Journal of

Clinical Medicine

Article
Association of Metabolic Syndrome with Sensorineural
Hearing Loss
Hwa-Sung Rim 1 , Myung-Gu Kim 2 , Dong-Choon Park 3 , Sung-Soo Kim 4 , Dae-Woong Kang 1 ,
Sang-Hoon Kim 1 and Seung-Geun Yeo 1, *

1 Department of Otolaryngology—Head & Neck Surgery, School of Medicine, Kyung Hee University,
Seoul 02454, Korea; marslover@naver.com (H.-S.R.); kkang814@naver.com (D.-W.K.);
hoon0700@naver.com (S.-H.K.)
2 Department of Otorhinolaryngology, Samsung Changwon Hospital, Sungkyunkwan University School of
Medicine, Changwon 51353, Korea; mgent.kim@samsung.com
3 St. Vincent’s Hospital, The Catholic University of Korea, Suwon 16247, Korea; dcpark@catholic.ac.kr
4 Department of Biochemistry and Molecular Biology, School of Medicine, Kyung Hee University,
Seoul 02447, Korea; sgskim@khu.ac.kr
* Correspondence: yeo2park@gmail.com; Tel.: +82-2-958-8980; Fax: +82-2-958-8470

Abstract: The prevalence of sensorineural hearing loss has increased along with increases in life
expectancy and exposure to noisy environments. Metabolic syndrome (MetS) is a cluster of co-
occurring conditions that increase the risk of heart disease, stroke and type 2 diabetes, along with
other conditions that affect the blood vessels. Components of MetS include insulin resistance, body
weight, lipid concentration, blood pressure, and blood glucose concentration, as well as other features
of insulin resistance such as microalbuminuria. MetS has become a major public health problem



affecting 20–30% of the global population. This study utilized health examination to investigate


whether metabolic syndrome was related to hearing loss. Methods: A total of 94,223 people who
Citation: Rim, H.-S.; Kim, M.-G.; underwent health check-ups, including hearing tests, from January 2010 to December 2020 were
Park, D.-C.; Kim, S.-S.; Kang, D.-W.;
evaluated. Subjects were divided into two groups, with and without metabolic syndrome. In addition,
Kim, S.-H.; Yeo, S.-G. Association of
Scopus, Embase, PubMed, and Cochrane libraries were systematically searched, using keywords
Metabolic Syndrome with
such as “hearing loss” and “metabolic syndrome”, for studies that evaluated the relationship between
Sensorineural Hearing Loss. J. Clin.
Med. 2021, 10, 4866. https://
the two. Results: Of the 94,223 subjects, 11,414 (12.1%) had metabolic syndrome and 82,809 did not.
doi.org/10.3390/jcm10214866 The mean ages of subjects in the two groups were 46.1 and 43.9 years, respectively. A comparison of
hearing thresholds by age in subjects with and without metabolic syndrome showed that the average
Academic Editor: George Psillas pure tone hearing thresholds were significantly higher in subjects with metabolic syndrome than
in subjects without it in all age groups. (p < 0.001) Rates of hearing loss in subjects with 0, 1, 2, 3, 4,
Received: 26 September 2021 and 5 of the components of metabolic syndrome were 7.9%, 12.1%, 13.8%, 13.8%, 15.5% and 16.3%,
Accepted: 20 October 2021 respectively, indicating a significant association between the number of components of metabolic
Published: 22 October 2021 syndrome and the rate of hearing loss (p < 0.0001). The odds ratio of hearing loss was significantly
higher in subjects with four components of metabolic syndrome: waist circumference, blood pressure,
Publisher’s Note: MDPI stays neutral
and triglyceride and fasting blood sugar concentrations (p < 0.0001). Conclusions: The number
with regard to jurisdictional claims in
of components of the metabolic syndrome is positively correlated with the rate of sensorineural
published maps and institutional affil-
hearing loss.
iations.

Keywords: metabolic syndrome; sensorineural hearing loss

Copyright: © 2021 by the authors.

