Biometals
https://doi.org/10.1007/s10534-022-00414-4
Serum copper‑to‑zinc ratio and risk of incident pneumonia
in caucasian men: a prospective cohort study
Setor K. Kunutsor · Ari Voutilainen ·
Jari A. Laukkanen
Received: 16 March 2022 / Accepted: 11 June 2022
© The Author(s) 2022
Abstract Serum copper (Cu) and zinc (Zn), essential micronutrients that have important immunomodulatory and antimicrobial properties, are biomarkers
of ageing. Serum Cu/Zn-ratio may be a more reliable marker for age-related degenerative conditions
compared with serum Cu or Zn alone. We aimed to
assess the association between Cu/Zn-ratio and the
risk of incident pneumonia in a prospective cohort
study. Serum levels of Cu and Zn were measured at
baseline using atomic absorption spectrometry in
2503 men aged 42–61 years in the Kuopio Ischemic
Heart Disease prospective cohort study. Hazard ratios
(HRs) with confidence intervals (CIs) were calculated
Supplementary Information The online version
contains supplementary material available at https://doi.
org/10.1007/s10534-022-00414-4.
S. K. Kunutsor
National Institute for Health Research Bristol Biomedical
Research Centre, University Hospitals Bristol and Weston
NHS Foundation Trust and the University of Bristol,
Bristol, UK
S. K. Kunutsor (*)
Musculoskeletal Research Unit, Translational Health
Sciences, Bristol Medical School, University of Bristol,
Learning & Research Building (Level 1), Southmead
Hospital, Bristol BS10 5NB, UK
e-mail: skk31@cantab.net
for incident pneumonia using Cox regression models. A total of 599 cases of pneumonia occurred during a median follow-up of 26.1 years. Serum Cu/
Zn-ratio and Cu were each linearly associated with
incident pneumonia. A unit increase in Cu/Zn-ratio
was associated with an increased risk of pneumonia
in analysis adjusted for potential confounders including C-reactive protein (HR 1.65; 95% CI 1.17–2.33).
The corresponding adjusted HR (95% CI) was 2.04
(1.22–3.40) for serum Cu. The association between
serum Zn and pneumonia was curvilinear. Compared
to the bottom tertile of Zn, the multivariable adjusted
HRs (95% CIs) for incident pneumonia were 0.68
(0.55–0.83) and 0.96 (0.79–1.16) for the middle and
top tertiles of Zn, respectively. Further analysis in the
same participants showed that Cu/Zn-ratio might be
a stronger risk indicator for pneumonia than serum
S. K. Kunutsor
Diabetes Research Centre, University of Leicester,
Leicester General Hospital, Gwendolen Road,
Leicester LE5 4WP, UK
A. Voutilainen · J. A. Laukkanen
Institute of Public Health and Clinical Nutrition,
University of Eastern Finland, Kuopio, Finland
J. A. Laukkanen
Institute of Clinical Medicine, Department of Medicine,
University of Eastern Finland, Kuopio, Finland
S. K. Kunutsor · J. A. Laukkanen
Department of Medicine, Central Finland Health Care
District Hospital District, Jyvaskyla, Finland
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C-reactive protein. In middle-aged and older Finnish
men, increased serum Cu/Zn-ratio and Cu concentrations are each linearly associated with an increased
risk of incident pneumonia.
Keywords Serum copper-to-zinc ratio · Serum
copper · Serum zinc · Pneumonia · Risk factor ·
Cohort study
Abbreviations
BMI
Body mass index
CAP
Community acquired pneumonia
CHD
Coronary heart disease
CI
Confidence interval
COPD Chronic obstructive pulmonary disease
Cu
Copper
HDL-C High-density lipoprotein cholesterol
HR
Hazard ratio
hsCRP High-sensitivity C-reactive protein
IQR
Interquartile range
KIHD
Kuopio Ischemic Heart Disease
SD
Standard deviation
SES
Socioeconomic status
Zn
Zinc
Introduction
Pneumonia, an inflammatory condition of the lung
tissue commonly caused by bacteria or viruses, can
be acquired in the community (community acquired
pneumonia, CAP) or in the hospital environment
(hospital acquired pneumonia). (Cilloniz et al. 2016b)
Community-acquired pneumonia is a leading cause of
hospitalization, morbidity, mortality, and associated
with significant health care costs. (Nair and Niederman 2011)The Global Burden of Disease Study 2019
reported that lower respiratory infections ranked as
the fourth leading cause of disability-adjusted lifeyears. (GBD Collaborators 2020) Despite the development of newer molecular tests for microbial identification of pathogens, pulmonary imaging facilities
and antimicrobial therapies for the management of
pneumonia over the last decade, the incidence of
pneumonia persistently remains high. (Cillóniz et al.
2018) Major contributors to the growing incidence
of pneumonia include increased life expectancy,
smoking, excessive alcohol consumption, respiratory
conditions such as asthma and chronic obstructive
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13
pulmonary disease (COPD), other chronic conditions
such as diabetes, kidney and liver disease, and immunosuppression. Cillóniz and others 2018; Nair and
Niederman 2011).
