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| Journal of the New Zealand Medical Association,
29-June-2007, Vol 120 No 1257
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Ethnic differences in the prevalence of new and known
diabetes mellitus, impaired glucose tolerance, and impaired fasting glucose.
Diabetes Heart and Health Survey (DHAH) 2002–2003, Auckland New
Zealand
Gerhard Sundborn, Patricia Metcalf, Robert Scragg, David
Schaaf, Lorna Dyall, Dudley Gentles, Peter Black, Rodney Jackson
In 2003, one in 23 New Zealand (NZ) adults had self-reported
diabetes mellitus.1 The prevalence of diabetes
was significantly higher in Māori (males 9.5%, females 6.7%) and Pacific
(males 8.0%, females 12.0%) populations [mostly of Samoan, Tongan, Niuean, or
Cook Islands origin; hereafter termed ‘Pacific’] than in NZ
Europeans (males 3.4%, females 2.4%).1 It has
been suggested that for every person diagnosed with diabetes there is another
person in the community undiagnosed.2
In November 2005, approximately 125,000 people had diagnosed
diabetes; therefore according to this prediction the true diabetic population in
NZ exceeds 250,000 people. It is predicted that more than 7,500 new people will
be diagnosed with diabetes in 2006 and that more than 1,700 deaths will be
attributable to diabetes.3
For Māori and Pacific, diabetes related-mortality rates
are 10 times higher than for European people.4
A recent review of the epidemiology of diabetes in NZ lists the prevalence of
known and undiagnosed diabetes from various NZ surveys and has called for a
nationally agreed strategic plan on how to best monitor and control
diabetes.5
Previous New Zealand studies and surveys that have been used
to estimate the prevalence of diabetes have used self-report data or have been
workforce surveys1,6–8 or are now more
than a decade old.9 Self-report surveys are
likely to underestimate diabetes prevalence as those who have diabetes and are
yet to be diagnosed will be missed (under-reporting) and workforce surveys will
be biased due to the ‘healthy worker’ effect. Moreover the majority
of these surveys included insufficient samples of Māori and Pacific to
allow for meaningful ethnic comparisons.
The South Auckland diabetes project (1991-4) is a useful
comparison for this study as it was also a population based survey and had good
numbers of both Māori and Pacific.10
The Diabetes Heart and Health (DHAH) survey has attempted to
overcome these problems by using a population-based study design that included
Glucose Tolerance Tests (GTT) for all non-diabetic participants. Targeted
sampling of Māori and Pacific was undertaken to generate large
representative samples of these communities.
The purpose of this study is to describe and compare ethnic
differences in the prevalence of new and known diabetes mellitus, and impaired
glucose tolerance and impaired fasting glucose levels.
MethodsThe DHAH survey was a cross-sectional study that
surveyed people aged 35–74 years, between January 2002 and December 2003.
All participants were selected from within the Auckland region. Of the 4049
participants, 1014 were Māori, 1011 were Pacific, and 1745 were of European
ethnicity.
Adults were recruited using two sampling frames: one
was a cluster sample where random starting point addresses were obtained from
Statistics New Zealand and the probability of selection was proportional to the
number of people living in that mesh block (response rate 61.3%); and the other
was a random sample taken from the November 2000 Auckland electoral rolls
stratified into 5-year age bands and included all people living in the Auckland
area, with the exception of the Franklin and Rodney electorates (response rate
65%). Participants were interviewed in places close to where they lived and all
completed a self-administered questionnaire and a series of health measures. The
19 people who refused to have a GTT were excluded. Asians were also
excluded.
Classification of ethnicity first gave priority to
Māori ethnicity and followed an ‘ever-Māori’ approach used
to improve undercounts in health data sets,11
followed by Pacific and Asian while all other participants formed the European
comparison group, as used by Statistics New
Zealand.12 Ethical approval was obtained from
the Health and Disability Ethics Committees.