Licensee MDPI, Basel, Switzerland.
1. Introduction
This article is an open access article
distributed under the terms and
Sensorineural hearing loss is an important public health problem whose prevalence
conditions of the Creative Commons
has increased as life expectancy has become longer [1]. For example, the Global Burden of
Attribution (CC BY) license (https:// Disease Study reported that the prevalence of hearing loss increased from 14.33% in 1990
creativecommons.org/licenses/by/ to 18.06% in 2015 and that hearing loss is the fifth most frequent cause of disability in both
4.0/). developed and developing countries [2]. Approximately 50% of people aged over 70 years

J. Clin. Med. 2021, 10, 4866. https://doi.org/10.3390/jcm10214866 https://www.mdpi.com/journal/jcm

J. Clin. Med. 2021, 10, 4866 2 of 11

and 80% of those aged over 85 years experience hearing loss, which affects their ability to
communicate and their social lives [1]. The pathophysiological mechanisms of hearing loss
are complex, although several risk factors have been reported to contribute to hearing loss,
including genetic factors, inflammatory processes, systemic diseases, noise, medications,
oxidative stress, and aging [3].
Metabolic syndrome is a disease that includes hypertension, central obesity, hyper-
lipidemia, and diabetes [4]. Metabolic syndrome was found to be associated with various
clinical conditions, including stroke, myocardial infarction, death from cardiovascular
disease, and diabetes [5,6]. Studies have recently reported that metabolic syndrome may be
associated with hearing loss [3,7–12]. Using health examination data, this study therefore
investigated the relationships between metabolic syndrome and hearing loss. In addi-
tion, studies published on this relationship were subjected to comparative analysis and a
systematic review.

2. Materials and Methods

2.1. Study Population
This study included a total of 94,223 people, ranging in age from their 20s to their
70s, who underwent health checkups, including hearing tests, at a tertiary university
hospital from January 2010 to December 2020. Height, weight, body mass index, waist
circumference and blood pressure (both systolic and diastolic) were measured in all subjects,
and all subjects underwent hearing and blood tests and pulmonary function tests (PFT).
Subjects without hearing test results and those with a ≥ 20 dB difference between the two
hearing thresholds, a history of surgery for otitis media, or suspected central disease were
excluded. The study protocol was approved by the Institutional Review Board of Kyung
Hee University Medical Center (KMC 2019-07-065).

2.2. Definition of Metabolic Syndrome

The criteria for metabolic syndrome were defined according to the revised National
Cholesterol Education Program Adult Treatment Panel III: [13] (1) waist circumference
(WC) > 90 cm for men or > 80 cm for women, (2) fasting blood sugar > 100 mg/dL or
a diagnosis of diabetes, (3) BP >130/85 mmHg or a diagnosis of high blood pressure,
(4) triglyceride concentration > 150 mg/dL, and (5) high-density lipoprotein-cholesterol
(HDL-C) < 40 mg/dL for men or < 50 mg/dL for women. Subjects with three or more of
these five factors were defined as positive for metabolic syndrome.

2.3. Hearing Test

Hearing threshold tests were performed at frequencies of 500, 1000, 2000, 3000, 4000,
and 6000 Hz, measured in that order, followed by a retest at 1000 Hz, with air conduction
measured at a frequency of 500 Hz. Pure tone audiometry was performed using the
standard hexadecimal method, by measuring the air conduction values of each ear at four
frequencies (500, 1000, 2000, and 4000 Hz) and calculating their sum using the equation:
(500 Hz + 2 × 1000 Hz + 2 × 2000 Hz + 4000 Hz)/6 [14–17].

2.4. Research on Hearing Loss and Metabolic Syndrome

To systematically review the results of studies on the correlation between hearing loss
and metabolic syndrome, the Scopus, Embase, PubMed, and Cochrane library databases
were searched using the keywords “hearing loss” and “metabolic syndrome”. A total of
1373 papers were identified initially, including 390, 612, 364, and seven, respectively, in
these databases. Of these, 1300 papers were excluded due to lack of relevance in the title,
and an additional 44 papers were excluded due to duplication of papers. Of the 29 papers
reviewed in full, 17 were included in the systematic analysis (Table 1).
J. Clin. Med. 2021, 10, 4866 3 of 11

Table 1. Studies assessing the association between hearing loss and metabolic syndrome.