Though pneumonia constitutes a substantial global
public health burden, it is a preventable cause of
death and disability. Older age is a major risk factor for pneumonia, (Torres et al. 2013) which is a
leading cause for hospitalization and subsequent
mortality among this population group. (Jackson
et al. 2004) This is due to the physiological changes
associated with aging such as age-related weakening of the immune system (immunosenescence)
(Haase and Rink 2009) as well as the high prevalence of chronic disease in older people. (Cilloniz
et al. 2016a) With increasing life expectancy, there is
increasing research focussed on identifying biomarkers of ageing,(Engelfriet et al. 2013) which could
be clinically relevant for preventing aging-related
diseases such as pneumonia. Copper (Cu) and zinc
(Zn), essential micronutrients involved in several
cellular processes such as nucleic acid synthesis,
enzymatic reactions, oxidoreductases, inflammation,
mitochondrial electron transport, cell replication and
repair,(Chimienti 2013; Festa and Thiele 2011) have
been identified as biomarkers related to aging as they
appear to be mostly related to inflammatory parameters than the nutritional ones. (Malavolta et al.2010)
They have important immunomodulatory and antimicrobial properties(Malavolta et al. 2015) and are relevant for the development, regulation and maintenance
of the immune and antioxidative defence system.
(Stafford et al. 2013) Copper is involved in various
biological processes, and its insufficiency, deficiency,
or toxic levels can lead to many disease states. (DiNicolantonio et al. 2018) Zinc deficiency contributes to
frailty, disability and an increased incidence of agerelated degenerative diseases such as cancer, infections and atherosclerosis. (Mocchegiani 2007).
Among serum micronutrients, concentrations of
Cu and Zn are strictly regulated by compensatory
mechanisms that act to stabilize them within certain
ranges of nutritional intake.(Malavolta et al. 2015)
Serum concentrations of Cu and Zn are only slightly
affected by nutritional changes unless during severe
deficiency or supplementation use.(Malavolta et al.
2015) However, in the presence of pathological
changes such as inflammatory conditions, there is a
decrease in serum Zn concentrations and an increase
Biometals
in serum Cu concentrations, and thus they are biologically interrelated.(Sullivan et al. 1979) The typical
presentation of several age-related chronic diseases
is an increase in the Cu-to-Zn ratio (Cu/Zn-ratio).
(Malavolta et al. 2015) It has been suggested that the
serum Cu/Zn-ratio may be a more reliable marker of
pathological outcomes, compared to the use of Cu
or Zn alone.(Malavolta et al. 2015) High serum Cu/
Zn-ratio has been shown to be associated with an
increased risk of cardiovascular mortality, (Leone
et al. 2006) cancer, (Leone et al. 2006) all-cause
mortality(Malavolta et al. 2010) as well as infectious disease. (Laine et al. 2020) Though the previous study by Laine and colleagues evaluated infection
outcomes, the specific outcome of pneumonia was not
assessed. (Laine et al. 2020) To our knowledge, the
prospective association between serum Cu/Zn-ratio
and the risk of the specific outcome of pneumonia has
not been previously explored. Our principal aim was
to assess the nature and magnitude of the prospective association of serum Cu/Zn-ratio with pneumonia risk, using a population-based prospective cohort
of 2503 middle-aged and older Finnish men. A secondary aim was to assess the individual associations
of serum Cu and Zn with incident pneumonia risk.
Furthermore, given that C-reactive protein (CRP) is
a major inflammatory marker, we also evaluated the
association of serum high sensitivity CRP (hsCRP)
with pneumonia risk in the same set of participants to
make comparisons.
Methods
This study was conducted in accordance with
STROBE (STrengthening the Reporting of OBservational studies in Epidemiology) guidelines for reporting observational studies in epidemiology (Supplementary File 1).
Study design and participants
The Research Ethics Committee of the University of
Kuopio approved the study protocol and each study
participant provided written informed consent. All
study procedures adhered to the Declaration of Helsinki. Participants included in this study were part
of the Kuopio Ischaemic Heart Disease Risk Factor Study (KIHD), a population-based prospective
cohort study that was set up to investigate risk factors
for atherosclerotic cardiovascular disease and other
related diseases. The study design and recruitment
methods have been described in detail in previous
reports. (Kunutsor et al. 2016b, c; Laukkanen et al.
2018) Briefly, participants included in the KIHD
comprised a representative sample of men aged 42,
48, 54 or 60 years living in the city of Kuopio and
its surrounding rural communities in eastern Finland.
During recruitment, a total of 3433 men were potentially eligible and of these, 3235 were found to be eligible for inclusion into study. Of this number, 2682
volunteered to participate and 553 did not respond
to the invitation or declined to give informed consent. Baseline examinations were performed between
March 1984 and December 1989. From this analysis,
we excluded those with missing data on the exposures
and potential confounders (n = 179). The current analysis included 2503 men with complete information on
serum measurements of Cu and Zn, relevant covariates, and incident pneumonia events.
