Shift work and metabolic syndrome, diabetes mellitus and ischaemic heart disease.

IJOMEH 2010;23(3) 287
R E V I E W P A P E R S
International Journal of Occupational Medicine and Environmental Health 2010;23(3):287 – 291
DOI 10.2478/v10001-010-0032-5
SHIFT WORK AND METABOLIC SYNDROME,
DIABETES MELLITUS AND ISCHAEMIC
HEART DISEASE
DOROTA SZOSLAND
Nofer Institute of Occupational Medicine, Łódź, Poland
The National Centre for Workplace Health Promotion
Abstract
Shift work is affecting 20% to 25% employees and is becoming increasingly prevalent in contemporary life all over Europe
and USA. It is associated with several health problems, such as e.g. metabolic syndrome, diabetes mellitus and cardiovas-
cular disease. These diseases are possibly due to an impairment of biological rhythm. The metabolic syndrome is a complex
of interrelated risk factors for cardiovascular disease and diabetes. Higher prevalence of the metabolic syndrome has been
demonstrated among shift workers. Rotating shift work has an impact on each component of metabolic syndrome. Shift
work might also have an impact on metabolic variables, and be a risk factor for type 2 diabetes. Only a few studies reported
prevalence of impaired glucose metabolism and diabetes mellitus in relation to shift work. There is rather strong evidence
in favour of association between shift work and coronary heart disease and that has been repeatedly demonstrated during
over 20 years of research. Recent data increasingly reveal relations between shift work and plasma resistin, ghrelin, leptin
and adiponectin.
Key words:
Shift work, Metabolic syndrome, Diabetes mellitus, Coronary heart disease
Address reprint request to D. Szosland, Nofer Institute of Occupational Medicine, The National Centre for Workplace Health Promotion, św. Teresy 8, 91-348 Łódź,
Poland (e-mail: szosland@imp.lodz.pl).
INTRODUCTION
Shift work is affecting 20% to 25% employees in manufac-
turing industries and is becoming increasingly prevalent
in contemporary life all over Europe and USA. Continu-
ous growth of shift worker numbers is observed also in the
service sector [1]. The proportion of shift workers remains
relatively constant up to 45 years of age but declines con-
siderably among older workers [2].
Shift work
There is more than one definition of shift work. Council
Directive 93/104/EC of 23 November 1993 concerning
certain aspects of the organization of working time (as
amended by Directive 2000/34 of 22 June 2000) defines
shift work a “any method of organizing work in shifts
whereby workers succeed each other at the same work sta-
tions according to a certain pattern, including a rotating
pattern, and which may be continuous or discontinuous,
entailing the need for workers to work at different times
over a given period of days or weeks” [3,4].
In general, the term “shift work” is quite vague and in-
cludes any organization of working hours that differ from
the traditional diurnal work period; sometimes it is syn-
onymous of irregular or odd working hours [5].
Another definition of shift work is given by Grosswald.
Shift work refers to a job schedule in which employees work
hours other than the standard hours of 8 a.m. to 5 p.m. or
a schedule other than the standard workweek — Monday
through Friday in the United States [6].
There is no precise definition of shift work. Most studies
classify shift workers as anyone working outside regu-
lar daytime hours (i.e. between approximately 7 a.m.
and 6 p.m., Monday through Friday). Under this defini-
tion, shift workers include all people working evening

