a Department of Family and Community Medicine, College of Medicine and Medical Sciences, Arabian Gulf University, PO Box 22979, Manama, State of Bahrain.
b Epidemiology Unit, Department of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK.
Reprint requests to: Dr Faisal Al-Mahroos, PO Box 2527, Manama, State of Bahrain. E-mail: fmahroos{at}batelco.com.bh
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Abstract |
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Methods A cross-sectional survey of 2120 Bahrainis aged 4069 years.
Results The age-adjusted prevalence of hypertension (defined as current treatment for hypertension, systolic blood pressure 160 mmHg or diastolic blood pressure
95 mmHg) rose with increasing degrees of glucose intolerance. Age- and sex-standardized prevalence of hypertension was 21% (95% CI : 1924%) in those with normal glucose tolerance, 31% (95% CI : 2736%) in those with impaired glucose tolerance, and 38% (95% CI : 3442%) in those with diabetes. In a multivariate analysis adjusting for age and sex, raised blood pressure was independently associated with waist girth, plasma cholesterol, glucose intolerance, family history of hypertension and (in women) postmenopausal status. There was an inverse relationship between blood pressure and educational status that was independent of other variables. This association parallels the inverse relationship of diabetes to educational level and is consistent with low educational level being a marker for socioeconomic deprivation in early life in this population.
Conclusion The high prevalence rates of hypertension and diabetes in Bahrainis are manifestations of a pattern of metabolic disturbances that includes raised plasma cholesterol levels. Both hypertension and diabetes are associated with low educational status, which in this population is a marker for socioeconomic deprivation in early life. This suggests that the risk of hypertension may be set by environmental factors in early life.
Keywords Hypertension, diabetes mellitus non-insulin dependent, obesity, cholesterol, socioeconomic factors, Arabia
Accepted 22 July 1999
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Introduction |
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In a nationwide survey of diabetes and cardiovascular risk factors in the State of Bahrain, we found that prevalence rates for type 2 diabetes in Bahrain were among the highest in the world.5 This is consistent with the results of a recent survey in Saudi Arabia.6 However, in Bahrainis the pattern of metabolic disturbances associated with glucose intolerance differed from that usually observed in European populations: diabetes and impaired glucose tolerance were associated with raised plasma cholesterol and triglyceride levels rather than with low HDL cholesterol levels.5
Similar findings have been reported from Kuwait, where both plasma total cholesterol and apolipoprotein B levels were markedly higher in diabetic than in non-diabetic women.7 Even in non-diabetic Bahrainis, a positive family history of diabetes was associated with raised plasma cholesterol levels, suggesting an underlying metabolic disturbance in this population in which raised plasma cholesterol levels are accompanied by insulin resistance, as in familial combined hyperlipidaemia.8 This contrasts with findings in other populations, in which glucose intolerance and insulin resistance are not associated with raised cholesterol levels.911
The objectives of this study were to examine whether hypertension is independently associated with glucose intolerance, plasma cholesterol and other cardiovascular risk factors in Bahrainis, and to determine whether the pattern of risk factors associated with hypertension in Bahrainis varies from that described in other populations.
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Methods |
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An initial invitation was sent to 4060 individuals, inviting them to participate in a screening survey for hypertension and other cardiovascular risk factors. Non-responders were contacted once again by a letter or a visit from a community nurse with the research team. Of the 4060 individuals invited, 382 were established to be not resident at the address given because they had moved, had died or were living abroad, 917 refused to participate and 41 were excluded as too ill or disabled to participate. For 592 individuals no reply was received. A sub-sample of 120 of these households was contacted during the survey to determine the reason for non-participation. The reasons given were as follows: 77 (64%) not interested in participating or too busy; 37 (31%) not resident at the address given, travelling overseas or detained by security forces; 6 (5%) too ill to participate.
Applying these proportions to the remaining 470 households from whom no reply was received, we can estimate that the denominator for the response ratethe number of individuals who received invitation letters and were eligible to participatewas 3419. Of these 2128 attended for initial examination, giving an estimated response rate of 62%. Of these 2128 participants, 8 were missing data on blood pressure or questionnaire items required to assign the blood pressure category and 30 were outside the age ranges originally defined for men and women. The analysis is therefore based on 2090 individuals.
Data collection
Respondents attended the health centre for an initial examination which included anthropometry and blood pressure measurement. A questionnaire on demographic background, medical history, and health-related behaviour was administered in Arabic by a nurse. This included items on physical activity at work, usual weekly distance walked or cycled, and leisure-time physical activity. Blood pressure levels were recorded by a single nurse in each health centre, trained specially for the survey. Blood pressure in the right arm was measured after the subject had been sitting quietly for 5 min. Mercury sphygmomanometers were used with a large cuff to ensure adequate encircling of the arm in all subjects. Weight, height, waist girth and hip girth were recorded with the subjects wearing light clothing and without shoes. Waist girth was measured at the midpoint between the iliac crest and the lower margin of the ribs, and the hip as the circumference at the level of the greater trochanters. An approximate indicator of the waist level was obtained by asking the subject to bend sideways.