Measurement of covariates and outcome
ascertainment
Blood sample collection and measurement of blood
biomarkers, physical measurements, and assessment of lifestyle characteristics, medical history
and dietary intakes have been described in detail in
previous reports. (Abdollahi et al. 2019; Kunutsor
and others 2016a; Kunutsor and Laukkanen 2016;
Salonen et al. 1992) Participants fasted overnight and
abstained from drinking alcohol for at least 3 days
and from smoking for at least 12 h before blood
samples were taken between 8 and 10 a.m. Serum
hsCRP measurements were made with an immunometric assay (Immulite High Sensitivity C-Reactive
Protein Assay; DPC, Los Angeles, CA, USA). Measurements of serum Cu and Zn concentrations were
made from frozen serum samples stored at −20° C
for 1–5 years, using the PerkinElmer 306 atomic
absorption spectrophotometer (Norwalk, Connecticut, USA). Self-administered questionnaires were
used to assess medical history and lifestyle characteristics such as smoking and alcohol consumption.
(Salonen et al. 1992) Socioeconomic status (SES)
was assessed using self-reported questionnaires via
a summary index that combined income, education,
occupational prestige, material standard of living and
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Biometals
housing conditions. The composite SES index ranged
from 0 to 25, with higher values indicating lower
SES. (Jae et al. 2020) The consumption of foods was
assessed with the use of a 4-day guided food record,
during three weekdays and one weekend day using
household measures. Instructions were provided and
completed food records were checked by a nutritionist together with the participant, to ensure accuracy.
Leisure-time physical activity was assessed from a
12 month physical activity history modified from the
Minnesota Leisure-Time Physical Activity Questionnaire. (Taylor et al. 1978).
Incident cases of pneumonia that occurred from
study entry to 2018 were included in this analysis.
The diagnoses of pneumonia cases were made by
qualified physicians based on the International Classification of Diseases (ICD) codes used in clinical practice (ICD-8 codes 485; ICD-9 codes 480–483, and
485; ICD-10 codes J15, and J18) and were collected
by linkage to the National Hospital Discharge Register (THL/93/5.05.00/2013). (Kunutsor et al. 2016a;
Kunutsor et al. 2016b).
Statistical analysis
Variables with skewed distributions (e.g., alcohol
consumption, physical activity, and hsCRP) were
natural log transformed to achieve approximately
symmetrical distributions. Baseline characteristics
were presented as means ± standard deviation (SD) or
median (interquartile range, IQR) for continuous variables and n (percentages) for categorical variables.
In linear regression models adjusted for age, Pearson
correlation coefficients were calculated to assess the
cross-sectional associations of serum Cu/Zn-ratio
with various continuous risk markers; for categorical variables, the percentage differences in mean values of serum Cu/Zn-ratio for a category versus its
reference were calculated. Hazard ratios (HRs) with
95% confidence intervals (CIs) for incident pneumonia were estimated using Cox proportional hazard
models and these were adjusted for in three models:
(Model 1) age; (Model 2) Model 1 plus body mass
index (BMI), smoking status, history of type 2 diabetes, prevalent coronary heart disease (CHD), history
of asthma, history of chronic bronchitis, history of
tuberculosis, alcohol consumption, SES, leisure-time
physical activity, total energy intake, intake of fruits,
berries and vegetables, and intake of processed and
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unprocessed red meat; and (Model 3) Model 2 plus
hsCRP (a potential mediator of the association). The
selected confounders were based on their previously
established roles as risk factors for pneumonia, evidence from previous research, or their potential as
confounders based on known associations with incident pneumonia and observed associations with the
exposures using the available data. (Groenwold et al.
2011) To explore potential nonlinear dose–response
relationships between the exposures and incident pneumonia risk, we constructed multivariable
restricted cubic splines (RCSs) with knots at the 5th,
35th, 65th, and 95th percentiles of the distribution
of the exposures as recommended by Harrell. (Harrell 2001) Serum Cu/Zn-ratio and Cu were modeled
as both continuous (per unit increase) and categorical (tertiles) variables given evidence of linear relationships with pneumonia risk using multivariable
RCS curves; serum Zn was modeled as tertiles given
evidence of a nonlinear relationship. We constructed
Kaplan–Meier curves for tertiles of serum Cu/Znratio and compared them using the log rank test. We
used formal tests of interaction tests to assess statistical evidence of effect modification by clinically
relevant characteristics. To minimize any bias due
to reverse causation, sensitivity analysis involved
excluding the first two years of follow-up. All statistical analyses were conducted using Stata version MP
17 (Stata Corp, College Station, Texas).
Results
Baseline characteristics
The overall mean (SD) age of study participants at
recruitment was 53 (5) years. The means (SDs) of
serum Cu/Zn-ratio, Cu and Zn were 1.21 (0.27),
1.11 (0.18) and 0.94 (0.12), respectively. Significant weak and positive correlations were observed
between serum Cu/Zn-ratio and age, alcohol consumption, and SES; whereas, the correlation was
stronger for hsCRP (r = 0.42). Significant weak and
inverse correlations were observed with physical
activity and intake of fruits, berries and vegetables.
