Ethnicity as an epidemiological determinant—crudely racist or crucially important?

Nish Chaturvedi

Department of Epidemiology and Public Health, Imperial College School of Medicine at St Mary's, Norfolk Place, London W2 1PG, UK. E-mail: n.chaturvedi{at}ic.ac.uk

The study of disease variation between populations is a central tenet of epidemiological enquiry, which has both generated and tested aetiological hypotheses.1 Yet such inter-ethnic group comparisons have attracted considerable criticism even within the epidemiological community, due in part to suspicions of a covert racist agenda.2–4 Recent developments in genetic research may, to some minds, have broadened the value of ethnic group comparisons, whereas to others, they are an unhappy reminder of eugenics. In addition, such research has often emphasized high rates of disease in minority ethnic groups, for example diabetes, sexually transmitted diseases and certain cancers, with implications that such disease rates (and therefore these ethnic groups) are a problem. It then follows that these differences are best addressed by the imposition of changes in practices, which may not be appropriate, and implicitly encourages assimilation of minority ethnic groups to the Western norm. This issue of the International Journal of Epidemiology is timely in its focus on the epidemiological examination of ethnicity and its association with health and disease. But before discussing the implications of this work, we should first reconsider what we mean by ethnicity.

The terms race and ethnicity are often used interchangeably, but they mean quite different things. ‘Race' had been associated with supposed biological variation, which to many observers essentially meant distinct genetic differences between groups. This notion has been discredited, with a clear demonstration that genetic variation between ‘racial' groups is small, and considerably less than that between individuals from the same supposed racial group.5,6 Racial categorization based on these criteria are therefore inaccurate and misleading, yet studies persist in using this term.7

In contrast ‘ethnicity', the increasingly preferred term, is a complex construct of, assuredly biology, but also culture, language, religion, and, importantly for epidemiologists, distinct health beliefs and health behaviours. Therefore ethnicity cannot be treated as if it were another epidemiological exposure, such as smoking or cholesterol levels, as it encompasses a range of exposures, both biological and environmental.

Clearly this could be problematic when attempting to determine the true aetiological agent, but this is both a strength and a weakness. Firstly, the complex nature of the term encourages the use of a well-defined composite, which can be as precise as the study question requires. This means that simple demonstrations of ethnic differences in disease can provide distinct aetiological clues, given an understanding of ethnic variations in health behaviours and beliefs. For example, certain ethnic groups, including those of African and South Asian descent, have a much lower rate of sudden infant death syndrome than others.8 A likely explanation is that these particular ethnic groups are more likely to nurse infants supine rather than prone, with the latter practice being generally encouraged in the past by Western beliefs.9 The fall in cot deaths since the introduction of clear advice on the supine nursing of infants has confirmed this hypothesis. This observation was made by the simple, quick and relatively cheap analysis of routine data on infant mortality by ethnic group, supplemented by knowledge of differences in nursing habits. If such ethnic differences did not exist, and we wished to study reasons for cot deaths, we would need to rely on expensive and methodologically difficult case-control studies to demonstrate the same phenomenon.

A further strength of research comparing disease risks between ethnic groups is that it may break the confounding due to highly correlated exposures and outcomes observed in many populations. People of Black African descent worldwide have high rates of hypertension and hypertensive end organ damage, most notably stroke, compared to Europeans.10 Yet these same populations appear to enjoy a degree of protection from coronary heart disease, well documented in the UK.11,12 The relative importance of certain exposures for stroke and heart disease—which are highly correlated in European populations—may more easily be disentangled in populations where this correlation is not so tight.

A second example is in the field of cancer epidemiology. Dietary factors are clearly important in the aetiology of several cancers, but are confounded by smoking status and alcohol intake. In general, populations with unhealthy diets have high smoking rates. Further, dietary exposures are notoriously hard to measure, and also demonstrate considerable colinearity. For example, high fibre intake is associated with a high intake of vitamin C, and with other healthy behaviours, such as low smoking rates, absence of obesity and increased physical activity, so it becomes almost impossible to disentangle the real aetiological agent from its confounders. However, ethnic groups are characterized by very different dietary patterns, and by different relationships between exposures of interest and confounders, so ethnic group comparisons offer a relatively simple yet power-ful way of exploring and defining true aetiological agents. For example, different Chinese dialect groups in Singapore have different rates of cancer of the oesophagus,13 and Muslims have low rates of cervical cancer compared with Punjabis.14 Coupled with information on diet, smoking and alcohol intake, these inter-ethnic group comparisons are a powerful way of generating hypotheses for cancer causality.

Migrant studies of one ethnic group to a new location have also proved valuable in establishing disease aetiology. Migrants in general bring their inherent disease risk to the new country, but in many cases they adopt the new country's disease profile within a few generations.15,16 This has demonstrated very clearly the importance and mutability of environmental exposures, compared to genetic factors. This is not to say that genetic susceptibility is not important, but it implies that if genetic factors do play an important role in common chronic conditions, then it is likely to be due to interactions with environmental factors, rather than the genetic factor per se. This understanding has generated interest in large population collections to explore such interactions. Ethnic origin poses a challenge to such genetic collections, as hidden admixture and population stratification may result in spurious associations, and has resulted in a search for alleles at marker loci, which could help to assign ancestry. This has clear implications for the future of ethnicity and genetics research. For example, the susceptibility of South Asian people to tuberculosis may be associated with an interaction between vitamin D deficiency and vitamin D receptor polymorphisms,17 indicating that interventions for prevention and treatment should differ. But in the field of pharmacogenetics, for example, the idea that people with a certain genetic make up may respond less well, on average, to treatment, has implications that treatment may be withheld from some groups, perhaps defined by ethnicity.