From those who were already diagnosed diabetic, a single non-fasting blood sample was taken at the first visit. For all participants who had not been previously diagnosed as diabetic, a second visit after an overnight fast of at least 10 hours was arranged. At this visit, venous samples were taken in the fasting state and 2 hours after drinking a specially-made soft drink containing 75g anhydrous dextrose equivalent dissolved in 300 ml water. Total cholesterol, HDL cholesterol and triglyceride levels were measured on EDTA plasma using a Cobas Miras analyser and Unimate 7 diagnostic kits (Roche, Switzerland).
Data analysis
Hypertension was defined as systolic blood pressure (SBP) 160 mmHg, diastolic blood pressure (DBP)
95 mmHg or being on current antihypertensive therapy in accordance with WHO criteria.12 Participants who did not have definite hypertension by these criteria were classified as normotensive (SBP
140 and DBP
90 mmHg) or borderline hypertensive (SBP >140 mmHg or DBP >90 mmHg).
Education level was grouped into five categories: illiterate, literate with no formal schooling, primary school only, secondary school only and higher education. The diabetic category includes men and women treated for diabetes and those with 2-hour glucose of 11.1 mmol/l. Impaired glucose tolerance was defined as 2-hour glucose 7.811.0 mmol/l according to WHO 1985 criteria.13 Plasma triglyceride values were log-transformed before analysis, and geometric means are given in the Tables. The low density lipoprotein (LDL) cholesterol was calculated from the Friedewald equation14 in the 1591 participants who underwent glucose tolerance testing and whose plasma triglyceride did not exceed 4.5 mmol/l. This excludes 38 participants whose plasma triglyceride levels were >4.5 mmol/l, and 410 participants with a history of diabetes who were not asked to fast for their blood test.
The Stata 5.0 package (Stata Co., Texas) was used for all statistical analyses. Age- and sex-standardized prevalence rates are based on the age and sex distribution of the entire study sample. Systolic blood pressure values were grouped into tertiles, allocating individuals on antihypertensive therapy to the upper tertile. On the assumption that anyone who is on antihypertensive treatment would have been in the upper tertile if untreated, we can then analyse effects on SBP as if no one had been treated. Associations with tertile of SBP as dependent variable were examined by ordered logit regression (proportional odds regression).15 This method generalizes logistic regression to deal with dependent variables grouped into three or more ordered categories. For a variable grouped into tertiles, the model is that the odds ratio (OR) associated with a given change in the explanatory variables is the same whether the variable is dichotomized between the middle and upper tertiles or dichotomized between the lower and middle tertiles. In all analyses, associations were adjusted for age.
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Results |
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Discussion |
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The inverse relationship between hypertension and educational level cannot be accounted for by uptake of antihypertensive therapy, as the allocation of treated hypertensives to the upper tertile discounts effects of antihypertensive therapy. This relationship becomes statistically significant only after adjusting for obesity, because in this Arabian Gulf population there is a positive relationship between obesity and educational level5 in contrast to the inverse relationship between obesity and socioeconomic status seen in most western populations. The inverse relationship of hypertension to educational level parallels the inverse relationship of diabetes to educational level that we have reported previously in this dataset.5 A possible explanation for the association of low educational level with raised blood pressure and diabetes may be that low educational level is a marker for socioeconomic deprivation in early life. This would be consistent with evidence that impaired growth in early life predisposes to hypertension and diabetes in adult life.17,18 Another possibility is that behavioural determinants of blood pressure such as dietary sodium intake may also vary with educational level.
In Bahraini women postmenopausal status was associated with higher prevalence of both hypertension and diabetes, independently of age and body fat pattern. Associations between postmenopausal status and hypertension have been reported in some other populations19 but have not been consistently found to be independent of covariates such as body fat pattern.20 The association of hypertension with positive family history of diabetes, independent of family history of hypertension, suggest that in this population hypertension and diabetes share some of the same genetic determinants. As in other studies,21,22 the associations of diabetes and impaired glucose tolerance with hypertension were not accounted for by adjusting for measures of obesity or body fat distribution. Hypertension was associated with raised waist girth and waist/height ratio but not with raised waist/hip ratio, even though the variation in waist/hip ratio (as indexed by the standard deviation) was no less than in other populations where such associations have been detected. This suggests that, at least in Bahrainis, waist/hip ratio does not distinguish between a high-risk pattern of fat distribution associated with central adiposity and a low-risk pattern associated with peripheral adiposity. Hypertension and glucose intolerance are usually associated with raised plasma triglyceride and low HDL cholesterol levels.23 In Bahrainis raised plasma triglyceride levels were associated with treated hypertension but not with untreated hypertension, suggesting an effect of antihypertensive therapy. There was no relationship of blood pressure to plasma triglyceride levels after adjusting for glucose intolerance and plasma cholesterol levels. In contrast, the association of hypertension with raised plasma levels of total cholesterol and LDL cholesterol in Bahrainis persisted after adjusting for glucose tolerance and obesity. This contrasts with other populations in which hypertension and glucose intolerance are not independently associated with raised cholesterol levels after adjusting for obesity and body fat pattern.24 We have previously reported that glucose intolerance also is associated with raised plasma cholesterol in this population;5 similar findings have been reported in other Arabian Peninsula populations.7 This suggests that the metabolic syndrome underlying the high prevalence of type 2 diabetes in the Arabian Peninsula may differ from the insulin resistance syndrome described in other populations.4,23 Understanding the aetiology of the high prevalence of hypertension and diabetes in Arab populations may depend upon identifying underlying defects in lipid metabolism.
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