Values of serum Cu/Zn-ratio were significantly
higher in men who smoked compared with men
who did not smoke (Table 1).
Biometals
Table 1 Baseline characteristics of study participants and cross-sectional correlates of copper-to-zinc ratio
Characteristics
Mean ± SD or median (IQR) Pearson correlation
r (95% CI)a
Percentage difference (95% CI) in
values of percentage of Cu/Zn-ratio
per 1 SD higher or compared to
reference category of correlateb
Serum copper-to-zinc ratio
Serum copper, mg/l
Serum zinc, mg/l
Questionnaire/prevalent conditions
Age (years)
Alcohol consumption, g/week
History of type 2 diabetes, n (%)
No
Yes
Current smoking, n (%)
No
Yes
History of CHD, n (%)
No
Yes
History of asthma, n (%)
No
Yes
History of chronic bronchitis, n (%)
No
Yes
History of tuberculosis, n (%)
No
Yes
Physical measurements
BMI, kg/m2
SBP, mmHg
DBP, mmHg
Physical activity, KJ/day
Socio-economic status
Blood-based markers
Total cholesterol, mmol/l
HDL-C, mmol/l
Fasting plasma glucose, mmol/l
High sensitivity C-reactive protein, mg/l
Dietary intakes
Total energy intake, kJ/day
Processed and unprocessed red
meat, g/day
Fruits, berries and vegetables, g/
day
1.21 ± 0.27
1.11 ± 0.18
0.94 ± 0.12
–
–
–
–
–
–
53 ± 5
31.8 (6.2–91.0)
0.11 (0.07, 0.15)***
0.16 (0.12, 0.20)***
0.03% (0.02, 0.04)***
0.04% (0.03, 0.06)***
2404 (96.0)
99 (4.0)
–
–
Ref
0.01% (−0.05, 0.06)
1712 (68.4)
791 (31.6)
Ref
0.11% (0.09, 0.14)***
1886 (75.4)
617 (24.6)
–
–
–
–
–
2412 (96.4)
91 (3.6)
–
–
Ref
0.02% (−0.04, 0.08)
2314 (92.4)
189 (7.6)
–
–
Ref
0.01% (−0.03, 0.05)
2406 (96.1)
97 (3.9)
–
–
Ref
−0.01% (−0.07, 0.04)
26.9 ± 3.6
134 ± 17
89 ± 11
1204 (630–1999)
8.48 ± 4.23
−0.03 (−0.06, 0.01)
0.02 (−0.02, 0.06)
−0.01 (−0.05, 0.03)
−0.04 (−0.08, −0.00)*
0.13 (0.09, 0.17)***
−0.01% (−0.02, 0.00)
0.01% (−0.00, 0.02)
−0.00 (−0.01, 0.01)
−0.01% (−0.02, −0.00)*
0.04% (0.02, 0.05)***
5.90 ± 1.08
1.29 ± 0.30
5.35 ± 1.28
1.29 (0.71–2.48)
0.03 (−0.01, 0.06)
0.01 (−0.03, 0.05)
0.03 (−0.00, 0.07)
0.42 (0.38, 0.45)***
0.01% (−0.00, 0.02)
0.00% (−0.01, 0.01)
0.01 (−0.00, 0.02)
0.11% (0.10, 0.12)***
9855 ± 2595
145 ± 77
0.00 (−0.04, 0.04)
0.04 (−0.00, 0.08)
0.00% (−0.01, 0.01)
0.01% (−0.00, 0.02)
251 ± 156
−0.12 (−0.16, −0.08)*** −0.03 (−0.04, −0.02)***
Ref
0.03% (0.00, 0.05)*
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Biometals
Table 1 (continued)
BMI, body mass index; CHD, coronary heart disease; DBP, diastolic blood pressure; HDL-C, high-density lipoprotein cholesterol;
SD, standard deviation; SBP, systolic blood pressure
a
Pearson correlation coefficients between serum Cu/Zn-ratio and the row variables
b
Percentage change in values of serum Cu/Zn-ratio per 1-SD increase in the row variable (or for categorical variables, the percentage
difference in mean values of serum Cu/Zn-ratio for the category versus the reference); asterisks indicate the level of statistical significance: *, p < 0.05; **, p < 0.01; ***, p < 0.001
(b)
H a z a r d r ati o of P n e u m o ni a
H a z a r d r ati o of P n e u m o ni a
(a)
25.2
22.8
20.4
18.0
15.6
13.2
10.8
8.4
6.0
3.6
1.2
.8
1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8
Cu/Zn ratio
3 3.2
32.6
30.2
27.8
25.4
23.0
20.6
18.2
15.8
13.4
11.0
8.6
6.2
3.8
1.4
.5
.7
.9
1.1
1.3 1.5 1.7
Copper, mg/l
1.9
2.1
2.3
H a z a r d r ati o of P n e u m o ni a
(c)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
.5
.7
.9
1.1
Zinc, mg/l
1.3
1.5
Fig. 1 Restricted cubic splines of the hazard ratios of incident pneumonia with serum Cu/Zn-ratio, Cu and Zn A Serum
Cu/Zn-ratio and pneumonia; B Serum Cu and pneumonia; C
Serum Zn and pneumonia Dashed lines represent the 95% confidence intervals for the spline model (solid line). Models were
adjusted for age, body mass index, smoking status, history of
type 2 diabetes, prevalent coronary heart disease, history of
