a School of Clinical Medicine & Research, University of the West Indies.
b Ministry of Health, Barbados, West Indies.
c University Medical Center at Stony Brook, Stony Brook, NY, USA.
d The Johns Hopkins University School of Medicine, Baltimore, MD, USA.
M Cristina Leske, Department of Preventive Medicine, University Medical Center at Stony Brook, HSC L3 086, Stony Brook, NY 117948036, USA. E-mail: cleske{at}notes.cc.sunysb.edu
Abstract
Objective To examine the distribution and impact of diabetes, glycaemic status, and related factors, in a predominantly black adult Caribbean population.
Methods The study included 4709 people, or 84% of a simple random sample of Barbadian-born citizens aged 4084 years, examined between 1988 and 1992 and re-assessed 4 years later. Diabetes was evaluated according to physician-diagnosis and glycosylated haemoglobin (GHb). Associations were assessed by logistic regression analyses, cumulative mortality by product-limit methods and death-rate ratios by Cox proportional hazards regression.
Results Among the 4314 black participants, the prevalence of known diabetes, predominantly type 2, was 9.1% at 4049 years of age and increased to 24.0% at 7079 years. The overall prevalence was 17.5%, while it was 12.5% in mixed (black/ white; n = 184) and 6.0% in white/other participants (n = 133), only 0.3% had younger-onset. Additionally, 2% had GHb >10% (>2 SD over the mean) without diabetes history. Sulphonylureas were the most frequent treatment, while insulin use was infrequent. In black participants, diabetes was positively associated with age (OR = 1.03 per year; 95% CI : 1.021.04), diabetes family history (OR = 2.85, 95% CI : 2.393.40), hypertension (OR = 1.71, 95% CI : 1.422.05), obesity (BMI 25 kg/m2; OR = 1.74, 95% CI : 1.442.10), and high waist-hip ratio (WHR
0.92; OR = 1.29, 95% CI : 1.091.53). Ocular co-morbidities were increased among people with diabetes, as was 4-year-mortality (death rate ratio = 1.42, 95% CI : 1.101.83). There was a 9% increase in mortality for each 1% increase in GHb (death rate ratio = 1.09, 95% CI : 1.041.15).
Conclusions A markedly high prevalence of diabetes existed in the adult black population, affecting almost one in five people and increasing morbidity and mortality. Prevention strategies are urgently needed to reduce the adverse implications of diabetes in this and similar populations.
Keywords Type 2 diabetes, morbidity, mortality, Barbados
Accepted 11 July 2001
Rates of type 2 diabetes continue to rise globally, particularly among developing countries undergoing the epidemiological transition to chronic disease and ethnic minorities in industrialized countries.14 Excessive diabetes complications exist among Afro-Americans, who experience visual loss, lower extremity amputation and end-stage renal disease at rates 1.5 to 4 fold higher than seen among comparable white groups.58 In particular, the Caribbean is a middle-income region with markedly high and rising rates of diabetes.3,912
Barbados is a predominantly black Caribbean nation, where a large nationally representative adult cohort comprising people initially aged 4084 years has been studied prospectively since 1988.13 This cohort provides a unique opportunity allowing the description of the prevalence, risk factors and the public health implications of diabetes in this country; results that are potentially applicable to other similar populations. This report presents the results of these analyses.
Methods
The study population was the Barbados Eye Study cohort (BES, 19881992). The BES, funded by the National Eye Institute, included 4709 people, identified by a simple random sample of Barbadian-born citizens, aged 4084 years. The participation rate was 84%, and the demographic composition of BES participants closely resembled the census population.13 As described in detail elsewhere,13 the study protocol included an interview with questions on medical history and medication use; as well as anthropometric and blood pressure measurements. Weight was measured by balance beam scale, height by a wall-mounted height measure and abdominal (maximal circumference between the lower ribs and hip) and hip circumferences (at the level of maximal protrusion of the buttocks) by steel tape. A history of alcohol use was elicited by assessing consumption of either beer, wine or liquor at least once a month for a year or more, while smoking was defined as at least one cigarette daily for at least one year. Occupation was based on self-reported major lifetime work and was categorized as professional (e.g. managerial, technical) and non-professional (e.g. service, agricultural) groups. In addition, information was obtained about duration and level of schooling. Ocular measurements included visual acuity, perimetry and tonometry, fundus photography, and a comprehensive ophthalmological examination.
Blood pressure was recorded as the average of two measurements with the Hawksley random zero sphygmomanometer, following the Hypertension Detection and Follow-up protocol.14 Hypertension was defined as a systolic blood pressure 140 mmHg and/or a diastolic blood pressure
90 mmHg and/or a history of antihypertensive treatment.
