients and increase the risk of complications and mortality.

To our knowledge, this was the first study that had investigated the association between adipose tissue distribution and severity of COVID-19 infection. There were several limitations in our study. First, it is a retrospective, single center with a small sample of hospitalized patient, therefore larger multicenter studies are needed to confirm the results of this study. Second, the possibility of selection bias in patient inclusion in the study group must be considered, as abdominal CT scan was not a routine examination for COVID-19 patients. Besides, physical activity levels were not analyzed in our study, although they were widely associated with adipose tissue distribution in previous studies (16, 28) . The association between smoking with severity of COVID-19 was controversial, it was not analyzed in our study because the electronic medical record of smoking was not complete in our hospital during the outbreak period (37, 38) . Despite that, our study gave a new insight that visceral adiposity and high IMF deposition were significantly associated with the severity of COVID-19 infection. The assessment of adipose distribution and IMF deposition may help the risk stratification of patients with COVID-19 upon hospital Accepted Article admission.

Our findings show that patients with visceral adiposity or high IMF deposition were more likely to develop into critical illness when infected with COVID-19. Therefore, patients with abdominal obesity should be monitored more carefully when hospitalized.

Acknowledgments: Data described in the manuscript, code book, and analytic code will be made available upon request pending. Table1 Baseline characteristics of hospitalized patients with COVID-19 in Tongji 

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This article is protected by copyright. All rights reserved Variables were presented as number (percentage), the X 2 test or Fisher exact test were used to compare variables between different groups. A two-tailed p value less than 0.05 was considered statistically significant.

SAT, subcutaneous adipose tissue area; VAT, visceral adipose tissue area; SMA, skeletal muscle area;

IMF, intramuscular fat. 

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