The prevalence and social patterning of chronic diseases among older people in a population undergoing health transition. A 10/66 Group cross-sectional population-based survey in the Dominican Republic

With demographic ageing and the accompanying health transition, chronic diseases are assuming progressively greater significance in countries with low and middle incomes. They are already the leading cause of death in all world regions apart from sub-Saharan Africa. Of the 35 million chronic disease deaths in 2005, 80% occurred in countries with low or middle incomes [1], partly because most of the world's older people live in these regions - 60% now rising to 80% by 2050. However, changing patterns of risk exposure also contribute. Latin America exemplifies the third stage of health transition. As life expectancy improves, and high fat diets, cigarette smoking and sedentary lifestyles become more common, so cardiovascular diseases have maximum public health salience - more so than in stage two regions (China and India) where risk exposure is not yet so elevated, and in stage four regions (Europe) where public health measures have reduced exposure levels [2]. The INTERHEART cross-national case-control study suggests that risk factors for myocardial infarction operate equivalently in all world regions, including Latin America [3].

In a bibliometric analysis of low and middle income country journals covering the period 1998-2003 more than 40% of articles focused on chronic disease research, cardiovascular diseases and cancers being the most popular topics [4]. However, the Latin American region was underrepresented. Chronic disability and its determinants have received comparatively little attention, in research, policy or practice. While cardiovascular diseases and cancers contribute mainly to mortality, much of the burden of other chronic diseases (dementia, mental disorders, diabetes and stroke) arises from years lived with disability [5]. Despite the growing interest in chronic diseases in low and middle income countries [6, 7], there is limited information available on their prevalence and impact, and most comprehensive studies focus exclusively or mainly upon young and middle-aged adults [8‐10]. Older people contribute 72% of all deaths [5], and 14% of all years lived with disability linked to chronic diseases in low and middle income countries.

Health policy should be informed by precise estimates of disease prevalence and burden. These are lacking for most chronic diseases in most countries with low and middle incomes, particularly for older adults. The 10/66 Dementia Research Group studies in seven Latin American countries, India and China aim to chart the progress of the health transition and its impact upon older persons [11]. This report, from the Dominican Republic, is a comprehensive population-based study of the prevalence and social patterning of chronic disease diagnoses, risk factors and impairments among older people, and their contribution to disability.

In the current study we were able to ascertain the prevalence and distribution of a wide range of chronic disease diagnoses, impairments and underlying risk factors in a large population-based sample drawn from urban catchment areas in Santo Domingo, the capital city of the Dominican Republic. There have been very few previous reports of the prevalence of chronic diseases and their risk factors among older people, in countries with low and middle incomes in Latin America or other regions. In our study, the catchment areas were selected as typical examples of the lower income areas that account for much of Santo Domingo's population. However, findings from this survey cannot be generalised safely to other parts of the city or to the country as a whole. Not all diagnoses were ascertained with equivalent rigour; self-reported clinician diagnoses, for example stroke and heart disease, may have been under-reported with respect to other conditions such as dementia and depression that were identified through clinical interview, and hypertension and diabetes that were identified through blood pressure and fasting glucose measurement respectively. The overall proportion responding, 95%, was very high. However, only 74% of those agreeing to the survey also supplied fasting blood samples that were used to identify diabetes, hyperlipidaemia, metabolic syndrome and anaemia. While most sociodemographic and health variables were not associated with provision of a blood sample, women, depressed participants, and those with three or more limiting impairments were slightly over-represented among those giving blood samples suggesting some potential for non-response bias in the estimation of the prevalence and correlates of these metabolic and haematological disorders.

In the CARMELA study of cardiovascular risk in those aged 25-64 years in seven Latin American cities [8], the prevalences of hypertension, diabetes and metabolic syndrome were similar to developed countries in Venezuela, Argentina and Chile with a lower prevalence in the less developed Latin American countries. The Dominican Republic is just 90 minutes flying time from Miami but purchasing power parity per capita gross domestic product is only one fifth that of the United States of America ($9,200 versus $46,000). Demographic ageing is much less advanced in the Dominican Republic, with just 5.7% of its population aged 65 and over, compared with 12.6% in the United States of America. Nevertheless, the age and sex adjusted prevalence of dementia and stroke seem already to have reached levels seen in Europe and the United States of America. Hypertension is, if anything, slightly more prevalent in the Dominican Republic. The high prevalence of hypertension and stroke may be explained in part by the high levels of African racial admixture seen in many residents of our catchment areas. Only diabetes and the metabolic syndrome were marginally less prevalent than in the United States of America. At the same time, anaemia, a condition strongly linked to dietary deficiency and poverty is more than three times more common in the Dominican Republic. This pattern of morbidity illustrates the 'double burden' of disease in countries undergoing the health transition - as chronic diseases become more prevalent, infectious and nutritional disorders recede but continue to make important contributions to mortality and disability.

Predicting the impact of the demographic and health transitions requires an understanding of the influences of age, sex and socioeconomic status on disease frequency and underlying risk factor exposures. In the Dominican Republic the negative relationship with increasing age for all cardiovascular risk factors other than hypertension suggests either a cohort effect with increasing exposure levels to be anticipated in future aged cohorts, or selective mortality, in which case improved secondary prevention may lead over time to more chronic morbidity and disability. Most of the disorders that were positively associated with older age (arthritis, anaemia, dementia, stroke, limb impairments) also made strong independent contributions to disability. Thus, in this setting, the numbers of frail and dependent older persons are likely to increase rapidly with demographic ageing. We have previously shown that the age-specific prevalence of dependence in the Dominican Republic is already similar to that in developed nations [27]. Unfortunately, social protection for older people in the Dominican Republic is very poorly developed. Pension coverage has been among the poorest in the world. Our survey indicated a particularly high proportion of older people without children available to care [28], both because of infertility and outmigration.