The critics of ethnicity-based research have numerous concerns, not least that such research is ‘racist', and results in victim blaming. Examples are the infamous Tuskegee experiment of untreated syphilis in people of Black African descent in the US,18 at a time when evidence-based treatment options were available, and the ‘stop rickets' programme in the UK, which focused on individual dietary change, rather than mass vitamin D fortification of foods which had been previously applied to the general population.19

But those who would deny the value of ethnic group comparison studies may also be turning their backs on important work exploring equity in access to health care and public health preventive interventions. Work on the decline in avoidable deaths in Singapore,20 published in this issue of the IJE, show that benefits have largely been enjoyed by the majority Chinese population, while Indian men in particular were found to have increased rates of death from such avoidable conditions. In the US, it is also clear that poor access to appropriate care may be accounted for not just by the low socioeconomic status of people of Black African descent, but could also be due to health care workers' stereotypical responses to skin colour.21,22 It is clear therefore that access to high quality health care is of value in promoting good health, and that this access is socioeconomically and ethnically biased. By not performing this kind of work, such problems would largely remain ignored. Attendance for cervical and breast cancer screening is substantially lower in South Asian people compared with the general population.23 Although this may be entirely appropriate in view of the lower risks of disease in South Asians, nevertheless, if screening is viewed as important, as it may be for diabetes in this ethnic group, this may require different public health incentives to attend for screening, and is unlikely to be due to a reluctance to attend for preventive health care, or to a lack of understanding of its importance, as uptake of childhood immunization is high in these groups.24

A further concern revolves around the definitions used for ethnic groupings, with critics rightly pointing out that such definitions lump together people of varying cultural and health beliefs and health behaviours.25 The term Asian in the UK has been used to include people as varied as the socioeconomically deprived, heavy smoking and meat eating Bangladeshi population in East London, and the socioeconomically affluent, vegetarian non-smoking Punjabi Sikh population in West London, but this may not always be inappropriate. People from the Indian subcontinent share a predisposition to heart disease, such that whatever subgroup is considered, the risk of heart disease is greater in these populations than in people from Western Europe.26 So when determining why heart disease rates differ, it is acceptable to include all these groups. Indeed, it has a certain strength, in that given that we know about the considerable differences in health behaviours between these groups, the fact that an enhanced risk of CHD is shared across groups despite these differences indicates clearly that the explanation does not lie with dietary and smoking differences. But if we wished to study the reasons for variation of heart disease between these subgroups, then clearly such lumping would be inappropriate. Further, the examples of cancer aetiology described above benefit from a detailed exploration of ethnic subgroups.

Critics have also argued that the difficulty in defining ethnicity, and the fact that different definitions have been used in different studies, mitigates against this kind of research.2 But this criticism could, and has been applied to research on socioeconomic status. Few would deny that socioeconomic status has an important impact on health, and countless studies have attempted to quantify and determine reasons for this.27 Indeed in the US, ethnicity has often been used exclusively as a measure of socioeconomic status.28 Yet socioeconomic status too is a composite measure of income, education, attitudes and beliefs regarding health. It could therefore also be argued that it would be better to measure individual components of socioeconomic status in epidemiological studies. But this is to deny that socioeconomic status is anything other than these lifestyle variables. Subtle, harder to measure variables, and interactions between variables, may not be sought, unless socioeconomic status differentials, unexplained by these factors, were not demonstrated in the first place.

A further challenge to the study of different ethnic groups is the use of measurement instruments and normal values, which are usually derived from European populations. For example, the Rose angina questionnaire may be less valid in certain ethnic groups, as the understanding and expression of chest pain due to angina may differ, a point elegantly illustrated in this issue.29 The normal values of body mass index and left ventricular mass may not be applicable in non-European ethnic groups, and may also not be comparable between ethnic groups.30,31 This point is well illustrated by an article in this issue demonstrating that the density of breast tissue on mammography differs by ethnic group, with implications for defining normal values and describing future risks.32

The study of ethnic groups is of crucial value in determining the role of different exposures to disease risk, which could not be delivered by sole study of a single ethnic group, and is an easy way of providing aetiological clues. Further, the demonstration of ethnic group susceptibility to disease is of importance in directing public health interventions and health care resources, and in order to fully benefit from health care interventions, treatments may need to be tailored, by dose, type and intervention threshold, to a particular ethnic group. Just as we should not refuse to study socioeconomic differences in disease, simply because socioeconomic status is hard to define, and may, if not interpreted with sensitivity, be used as a weapon for victim blaming, so too we should not shy away from studying ethnic variations in disease.

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