Participants were requested to bring their medications to the clinic at the time of their study visit, which assisted the validation of diabetes therapy. Depending on age at self-reported physician diagnosis, participants were further categorized as having younger (<30 years of age) or older onset (30 years) diabetes. Younger-onset participants were considered to have type 1 diabetes mellitus if they were also receiving insulin, while older-onset diabetes was likely to be type 2. The computerized database system allowed follow-up of the cohort, and mortality information 4 years later was verified from death certificate data obtained from records held at the Ministry of Health, which included dates and specific causes of death.
Glycosylated haemoglobin (GHb) assays by affinity chromatography of venous whole blood15 using Glyc-Affin GHb kits (Isolab, Akron, Ohio, USA) were available for 3754 or 81% of the participants (GHb was not measured in the first months of the study). This assay measures all glycosylated haemoglobin variants; the reference range being 48%. Duplicate testing (using two aliquots of the same sample) of a random sample of laboratory determinations (n = 264) showed good reproducibility, with an intraclass correlation coefficient of 0.89.
Standard fundus photographs of the disc and macula were used to evaluate the presence of diabetic changes and were independently graded at the Fundus Photography Reading Center,16 using an adapted version of the modified Early Treatment Diabetic Retinopathy Study (ETDRS) Airlie House classification.17,18 Lens gradings of cataract were based on the Lens Opacities Classification System II (LOCS II) at the slit lamp, under maximum dilatation with tropicamide, using photographic standards to grade lens opacities as defined by a LOCS II score 2.19 Intraocular pressure was measured by Goldman applanation tonometry.
The frequency of known diabetes by self-reported ethnicity (categories being black, mixed [black and white], white, Asian, other) was determined. Age-gender-specific distributions of diabetes among black participants were estimated according to various definitions. To account for the incomplete GHb data, estimates were adjusted by applying the age-gender-specific rates of those with GHb measurements to those without. Factors associated with diabetes were first evaluated using Mantel-Haenszel analyses. Significant variables (P < 0.05) including age, gender, family history of diabetes, hypertension, obesity, as well as lifestyle (alcohol use and cigarette smoking) and socioeconomic variables (occupation and education), were then retained and evaluated in logistic regression models. Results are presented as adjusted odds ratios (OR) with associated 95% CI. Follow-up data on mortality among cohort members were examined, and the 4-year cumulative mortality by diabetes type and treatment was calculated by the product-limit method. Death rate ratios were based on Cox proportional hazards regression models, adjusting for age and gender or other confounding variables.
Results
Table 1 presents the distribution of known diabetes status (younger- or older-onset) by ethnicity. The overall prevalence of known diabetes was 17.0% and varied by ethnic group. Diabetes was reported by 17.5% (754/4314) of black, 12.5% (23/184) of mixed, and 6.0% (8/133) of white/other participants, with the majority being of older-onset. In logistic regression, adjusting for age and sex, the odds of older-onset diabetes was 5.5 (95% CI : 2.213.5) in black compared to white participants. In contrast, younger-onset diabetes was rare in this population (16/4631), the overall prevalence being 0.3% among black, 0.8% among white/other, and 0% among mixed race participants. Further analyses are limited to black participants, in view of the small numbers of people with diabetes among the other groups.
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The mean GHb of people without a diabetes history was 7.1% (SD = 1.5; median 6.9%). Two per cent of participants in this group had GHb levels >10%, which were >2 SD over the mean. Because it is likely that these participants had undiagnosed type 2 diabetes, Table 2 provides age-sex specific prevalence estimates based both on self-reported history alone, as well as on history and GHb >10% among black participants. Both estimates of diabetes prevalence increased with age and reached a peak at ages 6079 years, decreasing thereafter. Women had a higher age-specific prevalence than men in all age groups. Based on self-report alone, the prevalence increased from 9.1% at 4049 years of age to 24.0% at 7079 years of age, with an overall crude prevalence of 17.5%. When age-standardized to the 1990 world population20 using the direct method,21 the adjusted prevalence estimate was 16.1% (95% CI : 15.017.2%). When GHb >10% was also included, the crude overall prevalence was slightly higher at 19.4% and the age-standardized prevalence was 18.0% (95% CI : 16.819.1%). No differences were found between people with and without GHb data with respect to age, gender, and self-reported history of diabetes. After accounting for the incomplete GHb measurements, the diabetes prevalence estimates slightly increased to 19.9%.
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This study determined the patterns of diabetes, glycaemic status, related factors, and 4-year outcomes in a large, nationally representative adult Caribbean population. Prevalent known diabetes (principally type 2) varied with ethnic group, affecting 17.5% of black, 12.5% of mixed, and 6.0% of white/other participants (Table 1). The median duration of diabetes following diagnosis was 5 years. Almost three-fourths of those with known type 2 diabetes reported treatment with oral hypoglycaemic agents, principally sulphonylureas alone (45.0%), or combined with Metformin (23.6%). Increased age, diabetes history, hypertension, high BMI and high WHR, higher education and alcohol consumption were related to diabetes (Table 3
). People with diabetes had an increased risk of ocular complications and of 4-year mortality (death rate ratio = 1.42), the latter being highest among those with type 2 diabetes treated with insulin and younger-onset diabetes (Table 4
). In the overall population, each 1% increase in GHb predicted a 9% increased mortality over 4 years. A discussion of these findings follows.