In the United States of America, the age-adjusted prevalence of hypertension [29], diabetes [21] and the metabolic syndrome [22] is similar in men and women. However, women with diabetes and hypertension may be at greater risk of cardiovascular disease [30]. The prevalence of metabolic syndrome in the Dominican Republic was double in older women that of men, mainly due to a marked excess of obesity and dyslipidaemia among women, and a trend towards a higher prevalence of diabetes. This is consistent with findings among younger Latin Americans [8], but contrasts with the higher prevalence among men in many developed country studies, albeit that recent increases in prevalence have been more rapid among younger women [30]. In our study, older women were also more likely to have been diagnosed as hypertensive, but were generally better controlled. Despite the generally increased cardiovascular risk exposure among women, the prevalence of stroke was markedly higher among men in all age groups and there was little effect of sex on ischemic heart disease. The very low levels of smoking among women may have compensated.

Our most striking findings relate to health inequalities. In countries with high incomes, cardiovascular risk factors and diseases are typically associated with poverty. Our assessment of the effects of the effects of socioeconomic position on chronic disease outcomes may have been limited to some extent by the constrained variance in the predominately low social class catchment areas. Nevertheless, among older people in the Dominican Republic this pattern of association was observed, but only for the more age-related and disabling conditions (dementia, anaemia, depression and most of the chronic limiting impairments). Conversely, cardiovascular risk factors (hypertension, diabetes, dyslipidaemia, obesity and the metabolic syndrome) were each associated with relative affluence, and there was no socioeconomic gradient for ischaemic heart disease or stroke. These findings are consistent with associations observed in a nationally representative survey of Thais aged 50 years and older, where poor self-rated health and functional limitation were associated with lower levels of education, income and wealth, but self-reported diagnoses of hypertension and heart disease were over-represented in the more affluent [31]. Urbanization may have important influences on the socioeconomic patterning of health; in the nationally representative Mexican Health and Ageing Study activity limitation was consistently associated with socioeconomic disadvantage in urban but not in rural areas [32]. In Buenos Aires, in a survey of adults of all ages, hypertension diagnosis was associated with low education and income in both sexes, as were obesity and high body mass index in women but not men [33]. In high income countries metabolic syndrome tends to be inversely associated with socioeconomic position in middle age; few studies have included older persons, but in the United States of America NHANES the effect of socioeconomic position was limited to women aged 25-64 [34]. Little research has been conducted in countries with low and middle incomes, but our findings are consistent with reports from India of a higher prevalence of metabolic syndrome among better educated adolescents [35]. With regard to lifestyle risk factors, much more research has been conducted into associations between socioeconomic position and obesity, documenting the progress of the health transition in countries with low and middle incomes - obesity is no longer confined to the more affluent and inverse associations with socioeconomic position are increasingly observed, particularly for women living in countries with per capita GDP greater than $2500 [36]. In Brazil, for example, there was a clear shift in the burden of obesity towards the poor between 1975 and 2003; the prevalence in women remained stable overall, but increased by 26% in the poorest two-fifths and decreased by 10% among the richest three-fifths [37]. The health transition is therefore likely to accentuate health inequalities. In the United States of America, reductions over the last 30 years in hypertension and dyslipidaemia have not lessened socioeconomic differences, and those for smoking and diabetes have increased due to less smoking in high income groups and increases in diabetes prevalence among those with low incomes [38]. The effect of the health transition on health inequalities may be particularly marked among older people. In the longitudinal American's Changing Lives Study [39], socioeconomic differences in health are modest in youth, increase markedly across middle age into young old age, and decline thereafter, a pattern observed consistently in developed country studies. There is evidence from the American's Changing Lives study that compression of morbidity (the increasing tendency for the preservation of health and functioning into late old age) is mainly observed in the most educated, and that this effect is becoming more pronounced over time.

The prevalence of many chronic diseases and chronic disease risk factors is already nearly as high in low socioeconomic status neighbourhoods in Santo Domingo, Dominican Republic as in the USA. Cautious extrapolation from our cross-sectional data suggests that the overall prevalence of conditions that are strongly age-associated (dementia, stroke and arthritis) is likely to rise in coming years with continued demographic ageing. There is also scope for increases in the prevalence of those cardiovascular risk factors currently associated with relative affluence (diabetes, hypertension, dyslipidaemia, obesity and the metabolic syndrome), if, as has been observed in other settings undergoing the epidemiologic transition, these exposures begin to be concentrated instead among the more numerous economically disadvantaged sectors of the population. This scenario would threaten both an increase in the overall incidence and prevalence of cardiovascular diseases, and in the extent of health inequalities. It will be important to monitor the actual extent and direction of such trends with further epidemiological research, in particular to assess the effectiveness and equity of attempts to improve the prevention and control of chronic diseases. The generally poor health of older women in the Dominican Republic should be a matter of public health concern, and a focus for prevention activities. Controlling for age, women were likelier than men to be hypertensive, dyslipidaemic, obese and underactive and had a higher prevalence of the metabolic syndrome, depression, physical impairments and severe disability. More research into the prevalence and causes of anaemia in older people is indicated, and the government should consider evaluating screening and treatment programmes. Finally, more attention should be given to the social protection of older people, including income security and provision and financing of long term care [40].