All participants received information in the mail with
instructions not to eat any food from 10pm onwards the night before their survey
was scheduled and to drink water only. Included in the information pack was a
sterile urine container that was used to collect an early-morning urine sample
(midstream). Most participants were contacted by phone prior to their survey
appointment where instructions were explained again and any queries answered. On
arrival at the survey location (from 8am to 10am) and after initial consent was
given, a fasting blood sample was taken from all participants.
Participants were then asked whether they had been
diagnosed with diabetes, and if so how old they were when they were first told,
and what their current treatment was. Those who did not have previously
diagnosed diabetes mellitus were then asked to complete a 2 hour Glucose
Tolerance Test (GTT). This involved them having a drink consisting of 75g
glucose after their initial blood test. A final blood test was then scheduled to
be taken 2 hours after the first. During the wait-time, participants were asked
to fill in other survey questionnaires and to not physically exert themselves in
any way or consume any food or drink with the exception of water. Glucose
samples were collected into fluoride tubes and stored on ice until taken to the
lab for analyses.
Fasting blood samples were assayed using enzymatic
methods, plasma glucose was measured using commercial reagents (Roche Products
[NZ]), HbA1c was measured by high performance liquid chromatography, and
micro-albumin was measured using an immunoturbidmetric method.
Categorisation of glucose tolerance status was
evaluated by 1998 WHO criteria using fasting glucose ≥ 7.0 mmol/L or
2-hour post glucose load of ≥ 11.1 mmol/L for diabetes; fasting glucose
< 7.0 mmol/L and 2-hour glucose between 7.8 and 11.0 mmol/L for Impaired
Glucose Tolerance (IGT) and fasting glucose of 6.1- 6.9 mmol/L for Impaired
Fasting Glucose (IFG). All participants were then classified as
‘known’ (from their past history), ‘new’-ly diagnosed,
having ‘IGT’ or ‘IFG’ or ‘normal’ glucose
functioning.
Leisure exercise was assessed using a three-month
physical activity recall questionnaire.13 One
question asked if participants had engaged in any vigorous activity at least
once a week, in the past three months, long enough, that caused them to breathe
hard or sweat. The other question asked if they had engaged in any moderate
activity (that did not cause them to breathe hard or sweat).
Statistical analysis was undertaken using SAS version
9.1. Participant data were weighted according to the sampling frame that they
were obtained from and means, standard errors and prevalence’s calculated
using dual frame sampling
methodology.14–16 SAS survey procedures
(SURVEYMEANS, SURVEYREG, SURVEYFREQ AND SURVEYLOGISTIC) were used to calculate
weighted means, adjusted means, percentages and odds ratios,
respectively.17
The Rao-Scott modified Pearson Chi-squared test was
used where appropriate with the reference category being the Europeans, because
they constituted the largest sample. Odds Ratios were converted to Relative
Risks as described by Zhang and Yu.18
ResultsThe demographic characteristics are shown in Table 1. For the total study population, 48.0% were male;
27.0% aged <45 years, 26.4% aged 45–54 years, 24.4% aged 55–64
years, and 22.3% aged 65+ years; 26.9% of the participants were Māori,
26.8% were Pacific, and 46.3% were of European ethnicity.
The proportions of impaired fasting glucose (IFG), impaired
glucose tolerance (IGT), newly diagnosed, previously diagnosed (known), and
total diabetes are shown in Figure 1. Europeans had the lowest proportions,
Pacific had the highest, and Māori were intermediate for prevalence of all
diabetes states. Māori had 2.8 times higher and Pacific 4.1 times higher
prevalence of total diabetes mellitus compared to Europeans.
Figure 1. Prevalence of diabetes states by
ethnicity adjusted for age and sex
 Lifestyle, socioeconomic status and demographic
characteristics by diabetes status are presented in Table 2. These proportions
have been adjusted for age, sex, and ethnicity. Those with abnormal diabetes
status generally had significantly higher BMI, were older and exercised less.
Compared to those with new or known diabetes and IGT categories, the IFG
subgroup was not as distinctly differentiated from the ‘normal’
group.