Glycosylated haemoglobin is an index of average blood glucose concentration over 3 months and, although proposed as a diagnostic tool for diabetes, remains limited by lack of standardized laboratory methods.25,26 The BES, due to feasibility constraints, relied on self-reported history of physician-diagnosed diabetes and measurements of GHb, precluding the diagnosis of unrecognized diabetes or glucose intolerance based on current criteria.26 A significant underestimation of the true diabetes prevalence is therefore likely.25 Prevalent diabetes among BES white participants (7.5%, based on history and GHb >10%) is consistent with reports from predominantly white populations with similar age distributions,27,28 while younger-onset diabetes was rare in this study as expected.29 Use of GHb >10% for the estimation of unknown diabetes in this study suggests that 10% of cases were undiagnosed, considerably less than the 34% reported in Afro-Americans.2 Self-reported diabetes among black BES participants, however, mirrors a previous report,10 and accords with a prevalence of 9.9% in a younger Jamaican population (mean age 46.6 years).3
Established diabetes risk factors were demonstrated in the BES population. High rates of diabetes in the Caribbean result from lifestyle factors leading to obesity,3,911,30 which also confer an excess disease burden on women known to have high obesity rates.3,10,30,31 Diabetes risk in the BES population increased continuously for BMI, even at levels deemed safe'32 and below recognized cut points for overweight and obesity (>22.5 kg/m2 in this study), as found by others.33,34 While people with WHR above the median level of the population as compared to those below had approximately 30% higher odds of having diabetes, there was no linear trend toward consistent increase at higher levels. The negative association between diabetes and longer duration of education may reflect healthier lifestyles, while the negative association with ever consumption of alcohol might be explained by reporting bias, i.e. under-reporting of alcohol use among people with diabetes.
The median duration of diabetes of 5 years in the BES is consistent with other reports.2 In contrast to data from NHANES II,8 10% of BES participants with type 2 diabetes were being treated with insulin (37.2% in NHANES II) and 45.0% with sulphonylureas alone (29.8% in NHANES II). These treatment patterns were presumably influenced by the younger age of NHANES II compared to BES participants (mean ages of 52.5 and 62 years, respectively), associated with a higher prevalence of type 1 diabetes as well as differences in physician-patient factors. Glycaemic control was poor in BES, particularly among those with younger-onset and with type 2 diabetes receiving insulin or combination oral therapy, resulting in high prevalence rates of retinopathy among the former.22 There are few longitudinal studies on hyperglycaemia-related mortality in African-origin populations.35,36 This study confirms an increased mortality and provides further evidence that GHb concentrations are predictive of mortality across the entire population distribution, including non-diabetics and at levels in the normal' range.37 The 9% increased mortality for each 1% GHb increase in the BES contrasted with the 29% increased mortality for each 1% HbA1c increment in the EPIC-Norfolk study.37 Differences in GHb assay, study populations (both genders in BES versus men only in the EPIC-Norfolk study), and ethnicity are likely to have contributed to these differences.
Implications for Public Health
This study highlights the high prevalence of diabetes and related factors in an Afro-Caribbean population, with frequent ocular co-morbidities and increased mortality. Primary prevention of diabetes by lifestyle interventions, screening for individuals at high risk and prevention of disease complications in established disease through non-pharmacological and therapeutic interventions are all needed to reduce the high burden of diabetes and associated mortality in Afro-Caribbean and similar populations. Interventions leading to reductions in mean population GHb levels would be expected to reduce the associated excess mortality seen among pepole with and without diabetes.
Acknowledgments
This study was supported by grants EYO7625 and EYO7617 from the National Eye Institute, Bethesda, MD. The authors thank the Barbados Eye Studies participants and the Ministry of Health, Barbados, West Indies, for their role in the study.
Notes
The Barbados Eye Study Group: M Cristina Leske, Principal Investigator. Co-ordinating Center: M Cristina Leske; Suh-Yuh Wu; Leslie Hyman; Ho Cheung; Vito Squicciarini; Kanginn Peng; Barbara Springhorn; Kasthuri Sarma; Koumoudi Manthani. Data Collection Center: Anthea MS Connell;
Coreen S Barrow; Doreen Y Boyce; Yolande Babb; Anne Bradshaw; Jillia Bird, 19891991; Valda Griffith, 19881991; Hermes Nurse 19911992; Judith D Hall, 19911992; Carol Selleck, 19911992. Fundus Photography Reading Center: Andrew P Schachat; Judith Alexander; Noreen B Javornik; Cheryl J Hiner; Deborah A Phillips; Reva Ward-Strozykowski; Gregory Whitehead; Terry W George.
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