asthma, history of chronic bronchitis, history of tuberculosis,
alcohol consumption, socioeconomic status, leisure-time physical activity, total energy intake, intake of fruits, berries and
vegetables, and intake of processed and unprocessed red meat
Cu, copper; Zn, zinc
Association of serum Cu/Zn-ratio with pneumonia
multivariable RCS curve showed that the risk of
pneumonia increased linearly with increasing serum
Cu/Zn-ratio across the range 1.50–3.10 (p-value for
nonlinearity = 0.16) (Fig. 1A). The HR (95% CI) for
incident pneumonia per unit increase in serum Cu/
Zn-ratio was 2.07 (1.51–2.84) in analysis adjusted
A total of 599 incident cases of pneumonia were
recorded (annual rate 10.33/1000 person-years
at risk; 95% CI 9.53–11.19) during a median
(IQR) follow-up of 26.1 (16.7–30.8) years. A
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Table 2 Associations of serum copper, zinc and copper-to-zinc ratio with risk of pneumonia
Exposure
Events/total
Serum copper-to-zinc ratio
Per unit increase
599/2503
T1 (0.48–1.07)
180/835
T2 (1.08–1.27)
198/837
T3 (1.28–3.12)
221/831
Serum copper, mg/l
Per unit increase
599/2,503
T1 (0.46–1.02)
185/875
T2 (1.03–1.17)
200/826
T3 (1.18–2.32)
214/802
Serum zinc, mg/l
T1 (0.50–0.89)
248/911
T2 (0.90–0.98)
159/802
T3 (0.99–1.62)
192/790
Model 1
HR (95% CI)
P-value
Model 2
HR (95% CI)
P-value
Model 3
HR (95% CI)
P-value
2.59 (1.92–3.49)
Ref
1.13 (0.92–1.38)
1.52 (1.24–1.85)
< 0.001
2.07 (1.51–2.84)
Ref
1.07 (0.87–1.31)
1.32 (1.08–1.62)
< .001
1.65 (1.17–2.33)
Ref
1.01 (0.82–1.24)
1.15 (0.92–1.42)
.004
3.77 (2.44–5.82)
Ref
1.23 (1.01–1.50)
1.66 (1.36–2.02)
Ref
0.62 (0.51–0.76)
0.87 (0.72–1.05)
.25
< 0.001
< .001
.04
< .001
2.89 (1.83–4.56)
Ref
1.19 (0.97–1.45)
1.44 (1.18–1.77)
< .001
.15
Ref
0.67 (0.55–0.82)
0.94 (0.77–1.14)
.53
.007
< .001
.92
.22
.09
< .001
2.04 (1.22–3.40)
Ref
1.13 (0.92–1.38)
1.25 (1.00–1.55)
.006
.26
.05
< .001
.51
Ref
0.68 (0.55–0.83)
0.96 (0.79–1.16)
< .001
.66
CI, confidence interval; HR, hazard ratio; ref, reference; T, tertile
Model 1: Adjusted for age
Model 2: Model 1 plus body mass index, smoking status, history of type 2 diabetes, prevalent coronary heart disease, history of
asthma, history of chronic bronchitis, history of tuberculosis, alcohol consumption, socioeconomic status, leisure-time physical activity, total energy intake, intake of fruits, berries and vegetables, and intake of processed and unprocessed red meat
Model 3: Model 2 plus high-sensitivity C-reactive protein
p-value<.001
Cumulative Hazard Pneumonia
Fig. 2 Cumulative Kaplan–
Meier curves for pneumonia
during follow-up according
to tertiles of serum Cu/
Zn-ratio
0.80
0.60
0.40
0.20
0.00
0
3
6
9
12 15 18 21 24
Follow-up time (years)
27
30
33
Number at risk
T1 835 823 798 768 736 711 661 599 534 461 293 88
T2 837 816 789 760 711 671 631 574 500 416 305 60
T3 831 784 744 692 635 580 517 464 393 311 210 40
First tertile
Third tertile
for age, BMI, smoking status, history of type 2 diabetes, prevalent CHD, history of asthma, chronic
bronchitis or tuberculosis, alcohol consumption,
Second tertile
SES, leisure-time physical activity, total energy
intake, intake of fruits, berries and vegetables,
and intake of processed and unprocessed red meat,
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Subgroup
No. of
participants
No. of
cases
HR (95% CI)
p-value*
276
323
1.82 (1.16, 2.85)
2.54 (1.66, 3.91)
.28
1,252
1,251
295
304
2.13 (1.40, 3.24)
1.99 (1.26, 3.15)
.83
Alcohol consumption (g/week)
<31.8
≥31.8
1,252
1,251
309
290
2.50 (1.63, 3.83)
1.59 (1.01, 2.50)
.15
Socioeconomic status
<10
≥10
1,397
1,106
292
307
1.56 (0.99, 2.45)
2.90 (1.90, 4.41)
.05
Physical activity (kj/day)
<1204
≥1204
1,252
1,251
319
280
1.77 (1.16, 2.71)
2.46 (1.57, 3.85)
.29
History of type 2 diabetes