Associated
Author (Year) Study Design N Age (Years) Conclusions
Variables
MetS is associated with hearing
50.4 ± 16.6 thresholds in women; and CKD is
MetS or
Kang S.H. et al. (men) associated with hearing thresholds in
Cross-sectional 16,554 CKD/Hearing
(2015) [9] 49.2 ± 16.4 men and women. Subjects with MetS or
thresholds
(women) CKD should be closely monitored for
hearing impairment.
Significant associations between the
number of components of metabolic
Sun Y.-S. et al. MetS compo-
Cross-sectional 2100 ≤65 syndrome and hearing thresholds in US
(2015) [3] nents/SNHL
adults, with the strongest association
between low HDL and hearing loss.
WHR may be a surrogate marker for
Kang S.H. et al.
Cross-sectional 8198 54.7 ± 9.9 WHR/HL predicting the risk of hearing loss
(2015) [18]
resulting from metabolic syndrome.
Adults with DM and hypertension
showed greater hearing impairment in a
highly endogamous population.
Bener A. et al. Diabetic patients with hearing loss were
Cross-sectional 459 20–59 DM, HTN/HL
(2016) [19] likely to have high blood glucose and
other risk factors like hypertension,
retinopathy, nephropathy, and
neuropathy.
42.33 ± 7.49
Underweight and severe obesity were
Kim S.H. et al. (normal)
Cross-sectional 61,052 BMI/HL associated with an increased prevalence
(2016) [20] 49.89 ± 9.32
of hearing loss in a Korean population.
(HL)
53.2 ± 10.7 HbA1c level was associated with
Kang S.H. et al.
Cross-sectional 7449 56.7 ± 11.0 HbA1c/HL hearing impairment in nondiabetic
(2016) [21]
59.8 ± 10.8 individuals.
Metabolic syndrome itself was not an
independent risk factor for hearing
Lee H.Y. et al. MetS compo- impairment. Among its individual
Retrospective 16,779 ≥19
(2016) [22] nents/SNHL components, only increased fasting
plasma glucose was independently
associated with hearing impairment.
Possible associations between different
components of MetS (obesity,
Aghazadeh-Attari J. MetS compo- hypertension, hypertriglyceridemia,
Cross-sectional 11,114 20–60
et al.(2017) [12] nents/SNHL high fasting glucose levels, and waist
circumference) and SNHL in a
population of West Azerbaijan drivers.
High TG/HDL-C ratio was associated
Jung D.J. et al. TG/HDL
Cross-sectional 18,004 >40 with hearing impairment in a Korean
(2017) [23] ratio/SNHL
population
High prevalence of hearing loss among
diabetic adults at University of Nigeria
Nwosu J. N. et al.
Case-control 416 26–80 years DM/HL Teaching Hospital, Enugu. Hearing loss
(2017) [24]
was predominantly sensorineural and
often mild to moderate in severity.
Factors relevant
FRAs were associated with
abdominal fats
frequency-specific hearing losses
Lee Y. et al. (FRAs)/
Retrospective 2602 57.6 ± 7.3 according to sex. DM and visceral
(2017) [25] Age-related
adipose tissue (VAT) are particularly
hearing loss
important role for hearing.
(ARHL)
J. Clin. Med. 2021, 10, 4866 4 of 11

Table 1. Cont.

Associated
Author (Year) Study Design N Age (Years) Conclusions
Variables
MetS is associated with age-related
hearing impairment in women aged ≥
MetS components/
50 years. At 5-year follow-up,
Kim T.S. et al. age-related hearing
Prospective 1381 >50 high-frequency hearing loss tended to
(2017) [8] impairment
be greater in women with than without
(ARHI)
MetS, suggesting the need for hearing
evaluation in older women with MetS.
MetS, including its components central
61.1 ± 7.6
Han X. et al. MetS compo- obesity, hyperglycemia, and low HDL-C
Cross-sectional 18,824 66.6 ± 7.2
(2018) [7] nents/SNHL levels, is positively associated with
71.0 ± 7.7
hearing loss.
Among the components of MetS, low
HDL and high TG levels were especially
Jung D.J. et al. MetS compo- associated with hearing loss. Rather
Cross-sectional 17,513,555 >40
(2019) [11] nents/SNHL than assessing MetS, each MetS
component should be evaluated
individually.
MetS may be associated with hearing
Shim H.S. et al. MetS compo- loss, especially in subjects who meet
Cross-sectional 28,866 all age groups
(2019) [10] nents/SNHL four or five of the diagnostic criteria for
MetS.
Overweight and obesity are associated
BMI
Hu H. et al. with an increased risk of hearing loss,
Prospective cohort 48,549 20–64 (w/o Waist circum-
(2020) [26] with metabolically unhealthy status
ference)/SNHL
conferring an additional risk.
Kim J. et al. MetS MetS is associated with high-frequency
Cross-sectional 10,356 40–80
(2021) [27] components/HL hearing loss in subjects exposed to noise.
Abbreviations: MetS, metabolic syndrome; CKD, chronic kidney disease; SNHL, sensorineural hearing loss; HL, hearing loss, DM, diabetes
mellitus; HTN, hypertension; BMI, body mass index; WHR, waist hip ratio; HDL, high density lipoprotein; TG, triglyceride.