Table 2. Percentage and mean demographic
characteristics by diabetes status; adjusted for age, sex, and
ethnicity
*0.01 <p< 0.05; ** 0.001<p<0.01; ***
p<0.001 compared to ‘Normal’ group; †Not adjusted for sex;
‡Not adjusted for age; IGT: impaired glucose tolerance; IFG: impaired
fasting glucose; Mod-ex: Participated in moderate exercise at
least 1 × per week in past 3 months; Vig-ex: Participated
in vigorous exercise at least 1 x per week in past 3 months.;
Good+ health: Rated personal
health as good or better.
Table 3 compares the risk of having newly diagnosed or known
diabetes by ethnic group. Māori aged 45–54 and 55–64 years were
found to have significantly higher risk of known diabetes compared to Europeans
(RR: 6.4 and 4.1 respectively). For Pacific, all age groups had a significantly
higher risk of known diabetes than Europeans, with the highest being in the
55–64 year age group (RR: 9.3).
Only Pacific participants in the <45, and 45–55
year age groups were found to have a significantly higher risk of new diabetes
status (RR: 11.6 and 4.2 respectively) compared to Europeans.
Table 3. Relative risk (RR) of new and known
diabetes by ethnic group, adjusted for sex
*0.01 <p< 0.05; **0.001<p<0.01;
***p<0.001 compared to Europeans.
The highest proportion of new diabetes was observed in the
Pacific 45-54 age group of 6.9% (Europeans: 2.2%, Māori: 2.5%). The largest
proportion of previously diagnosed (known) diabetes was also reported by the
Pacific ethnic group, aged 55-64 of 38.2%. This is compared to Europeans 4.4%,
and Māori 17.4%.
The only significant difference in IGT was observed in the
Pacific ethnic group aged 45-54 years compared to the Europeans. For IFG the
only significant differences were observed in Māori aged 45-54, and Pacific
aged <45 and 45-54 years. Generally a clear trend was observed where Pacific
had the highest risk and Māori intermediate for both IGT and IFG compared
to Europeans. However two exceptions to this trend existed. The 55-64 IFG
Māori group and the 65+ IFG/IGT Pacific groups reported lower (but not
significantly different) risks compared to Europeans.
Table 4. Relative risk (RR) of impaired glucose
tolerance (IGT) and impaired fasting glucose (IFG) by age group and ethnicity,
adjusted for sex
*0.01 <p< 0.05; ** 0.001<p<0.01; ***
p<0.001 compared to Europeans.
Figure 2 shows the cumulative proportion of newly diagnosed
diabetes mellitus by age group and ethnicity. This graph shows that Māori
follow a similar trend to Europeans in spite of having a higher prevalence of
both new and known diabetes mellitus. The Pacific ethnic group however follow a
clearly different path. For Pacific, the same proportion of newly diagnosed
participants were diagnosed approximately ten years earlier compared to both
Māori and Europeans. For example 80% of newly diagnosed Pacific
participants had been diagnosed by the 45-54 age group. This occurred closer to
the 55-64 age-group for both Māori and Europeans.
Figure 2. Cumulative proportion of new diabetes
status by age and ethnicity
 Table 5 presents the findings of 2 multivariate models for
newly diagnosed diabetes mellitus. Model 1 shows that being Māori or
Pacific increases the odds of being newly diagnosed with diabetes mellitus
compared with Europeans. However, only Pacific ethnicity was statistically
significant. After adjustment for Body Mass Index (BMI) in Model 2 both odds
ratios for Māori and Pacific ethnicity dropped making Pacific ethnicity now
insignificant.
Table 5. Multivariate odds ratios (95% CI) for
‘newly’ diagnosed diabetes mellitus
Model 1 includes age, male, Māori and Pacific;
Model 2 includes Model 1 plus BMI.
Table 6 shows 2 multivariate models for previously diagnosed
(known) diabetes mellitus. Model 1 show 4 times higher odds for Māori and
6.6 times higher odds for Pacific of previously diagnosed diabetes compared to
Europeans. Adjustment for BMI in Model 2 saw a reduction of the odds in both
Māori and Pacific compared to Europeans, but did not eliminate ethnic
differences in people with previously diagnosed diabetes mellitus.