No
Yes
2,404
99
569
30
2.01 (1.46, 2.78)
3.85 (0.87, 17.00)
.40
Smoking status
Non-smokers
Current smokers
1,712
791
395
204
2.20 (1.47, 3.30)
1.89 (1.16, 3.09)
.64
History of CHD
No
Yes
1,886
617
414
185
1.77 (1.21, 2.58)
3.00 (1.72, 5.23)
.12
History of asthma
No
Yes
2,412
91
563
36
2.06 (1.49, 2.85)
2.22 (0.68, 7.26)
.91
History of chronic bronchitis
No
Yes
2,314
189
535
64
2.11 (1.51, 2.94)
1.81 (0.74, 4.43)
.76
History of tuberculosis
No
Yes
2,406
97
568
31
2.03 (1.47, 2.80)
3.12 (0.71, 13.66)
.58
Age at survey (years)
<55
≥55
1,332
1,171
Body mass index (kg/m2)
<26.0
≥26.0
.25
.5
1
2.5
5 7.5
15 25
HR (95% CI) per unit higher Cu/Zn-ratio
Fig. 3 Association between serum Cu/Zn-ratio and pneumonia risk across several clinically relevant subgroups CHD coronary heart
disease; CI confidence interval; Cu copper; HR hazard ratio; Zn zinc
which was attenuated to 1.65 (1.17–2.33) after
further adjustment for hsCRP (Table 2). The corresponding adjusted HRs (95% CIs) were 1.32
(1.08–1.62) and 1.15 (0.92–1.42) comparing the top
versus bottom tertiles of serum Cu/Zn-ratio. Cumulative hazard curves showed an increased risk of
pneumonia among men in the top tertile of serum
Cu/Zn-ratio compared with the other Cu/Zn-ratio
groups (p-value for log-rank test < 0.001; Fig. 2).
Association of serum hsCRP with pneumonia
Direct comparisons were made to the association of
serum hsCRP with pneumonia risk in the same set
of participants. Serum hsCRP was independently
Vol:. (1234567890)
13
associated with pneumonia risk (Supplementary File
2).
Association of serum Cu/Zn-ratio with pneumonia in
subgroups
The association between serum Cu/Zn-ratio and
pneumonia risk remained consistent across several clinically relevant subgroups except for marginal evidence of interaction by SES (p for interaction = 0.05); the association between serum Cu/
Zn-ratio and pneumonia risk was strong and positive in men with low SES but was modest in men
with high SES (Fig. 3).
Biometals
Associations of serum Cu and Zn with pneumonia
A multivariable RCS curve showed that the risk of
pneumonia increased linearly with increasing serum
Cu across the range 1.60–2.30 (p-value for nonlinearity = 0.36) (Fig. 1B). The HR (95% CI) for incident pneumonia per unit increase in serum Cu was
2.89 (1.83–4.56) in analysis adjusted for age, BMI,
smoking status, history of type 2 diabetes, prevalent CHD, history of asthma, chronic bronchitis or
tuberculosis, alcohol consumption, SES, leisure-time
physical activity, total energy intake, intake of fruits,
berries and vegetables, and intake of processed and
unprocessed red meat, which was attenuated to 2.04
(1.22–3.40) following further adjustment for hsCRP
(Table 2). The corresponding adjusted HRs (95% CIs)
were 1.44 (1.18–1.77) and 1.25 (1.00–1.55) comparing the top versus bottom tertiles of serum Cu.
A multivariable RCS curve showed a curvilinear relationship between serum Zn and pneumonia
risk (p-value for nonlinearity = 0.009) (Fig. 1C).
Compared to the bottom tertile of Zn, the HRs (95%
CIs) for incident pneumonia were 0.67 (0.55–0.82)
and 0.94 (0.77–1.14) for the middle and top tertiles
of Zn, respectively, in analysis that adjusted for age,
BMI, smoking status, history of type 2 diabetes,
prevalent CHD, history of asthma, chronic bronchitis
or tuberculosis, alcohol consumption, SES, leisuretime physical activity, total energy intake, intake of
fruits, berries and vegetables, and intake of processed
and unprocessed red meat (Table 2). The respective HRs (95% CIs) were 0.68 (0.55–0.83) and 0.96
(0.79–1.16) in further analysis adjusted for hsCRP.
The associations of serum Cu/Zn-ratio, Cu and Zn
with risk of pneumonia remained similar in analyses
that excluded the first two years of follow-up (Supplementary File 3).