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Metabolic Syndrome
The metabolic syndrome is a complex of interrelated risk
factors for cardiovascular disease and diabetes. Several
clinical definitions have been proposed.
The first criteria for definition for the diagnosis of meta-
bolic syndrome were proposed by World Health Organiza-
tion (WHO) in 1999 [12]. European Group for the Study
of Insulin Resistance (EGIR) was in the same year the next
one [13]. The presence of insulin resistance was a prerequi-
site in both definitions. National Cholesterol Education Pro-
gram (NCEP) Adult Treatment Panel III in 2001 (ATP-III)
does not obligatorily require impaired glucose regulation
or insulin resistance as an essential component [14]. These
three groups have agreed that the core components of the
metabolic syndrome are: obesity, hypertension, insulin resis-
tance and dyslipidemia. The levels set for each component
and their combination required to diagnose the metabolic
syndrome are slightly different in these three recommen-
dations. In 2005 International Diabetes Federation (IDF)
presented definition where visceral obesity was the neces-
sary requirement. The American Heart Association and the
National Heart, Lung and Blood Institute (AHA/NHLBI)
modified the NCEP criteria by decreasing the glucose cut-off
point [15]. All these definitions have been used by various au-
thors. IDF and AHA/NHLBI representatives agreed that the
abdominal obesity should not be a prerequisite for diagnos-
ing of the metabolic syndrome. Abdominal obesity is one of
the five criteria, while 3 of the 5 risk factors must be present
to diagnose the syndrome [16]. Recently, common criteria for
the clinical diagnosis of metabolic syndrome have been pro-
posed by several major organizations in the Joint Scientific
Statement in an attempt to unify all presented criteria [17].
Higher prevalence of the metabolic syndrome has been
demonstrated among shift workers [18–21]. A recent study
by De Bacquer et al. demonstrates that the risk of develop-
ment of the metabolic syndrome gradually increases inde-
pendently of the accumulated years of shift work. Rotating
shift work has an impact on each component of metabolic
syndrome [21]. Esquirol et al. show that shift work, regard-
less of models, was significantly associated with metabolic
syndrome [22]. The risk of developing metabolic syndrome
is high in night shift healthcare workers [23].
shift, night shift, rotating shifts, split shifts or on-call
schedules both during the week and on weekends [7].
There are thousands of shift systems that differ widely
in their structure. Among shifts, alternating day shifts
are most frequently used in Europe [8]. A major prob-
lem in numerous publications regarding shift work and
health problems is the lack of one common definition of
shift work.
Shift work is associated with several health problems, pos-
sibly due to an impairment of biological rhythm.
Over the last decades, Westernized countries have be-
come 24-hour societies. That is why the interest concern-
ing the influence of shift work on several health problems
has been increasing all the time.
Circadian rhythm
Humans have natural body rhythms which are regulated
by a “circadian clock” in the brain. These are called cir-
cadian rhythms. Circadian rhythm is an integrated body
rhythm that cycles approximately every 24 hours.
Circadian rhythmicity can be important in the patophys-
iology of numerous diseases. In discussing health prob-
lems in shift workers, the role of circadian rhythm is the
key issue. The circadian timing system proficiently coor-
dinates the physiology of living to match it to the envi-
ronmental or imposed 24-hour cycles. A wide range of
biological processes are regulated by the circadian clock,
including sleep-wake cycles, body temperature, energy
metabolism, cell cycle and hormone secretion. Circa-
dian regulation is important to maintain normal cellular
functions. A disruption of core clock genes can be dam-
aging to the organism’s well-being [9–11]. Shift work is
generally associated with chronic misalignment between
the endogenous circadian timing system and the behav-
ioural cycles, including sleep/wake and fasting/feeding
cycles. There is a large amount of data pointing to an as-
sociation between shift work and the prevalence of many
medical conditions. Work/rest schedules resulting from
shift work disrupt the circadian clock, causing symptoms
similar to jet lag. Generally, no complete physiological
(body function) adaptation takes place for the majority
of night shift workers.

SHIFT WORK AND METABOLIC SYNDROME, DIABETES AND CVD R E V I E W P A P E R S
IJOMEH 2010;23(3) 289
among shift workers has been repeatedly demonstrated
during over 20 years of research. There is rather strong
evidence in favour of association between shift work and
coronary heart disease [31–34].
Shift work can increase the risk of CVD by several mecha-
nisms. Besides of psychological and psychosocial factors,
physiological and biological mechanisms are crucial. They
are usually related to activation of the autonomic nervous
system, changed lipid, carbohydrate and other metabolic
parameters. These factors may lead to increase risk for
atherosclerosis, metabolic syndrome and type 2 diabetes.
Coronary heart disease, metabolic syndrome and carbo-
hydrate metabolism disturbances have some joint cause.
Unphysiologic timing of physical activity in relation to cir-
cadian rhythms is a possible explanation for the negative
impact on the cardiovascular system.
Recent data increasingly demonstrate relations between
shift work and elevated plasma resistin levels. Resistin
may play an important role in the pathogenesis of early
metabolic syndrome components in young men chronical-
ly exposed to circadian misalignment [35]. Its circulating
levels are predictive of coronary atherosclerosis [36]. Cir-
culating adiponectin, ghrelin and leptin play an important
role in human obesity, one of the components of metabol-
ic syndrome, by the long term regulation of body weight
and energy homeostasis [37]. Further studies in the shift
work population are needed to sort out an important role
of these in all elements of metabolic syndrome, diabetes
and coronary health disease which are closely related to
each other.
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