Table 6. Multivariate odds ratios (95% CI) for
‘known’ diabetes mellitus
Model 1 includes age, male, Māori and Pacific;
Model 2 includes Model 1 plus BMI.
DiscussionIt has been frequently reported that between a third and a
half of all diabetes in the community remains
undiagnosed2 and that this may be experienced
more by Pacific people.19 The common mantra
that ‘for every known case of diabetes there is another undiagnosed in
the community’, is not supported by our study and has
significant public health implications in estimating the projected burden of
undiagnosed diabetes in New Zealand by ethnic group.
In the current study, the proportion of Europeans that were
newly diagnosed with diabetes was equal to 46% of those with known diabetes.
This suggests that for every two European people with previously diagnosed
diabetes there approximately one (0.92) person in the community undiagnosed.
For Māori and Pacific, the proportions that were newly
diagnosed with diabetes were equal to 32% and 21% respectively, of those with
known diabetes. This suggests that for every three
Māori and every five
Pacific people with previously diagnosed diabetes there would one person in the
community undiagnosed (0.96 for Māori, 1.05 for Pacific).
In contrast, a study that measured new and known diabetes in
adults aged 40–70 years in South Auckland during 1991-94, found that the
proportion of new diabetes in Europeans was equal to 52% of known. For
Māori and Pacific, these proportions were 77% and 81%
respectively.10 These findings suggest that
diabetes screening for Māori and Pacific have improved considerably over
the past decade.
Prevalences of previously diagnosed diabetes found in this
study had similar ethnic patterns to those reported in the 2002/03 NZ Health
Survey. However the self-reported prevalences of previously diagnosed diabetes
tended to be considerably lower in the NZ Health Survey. In absolute terms, the
DHAH survey prevalences were higher than the NZ Health Survey data by
approximately 1.4% for Europeans, 4.0% for Māori, and 9.5% for Pacific
ethnic groups. These differences are in part due to the differing age structure
of each survey. The NZ Health survey sampled from 15 years and above compared to
35 years for the DHAH. These surveys were both conducted during 2002/03.
In contrast to a cross-sectional survey carried out in South
Auckland from 1992-199511 and the Workforce
Survey in 1988-1990,20 Pacific people now have
a poorer profile than the Māori population. The South Auckland study
reported age adjusted rates of known diabetes of 5.2% for Europeans, 7.3% for
Māori and 6.0% for Pacific peoples compared to 3.9%, 12.0%, and 19.5%,
respectively, in the DHAH.
For those with previously diagnosed (known) diabetes,
Pacific aged 55-64 reported the largest relative risk (RR: 9.33 compared to
Europeans), this compares to Pacific aged 50-54 (RR: 11.8) from the Workforce
Survey. Prevalences of known diabetes by age group and ethnicity followed
expected trends with Pacific people having the highest prevalences and
Māori intermediate.
However the patterns in RR were more mixed for new diabetes.
Māori had the highest RR for new diabetes in the 55-64 and 65+ age groups
with Pacific intermediate. For the <45 and 45-54 age groups Pacific had the
highest risk. This could be due to both earlier onset of diabetes in Pacific and
less robust screening for diabetes in Māori compared to Pacific in the
younger age groups. Risk of new diabetes in Māori ranged from
1.38–4.97 compared to Europeans and for Pacific they ranged from
0.99–11.61. The largest risk for new diabetes was found in the Pacific age
group of <45 years (RR: 11.61) which was also the case for the Workforce
Survey (RR: 9.5).
The Workforce Survey conducted during
1988-9021 reported newly diagnosed diabetes in
1.7% of Europeans, 9.7% Māori, and 7.7% Pacific
people.22 However, these were likely to be
lower than the general population as they were employees (healthy worker
effect). Prevalences found in the DHAH were 1.8% for Europeans, 3.8% Māori,
and 4.0% Pacific.