Discussion
Key findings
In this prospective evaluation of the relationship
between serum Cu/Zn-ratio and risk of incident
pneumonia using a cohort of middle-aged and older
Finnish men, elevated serum Cu/Zn-ratio was associated with an increased risk of incident pneumonia in
a linear dose–response manner. The association did
not differ across several clinically relevant subgroups,
except for evidence of effect modification by SES;
the association appeared to be stronger in men with
low SES. In separate evaluations of serum Cu and
Zn, Cu was positively associated with pneumonia in a
linear dose–response manner, whereas serum Zn was
inversely associated with pneumonia risk in a curvilinear manner. When serum Cu/Zn-ratio and Cu were
modelled as categorical variables, the associations
were attenuated on further adjustment for hsCRP,
reflecting the fact that inflammatory pathways are
involved in the development of pneumonia and hence
confirms the fact that inflammation is a potential
mediator of the observed association. Furthermore,
correlation analysis demonstrated a strong positive
correlation between serum Cu/Zn-ratio and hsCRP. If
serum hsCRP is a potential mediator, then adjusting
for it constitutes an overadjustment. Findings therefore suggest independent associations of serum Cu/
Zn-ratio, Cu and Zn with pneumonia risk. Furthermore, the associations remained persistent when the
first two years of follow-up were excluded. In further
analysis that assessed the association of serum hsCRP
with pneumonia risk in the same set of participants,
a relatively weaker association was demonstrated in
the analysis that modeled serum hsCRP as a continuous variable; which implies that serum Cu/Zn-ratio
may be a stronger risk indicator than serum hsCRP
for pneumonia risk.
Comparison with previous studies
To our knowledge, this is the first study to evaluate
the prospective association between serum Cu/Znratio and pneumonia risk, hence, we are unable to
compare the current findings in the context of previous work. However, several epidemiological observational studies have demonstrated associations
between serum Cu/Zn-ratio and several age-related
degenerative conditions such as cardiovascular mortality (Leone et al. 2006; Reunanen and others 1996),
HIV-1 mortality (Lai et al. 2001), cancer (Leone et al.
2006), knee chondrocalcinosis (He et al. 2020), and
all-cause mortality (Malavolta et al. 2010). In a recent
prospective evaluation, Laine and colleagues demonstrated an increased serum Cu/Zn-ratio and Cu concentration to be each associated with an increased
risk of incident infections; there was no evidence of
an association of Zn with incident infection, except
Vol.: (0123456789)
13
Biometals
when the analysis was limited to the first 10 years of
follow-up (Laine et al. 2020). However, the outcome
used in this evaluation comprised a comprehensive
list of infectious conditions including intestinal infectious diseases, other bacterial diseases, viral diseases,
diseases of the ear, other forms of heart disease, acute
respiratory infections, influenza, pneumonia, diseases
of the urinary system and male genital organs, and
infections of skin and subcutaneous tissue. Though
the commonest infection was pneumonia,(Laine
et al. 2020) it is uncertain which specific outcome/
outcomes could be driving the observed association, as estimates for cause-specific infections were
not reported. In a number of case–control studies
that were based on patients with bacterial, viral and
parasitic infections, serum Cu/Zn-ratio was demonstrated to be a potential prognostic marker (Asemota
et al. 2018; Kassu et al. 2006; Van Weyenbergh et al.
2004). Given that this is the first prospective study to
evaluate the association between serum Cu/Zn-ratio
and pneumonia, other large-scale prospective studies
are still needed to confirm the current findings.
Explanations for findings
Several mechanistic pathways may underline the
observed associations of serum Cu/Zn-ratio and Cu
and Zn concentrations with the risk of incident pneumonia. In addition to their roles in almost every cellular process in the human body,(Chimienti 2013; Festa
and Thiele 2011) Cu and Zn play important roles in
the optimal functioning of the immune system. (Stafford et al. 2013) Though Cu plays a beneficial role in
numerous biological processes, it can exhibit toxic
effects in high amounts. High levels of Cu could
increase the risk of infections such as pneumonia via
increased inflammation, given its close relationship
with ceruloplasmin, which is elevated during an acute
phase response,(Uriu-Adams and Keen 2005) in addition to the ability of Cu to serve as a nutrient for infectious microbes. (Besold et al. 2016) For almost six
decades, Zn has been known as an important factor
for the immune system;(Prasad et al. 1963) its role in
immune function has been consistently demonstrated
in several cellular studies. (Haase and Rink 2014) The
immune defence system relies on two major groups
of cells (innate and adaptive immune cells), which
also depend on Zn availability at multiple levels.
(Wellinghausen et al. 1997) The major roles played
Vol:. (1234567890)
13
by Zn in immunity include (i) signal transduction of
immune cells; (ii) its impact on immune cell function such as suppression of several T cell-mediated
immune reactions and formation of neutrophil extracellular traps; and (iii) “nutritional immunity”, a host
response designed to starve pathogens of essential
metals. (Haase and Rink 2014) Consequently, Zn
deficiency leads to impaired immune function and
an increased risk of infections. With advancing age,
there is a decrease in serum Zn concentrations due to
insufficient dietary Zn consumption, reduced intestinal absorption or increased losses (due to diarrhoea
or use of diuretics) (Mocchegiani et al. 2013) and/or
an increase in serum Cu concentrations (Baudry et al.