2.5. Statistics
Subjects were divided into two groups, those with and without metabolic syndrome,
for analysis of clinical results, and into six groups by age, 20s, 30s, 40s, 50s, 60s, and 70s
and over, for analysis of average hearing threshold and degree of hearing loss. To evaluate
risk factors related to hearing loss, subgroups were classified according to the number
of factors corresponding to the diagnostic criteria for metabolic syndrome. Continuous
variables were compared by t-tests and categorical variables by chi-square tests and Fisher’s
exact tests. Normal and adjusted odds ratios (ORs) of the relationships of hearing loss
with age and number of factors of metabolic syndrome were analyzed by univariate and
multivariate logistic regression analyses. All statistical analyses were performed using
IBM SPSS version 22 (IBM Corp., Armonk, NY, USA), with p values < 0.05 defined as
statistically significant.

3. Results
Of the 94,223 subjects, 11,414 (12.1%) had metabolic syndrome and 82,809 (87.9%)
did not. The mean ages of these two subgroups were 46.1 and 43.9 years, respectively.
Of the 11,414 subjects with metabolic syndrome, 8255 (72.3%) were men, whereas, of
the 82,809 subjects without metabolic syndrome, 45,200 (54.5%) were men, making the
percentage of men significantly higher in subjects with metabolic syndrome (p < 0.001).
Waist circumference, BP, and triglyceride and fasting blood glucose concentrations were
all significantly higher in subjects with metabolic syndrome than in subjects without it.
(p < 0.001) (Table 2).
J. Clin. Med. 2021, 10, x FOR PEER REVIEW 5 of 11
J. Clin. Med. 2021, 10, 4866 5 of 11

Table 2. Demographic characteristics of study subjects.

Table 2. Demographic characteristics of study subjects.
Non-MetS MetS
p-Value
n or MeanNon-MetS % or SD n or Mean MetS % or SD
Age 43.91 9.42 46.12 9.30 p-Value*,a
<0.0001
n or Mean % or SD n or Mean % or SD
Male 45,157 (54.53%) 8253 (72.31%) <0.0001 *,b
Sex AgeFemale 43.91 9.42 46.12 9.30 <0.0001 *,a
37,652 (45.47%) 3161 (27.69%)
Male 45,157 (54.53%) 8253 (72.31%) <0.0001 ,b
Waist
Sex circumference 82.00 7.86 93.40 8.15 <0.0001 **,a
Female
Systolic blood pressure 37,652
116.39 (45.47%)
11.18 3161
127.91 (27.69%)
11.97 <0.0001 *,a
Waist circumference 82.00 7.86 93.40 8.15 <0.0001 *,a,a
Diastolic blood pressure 70.96 8.99 79.11 9.29 <0.0001 *,a
Systolic blood pressure 116.39 11.18 127.91 11.97 <0.0001 * ,a
HDL 61.84 15.62 45.79 11.83 <0.0001 *
Diastolic blood pressure 70.96 8.99 79.11 9.29 <0.0001 *,a,a
TG
HDL 107.12
61.84 64.92
15.62 233.77
45.79 141.82
11.83 <0.0001 **,a
<0.0001
FPG
TG 90.89
107.12 14.14
64.92 109.39
233.77 28.33
141.82 <0.0001 **,a
<0.0001 ,a

BMI
FPG 23.26
90.89 3.00
14.14 27.50
109.39 3.24
28.33 <0.0001 *,a
<0.0001 * ,a
Abbreviations:BMIMetS, metabolic syndrome;
23.26 Non-MetS,
3.00 without metabolic syndrome;
27.50 3.24 HDL, high*,a
<0.0001
density lipoprotein cholesterol; TG, triglyceride; FPG, fasting plasma glucose; BMI, body mass
Abbreviations: MetS, metabolic syndrome; Non-MetS, without metabolic syndrome; HDL, high density lipopro-
index. * p valuesTG,
tein cholesterol; between Non-MetS
triglyceride; and MetS
FPG, fasting were
plasma tested BMI,
glucose; usingbody
the t-test a and chi-square b. *
mass index. * p values between
Statistically significant. p < 0.05. a b
Non-MetS and MetS were tested using the t-test and chi-square . * Statistically significant. p < 0.05.