The marked decrease in Māori of newly diagnosed
diabetes is interesting and may be the result of improved health and improved
access to healthcare with the emergence of many new Māori healthcare
providers over the past two decades, resulting in earlier detection of diabetes.
Measurement and classification of Māori ethnicity could also have
influenced this difference. This study used an ‘ever-Māori’
approach to assign ethnicity which decreases the likelihood of
under-reporting.11
A limitation of this study is that using Electoral role
based and cluster sampling frames did not allow for ethnic specific response
rates to be determined. Although the overall response rate was not as high as in
previous Auckland risk factor studies, it has been shown in the Atherosclerosis
Risk in Communities Study22 that response rates
lower than those in our study produced relatively small errors in the estimates
of prevalence of common cardiovascular disease risk factors.
Participants with known diabetes were more likely to engage
in moderate exercise when compared to those newly diagnosed. This suggests that
once a diagnosis is made increased physical activity may have been recommended
to these people.
Having received further tertiary education had a protective
association with diabetes. Levels of tertiary education were lower for all
categories of impaired glucose tolerance when compared to the
‘normal’ reference group. (Table 2) This may be due to education
leading towards higher socio-economic status, and also an increased awareness of
healthy lifestyles, diabetes risk factors and symptoms.
The difference observed in the cumulative proportions of new
diabetes between Pacific and non-Pacific ethnic groups (Figure 2) showed that a
larger proportion of Pacific people generally experience earlier onset of
diabetes. This difference equates to Pacific people being diagnosed up to 10
years earlier than Europeans and suggests that Pacific people will live with
diabetes and its complications significantly longer and/or have earlier
mortality. This figure could also imply that the Māori population may
follow a more similar disease profile/pattern to Europeans than Pacific.
It is important to note that the age at diagnosis is not
necessarily the age of development of diabetes, and that the time between
development and diagnosis may vary between ethnic groups.
BMI and age were found to be the most significant factors
associated with people newly diagnosed with diabetes, IGT, and IFG (Table 2, 5)
which supports the focus that many health campaigns have on prevention and
control of obesity to lower the prevalence of diabetes. Model 2 from Table 5,
showed that adjusting for BMI alone reduced ethnic differences in new diabetes
prevalence. However, although adjustment for BMI in people previously diagnosed
with diabetes mellitus reduced the odds in both Māori and Pacific people
compared to Europeans, it did not eliminate these ethnic differences (Table 6).
Increasing and maintaining health promotion programmes
centred on living healthy lifestyles (nutrition and activity) to keep a healthy
BMI will continue to be the most appropriate method to prevent and manage
diabetes in New Zealand.
Competing interests: None.
Author information: Gerhard
McDonald-Sundborn, Research Fellow in Pacific Health; Patricia Metcalf, Senior
Lecturer in Biostatistics; Robert Scragg, Associate Professor of Epidemiology;
David Schaaf, Senior Research Fellow in Pacific Health; Lorna Dyall, Senior
Lecturer in Māori Health; Dudley Gentles, Research Fellow in Māori
Health; Peter Black, Associate Professor of Medicine; Rodney Jackson, Professor
of Epidemiology, Section of Epidemiology and Biostatistics, School of Population
Health
University of Auckland, Auckland
Acknowledgments: This research was funded
by the Health Research Council of New Zealand and was carried out in the Section
of Epidemiology and Biostatistics/Section of Pacific Health, School of
Population Health, University of Auckland.
We also thank the participants who took part in this survey:
Wayne and Sola McDonald-Sundborn and Bruno and Vera Zarins for their
helpful discussions, comments, and feedback.
Correspondence: Gerhard Sundborn, Section
of Epidemiology and Biostatistics, School of Population Health, University of
Auckland, Private Bag 92019, Auckland 1. Fax: (09) 3737 503; email: g.sundborn@auckland.ac.nz
References:
This article was corrected on 26 October 2007 to
reflect the Erratum at http://www.nzma.org.nz/journal/120-1264/2797
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