2020) due to the presence of inflammatory conditions
commonly seen in old age. (Sullivan et al. 1979) This
consequently leads to an increase in the serum Cu/
Zn-ratio. Given that an increased serum Cu/Zn-ratio
is commonly seen in older people, there is also a possibility that our findings of an increased risk of pneumonia with an increased serum Cu/Zn-ratio could
be due to reverse causation. However, this may be
unlikely given that the findings were essentially similar on excluding the first two years of follow-up.
Implications of findings
The overall evidence suggests that serum Cu/Zn-ratio,
Cu and Zn could be risk markers for incident pneumonia. Whether there is a causal relevance to these
presented relationships would need to be proved using
appropriate study designs such as randomised controlled trials and Mendelian randomisation studies.
Nevertheless, the findings are clinically relevant. It has
previously been suggested that the serum Cu/Zn-ratio
may be a valuable predictive marker for pathological
outcomes, and might be comparable or even superior
to other well established inflammatory markers such
as CRP and erythrocyte sedimentation rate. (Malavolta et al. 2015) Indeed, our analysis showed that
serum Cu/Zn-ratio might be a potentially stronger risk
indicator for pneumonia risk than hsCRP. An increment of the serum Cu/Zn-ratio above 2.0 in older people has been reported to commonly reflect an inflammatory response or decreased nutritional Zn status.
(Malavolta et al. 2010) Measurement of the serum
Cu/Zn-ratio as well as serum Cu and Zn concentrations could be used to identify individuals at high risk
of serious infections such as pneumonia. However,
Biometals
formal risk prediction analyses are needed to assess
the value of these potential risk predictors. Since Zn
deficiency in old age is commonly due to insufficient
dietary Zn consumption, reduced intestinal absorption or increased losses,(Mocchegiani et al. 2013) its
supplementation could help alleviate the deficiencies, which could ultimately provide optimal levels
of serum Cu/Zn-ratio in at-risk individuals. There is
consistent evidence that preventive Zn supplementation reduces the risk of morbidity and mortality from
infectious diseases such as pneumonia, diarrhoea and
malaria. (Bates et al. 1993; Yakoob et al. 2011).
Strengths and limitations
Apart from being the first prospective evaluation
of the association between serum Cu/Zn-ratio and
the specific outcome of pneumonia, other strengths
include (i) the representativeness of the general Finnish middle-aged to older male population, (ii) employment of a relatively large cohort, (iii) the long-term
follow-up of the cohort, and (iv) the comprehensive
analyses including adjustment for a panel of potential confounders, assessment of the dose–response
relationships, evaluation for effect modification on
the association using several clinically relevant characteristics and sensitivity analysis. Several limitations
of this study deserve consideration. They include (i)
the inability to generalise findings to other populations, women and other age groups; furthermore,
evidence suggests that there may be gender differences in the concentrations of Cu and Zn;(Olsen et al.
2012) (ii) the possibility that regression dilution bias
could have underestimated the associations due to the
use of single baseline measurements of the exposures
and the long-term follow-up period; (iii) serum Cu
concentrations may not accurately reflect actual Cu
status, given that leucocyte Cu measurement is considered to be a more reliable index of Cu status in the
body;(DiNicolantonio et al. 2018) and (iv) the potential for biases such as residual confounding and reverse
causation as with all observational cohort studies.
men, consistent with linear dose–response relationships. The relationship between serum Zn and pneumonia is inverse and curvilinear. Furthermore, serum
Cu/Zn-ratio might be a stronger risk indicator for
pneumonia than hsCRP, a major inflammatory marker.
Acknowledgements We thank the staff of the Kuopio
Research Institute of Exercise Medicine and the Research Institute of Public Health and University of Eastern Finland, Kuopio, Finland for the data collection in the study.
Author contributions S.K.K. conceived and planned the study
and methodology, conducted data curation, carried out the statistical analysis and prepared an original draft; S.Y.J. conceived
and planned the study; J.A.L. conceived and planned the study
and methodology; all authors contributed to writing, reviewing,
and editing of the manuscript, provided insights on the topic, discussed the results and critically revised the manuscript.
Funding JAL acknowledges support from The Finnish Foundation for Cardiovascular Research, Helsinki, Finland.
Data availability The data that support the findings of this
study are available from the Principal Investigator (J.A.L.)
upon reasonable request.
Declarations
Conflict of interest
ing interests.
The authors declare there are no compet-
Ethical approval The Research Ethics Committee of the
University of Eastern Finland approved the study (reference
#:143/97), and each participant gave written informed consent.
All study procedures were conducted according to the Declaration of Helsinki.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits
use, sharing, adaptation, distribution and reproduction in any
medium or format, as long as you give appropriate credit to the
original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The
images or other third party material in this article are included
in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not
included in the article’s Creative Commons licence and your
intended use is not permitted by statutory regulation or exceeds
the permitted use, you will need to obtain permission directly
from the copyright holder. To view a copy of this licence, visit
http://creativecommons.org/licenses/by/4.0/.
Conclusions
An increased serum Cu/Zn-ratio and serum Cu concentrations are associated with an increased risk of
incident pneumonia in middle-aged and older Finnish
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