A comparison
comparisonof ofhearing
hearingthresholds
thresholds byby
age in subjects
age with
in subjects andand
with without metabolic
without syn-
metabolic
drome showed that the average pure tone hearing thresholds were significantly
syndrome showed that the average pure tone hearing thresholds were significantly higher higher in
subjects with metabolic syndrome
in subjects with metabolic syndrome than in subjects without it in all age groups.
in subjects without it in all age groups. (p <
(p < 0.001)
0.001) (Figure(Figure
1). In 1). In addition,
addition, comparisons
comparisons by age by age confirmed
group group confirmed
that thethat the per-
percentages
centages
of subjectsof with
subjects with loss
hearing hearing
wereloss wereinhigher
higher in subjects
subjects with metabolic
with metabolic syndrome syndrome
than in
than in subjects
subjects withoutwithout
metabolic metabolic
syndrome,syndrome,
and thereandwas
there was a significant
a significant statistical
statistical differ-
difference in
ence in the groups in their 30s, 40s, and 50s. (p < 0.05)
the groups in their 30s, 40s, and 50s. (p < 0.05) (Table 3). (Table 3).

50 *
45
40
*
35
30 *
PTA(dB)

25 *
* * Non-MetS
20
15 MetS
10
5
0
20–29 30–39 40–49 50–59 60–69 ≥70
Age

Figure
Figure 1. PTA 1. PTA threshold
threshold according
according to age.toAbbreviations:
age. Abbreviations:
MetS,MetS, metabolic
metabolic syndrome;
syndrome; Non–MetS,
Non–MetS, without
without meta-
metabolic
bolic
syndrome; syndrome;
PTA, PTA,
pure tone pure tone*audiometry.
audiometry. * Statistically
Statistically significant. p <significant.
0.05. p < 0.05.

Subjects in this study were divided into subgroups according to the number of com-
ponents belonging to the diagnostic criteria for metabolic syndrome, and the proportions
of subjects diagnosed with hearing loss in each group were compared. Rates of hearing
loss in subjects with 0, 1, 2, 3, 4, and 5 of the components of metabolic syndrome were 7.9%,
12.1%, 13.8%, 13.8%, 15.5%, and 16.3%, respectively, indicating a significant association
between the number of components of metabolic syndrome and the rate of hearing loss.
(p < 0.0001) (Table 4).
J. Clin. Med. 2021, 10, 4866 6 of 11

Table 3. Comparison of hearing by age groups in MetS and non-MetS subjects.

Age Non-MetS MetS

p-Value a
(Years) Normal Hearing Loss Total Normal Hearing Loss Total
20–29 4645 (99.06%) 44 (0.94%) 4689 272 (98.19%) 5 (1.81%) 277 0.1943
30–39 20,996 (97.78%) 477 (2.22%) 21,473 2309 (96.81%) 76 (3.19%) 2385 0.0041 *
40–49 32,068 (92.98%) 2422 (7.02%) 34,490 4465 (91.48%) 416 (8.52%) 4881 0.0002 *
50–59 14,199 (79.26%) 3715 (20.74%) 17,914 2316 (76.84%) 698 (23.16%) 3014 0.0028 *
60–69 1828 (56.86%) 1387 (43.14%) 3215 341 (53.36%) 298 (46.64%) 639 0.1064
≥70 197 (29.1%) 480 (70.9%) 677 40 (23.53%) 130 (76.47%) 170 0.1532
Abbreviations: MetS: metabolic syndrome; Non-MetS, without metabolic syndrome. * p values between Non-MetS and MetS were tested
using the Fisher’s exact test a . * Statistically significant. p < 0.05.

Table 4. Comparison of the groups of subjects with normal hearing and hearing loss.

Subject Group
Normal Hearing (n) Hearing Loss (n)
p-Value a,b
n (%) PTA(dB) SD n (%) PTA(dB) SD
Non-MetS 38,069 (92.0%) 11.11 4.96 3293 (7.9%) 38.70 15.44 <0.0001 *
Non-MetS
22,635 (87.9%) 12.25 5.22 3109 (12.1%) 38.72 14.75 <0.0001 *
(1 factor)
Non-MetS
13,272 (86.2%) 12.84 5.23 2123(13.8%) 38.16 13.83 <0.0001 *
(2 factors)
MetS (3 factors) 6900 (86.2%) 13.19 5.20 1098 (13.8%) 37.79 13.56 <0.0001 *
MetS (4 factors) 2396 (84.5%) 13.34 5.12 438 (15.5%) 37.56 13.56 <0.0001 *
MetS (5 factors) 449 (83.7%) 13.65 5.28 87 (16.3%) 38.86 12.95 <0.0001 *
Results are reported as mean ± SD or as n (%). Abbreviations: MetS, metabolic syndrome; Non-MetS, without metabolic syndrome; PTA,
pure tone audiometry. * p values between Normal hearing and Hearing loss were tested using the t-test a and chi-square b . * Statistically
significant. p < 0.05.

To correct for the effects of age and gender on the relationship between BMI (Body
Mass Indx) and hearing loss, multivariate analysis was performed to determine the normal
and corrected odds ratios of hearing loss. The odds ratio of hearing loss was 1.89-fold
higher in women than in men, and it was 1.13-fold higher as age increased by 1 year
(p < 0.0001). Relative to subjects with 0 components of metabolic syndrome, the odds ratios
of hearing loss in subjects with 3, 4, and 5 components of metabolic syndrome were 1.06
(p = 0.1119), 1.29. (p < 0.0001), and 1.21 (p = 0.1410), respectively (Table 5).

Table 5. Crude and adjusted odds ratios of hearing loss among subjects with metabolic syndrome.

Univariable Analysis Multivariable Analysis a

OR 95% CI p-Value a OR 95% CI p-Value b
Sex 1.97 1.89 2.06 <0.0001 * 1.89 1.80 1.99 <0.0001 *
Age 1.13 1.13 1.14 <0.0001 * 1.13 1.13 1.14 <0.0001 *
MetS (3 factors) 1.38 1.29 1.48 <0.0001 * 1.06 0.99 1.14 0.1119
MetS (4 factors) 1.59 1.43 1.76 <0.0001 * 1.29 1.15 1.44 <0.0001 *
MetS (5 factors) 1.68 1.34 2.12 <0.0001 * 1.21 0.94 1.56 0.1410
Abbreviations: MetS, metabolic syndrome; OR, odds ratio; CI, confidence interval. * p values were tested using
univariate a and multivariate b logistic regression analyses. * Statistically significant. p < 0.05. a Including the
variables age, sex, and metabolic syndrome.

When analyzing the influence of each component of metabolic syndrome, the odds
ratio of hearing loss was significantly higher in the group with four factors: waist cir-
cumference, blood pressure and triglyceride, and fasting blood sugar concentrations
(p < 0.0001) (Table 6).
J. Clin. Med. 2021, 10, 4866 7 of 11

Table 6. Association between hearing loss and metabolic syndrome.

Univariable Analysis Multivariable Analysis

Risk Factors
OR 95% CI p-Value a OR 95% CI p-Value b
Waist circumference 0.96 0.92 1.01 0.1003 1.09 1.04 1.15 0.0009 *
Systolic BP 1.72 1.64 1.80 <0.0001 * 1.12 1.06 1.18 <0.0001 *
Triglyceride level 1.25 1.19 1.31 <0.0001 * 1.08 1.03 1.13 0.0037 *
HDL-C level 1.20 1.13 1.28 <0.0001 * 1.05 0.98 1.12 0.1641
FPG level 2.14 2.05 2.24 <0.0001 * 1.14 1.09 1.20 <0.0001 *
Abbreviations: Systolic BP, systolic blood pressure; HDL-C, high density lipoprotein cholesterol; FPG, fasting
plasma glucose; OR, odds ratio; CI, confidence interval. * p values were tested using univariate a and multivariate
b logistic regression analyses. * Statistically significant. p < 0.05.

4. Discussion
This cross-sectional study involving a large population assessed the relationship
between metabolic syndrome and hearing loss. This study is based on health examination
data for relatively healthy patients. In this study, 12.1% or 11,413 subjects met the diagnostic
criteria for metabolic disease. First, due to the nature of this study, we cannot rule out the
possibility that the study population was biased toward people interested in health. In
addition, it is possible that there were fewer subjects who met the criteria for metabolic
syndrome because at the time of assessment subjects may have been taking medications to
control blood pressure, blood sugar, and/or other blood levels. However, the results of this
study are meaningful because sufficient number of subjects, 94,223 was included.
To compare the incidence of hearing loss in adults with and without metabolic syn-
drome after adjustment for age, the rates of hearing loss were analyzed in subjects with 3,
4, and 5 of the components of metabolic syndrome, and the factors most closely related to
hearing loss were identified.
The result of this study demonstrated an association between hearing loss and
metabolic syndrome. The threshold of PTA was significantly higher in the group with
metabolic syndrome in Table 2, and the rate of hearing loss was also higher in the group
with metabolic syndrome in Table 3. In addition, Table 5 shows that although statisti-
cal significance was not observed for 3 and 5 factors in the multivariable analysis, this
can be attributed to a decrease in the number of relevant groups, and the odds ratio of
hearing loss tends to increase according to the number of metabolic syndrome factors.
In particular, after adjustment for other factors, four specific components of metabolic
syndrome—waist circumference, blood pressure, and triglyceride and fasting blood glu-
cose concentrations—were strongly associated with hearing loss.
Although the mechanism(s) underlying the correlation between metabolic syndrome
and hearing impairment remain undetermined, peripheral vascular disorders may be
associated with both of these conditions.
Analysis of data from the US National Nutrition Survey showed a link between hear-
ing loss and high blood pressure [28]. Hypertension causes hemorrhage in the inner ear,
leading to reductions in capillary blood flow and oxygen supply and resulting in progres-
sive or sudden sensorineural hearing loss [29]. Hypertension also reduces blood flow by
reducing the inner diameter of blood vessels in the inner ear through atherosclerosis [30].
Diabetes is associated with microvascular and neuropathy complications affecting the
retina, kidneys, peripheral arteries, and peripheral nerves [31,32]. Pathological changes in
diabetes can cause sensorineural hearing loss through damage to the blood vessels and
nervous system of the inner ear. In autopsy studies of diabetic patients, changes in labyrinth
artery, spiral ganglion, cochlear blood vessels, and cranial nerve 8 were observed [33].
Correlations have been observed between diabetes and hearing loss, [19,21,24] and high
fasting blood glucose concentrations were found to be independently associated with
hearing loss [7,34].
Recent studies have shown that increases in triglyceride concentrations are closely
associated with reduced hearing function [35]. Vacuolar edema and degeneration of the
vascular striatum were reported as pathological changes associated with dyslipidemia in
J. Clin. Med. 2021, 10, 4866 8 of 11

guinea pigs fed a lipid-rich diet [36,37]. Decreased nitric oxide production and increased
reactive oxygen levels due to dyslipidemia can lead to hearing damage [22,37–39]. Further-
more, HDL was shown to have anti-inflammatory, antioxidant, and anti-apoptotic effects
that can attenuate the pathological changes caused by dyslipidemia [34,40] High TG/HDL
ratio has been associated with hearing loss [23] as has low HDL [3].
Obesity has been associated with hearing loss in both humans and animals. Central
obesity, increased waist circumference, and increased hearing threshold after BMI cor-
rection of the content of visceral adipose tissue were found to be related [20]. Obesity,
as determined by BMI, has been associated with an increased risk of hearing loss [26].
Moreover, abdominal lipid-related factors were reported to be associated with hearing loss
at specific frequency bands [25]. Abdominal adipose tissue has also been associated with
hearing loss in women, [8] and weight-hip ratio (WHR) may be an indicator of the risk of
hearing loss [18].
Seventeen previous studies have examined the relationship between metabolic dis-
eases and hearing loss. Nine of these studies analyzed the correlation between the overall
components of metabolic syndrome and hearing loss; four analyzed the correlations be-
tween indices such as BMI, WHR, and factors relevant to abdominal fats (FRA) and hearing
loss; three analyzed the correlations of diabetes DM and HbA1c concentration with hearing
loss, and one analyzed the correlation of TG/HDL ratio with hearing loss. Of these 17 stud-
ies, 12 were cross-sectional in design, two were prospective studies, two were retrospective
studies, and one as a case-control study. Most of the study subjects were middle-aged.
One study reported that metabolic syndrome was related to high-frequency hearing
loss in a noise-exposed population [27]. Two studies found that metabolic syndrome was
related to hearing loss in elderly women [8,9]. Three studies reported that the percentage
of subjects with hearing loss increased as the number of diagnostic factors for metabolic
syndrome increased, with the rate of hearing loss being high in patients with four or
five factors [3,10,11]. A study in drivers from West Azerbaijan showed the importance of
analyzing each component of metabolic syndrome, not metabolic syndrome itself, finding
a correlation between each component of metabolic syndrome and hearing loss [12]. One
study found that metabolic syndrome itself was not an independent risk factor for hearing
loss; rather, increased fasting plasma glucose concentration was the only independent risk
factor for hearing loss [22]. In comparison, another study found that metabolic syndrome
itself and several of its specific components, such as central obesity, hyperglycemia, and
low HDL, were positively associated with hearing loss [7].
Many studies have found a correlation between HDL and hearing loss. For example,
one study in US adults found that low HDL correlated with hearing loss, with low HDL
being most responsible for the correlation between hearing loss and metabolic syndrome. [3]
Two studies in the Korean population reported that low HDL and high TG were highly
correlated with hearing loss, [11,23] and one study in a Chinese population reported
that hearing loss was positively correlated with low HDL, hyperglycemia, and central
obesity [7].
Several studies also observed relationships between obesity-related anthropometric
indices and hearing loss. For example, one study found that WHR may be a surrogate
marker for predicting the risk of hearing loss, [18] whereas another study suggested that
FRAs were associated with hearing loss at specific frequencies, as determined by sex and
the presence of diabetes, and that visceral adipose tissue (VAT) is particularly important
role for hearing [25]. Two studies found relationships between BMI and hearing loss,
with one finding that underweight and severe obesity were associated with an increased
prevalence of hearing loss in a Korean population, and the other reporting that overweight
was associated with an increased risk of hearing loss in a Japanese population [26].
This study had several limitations. First, because it was a cross-sectional study, the
causative relationships between hearing loss and components of metabolic syndrome
could not be determined. Second, because these data were from health examinations, the
correlation between the prevalence of metabolic syndrome and hearing loss could not be
J. Clin. Med. 2021, 10, 4866 9 of 11

accurately determined. Third, this study did not evaluate subjects who received medical
treatment for hearing loss, nor did it evaluate factors contributing to hearing loss, such as
noise, ototoxic drugs, otitis media, and family history of hearing loss.

5. Conclusions
This cross-sectional study involving a large population analyzed the association
between metabolic syndrome and hearing loss. Hearing loss showed a positive correlation
with the number of diagnostic factors for metabolic syndrome, especially in subjects with
four specific factors: high waist circumference, blood pressure, triglyceride concentration,
and fasting blood glucose concentration.
This study highlights the importance of control of metabolic syndrome in management
of hearing loss. Subjects with metabolic diseases should therefore undergo regular hearing
tests and, if necessary, hearing rehabilitation along with the management and treatment of
their metabolic diseases. In addition, patients and health professionals may not be aware
of this information regarding association between metabolic syndrome and hearing loss;
hence, this can be included in part of health education.

Author Contributions: Original draft preparation, writing—review and editing, and data curation,
H.-S.R.; conceptualization, investigation, and data curation, M.-G.K. and D.-C.P.; methodology
and resources, S.-H.K. and D.-W.K.; methodology and data curation, D.-C.P., S.-S.K., and D.-W.K.;
conceptualization, writing—review and editing, and supervision, S.-G.Y. All authors have read and
agreed to the published version of the manuscript.
Funding: This work was supported by the National Research Foundation of Korea (NRF) grant
funded by the Korean government (NRF-2019R1A2C1086807).
Institutional Review Board Statement: The study was conducted according to the guidelines of the
Declaration of Helsinki, and approved by the Institutional Review Board of Kyung Hee University
Hospital (IRB No. 2019-07-065).
Informed Consent Statement: Patient consent was waived due to Cross-sectional design of
this study.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author. The data are not publicly available because the health examination data of a
private hospital was used.
Conflicts of Interest: The authors declare no conflict of interest.

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