South Asians, ethnicity and cardiovascular disease:no easier to unravel than genetics
South Asians (people from India, Pakistan, Bangladesh, Nepal, Maldives and Sri Lanka) seem to have high rates of heart attacks and stroke (collectively known as cardiovascular disease) that is not explained by widely-known risk factors such as high blood pressure or smoking. When you consider that the Indian subcontinent accounts for over 1.4 billion people and carries the greatest global burden of cardiovascular disease, it is important to understand what puts these people at increased risk, even when they migrate to other countries.
I have spent this week at Northwestern University in Chicago, learning about their great programme of clinical research into the causes of cardiovascular disease. I heard of my favourite ever acronym for a study, “MASALA” (The Mediators of Atherosclerosis in South Asians Living in America) which is recruiting 1000 South Asians in Chicago and San Francisco and will compare risk factors and development of heart disease. Interestingly, whereas study of South Asians has been at the forefront of research regarding ethnicity and cardiovascular disease in the UK, study of ethnic disparities in health has tended to focus on African American populations.
It is well-known that risk of stroke and risk of heart disease are linked to socio-economic status. In a London-based study of 1400 South Asian men, deaths from heart attacks and stroke were more likely in men whose fathers had “manual” occupations, or in men who had completed less than 11 years of formal education. This effect was more marked in men who themselves were engaged in manual work, and the authors concluded that “childhood socioeconomic position”, together with adult socioeconomic position cumulatively influenced the risk of dying of from cardiovascular disease. Studies in Scotland have shown similar results. There is definitely a relationship with socioeconomic deprivation and heart disease in Indians living in India How do we best measure “social disadvantage”? Do we need a “social disadvantage index”, as has been proposed by Canadian researchers led by Salim Yusuf? They found that social disadvantage was higher among older people, women, and non-white ethnic groups. The jury is still out regarding the best way to measure socioeconomic status, but it does appear that ethnic minorities, including South Asians, tend to suffer from a greater degree of social deprivation, which may contribute to cardiovascular disease. However, results of the 2010 US census show that Indians living in America have higher income than other ethnic groups and are the fastest growing ethnic minority, so socioeconomic status is not telling the whole story about the high rates of cardiovascular disease in America.
What about migration? The Indian Migrant Study and other studies suggest that South Asians who migrate within their countries from rural to urban areas, as well as those who move to Western countries, seem to have increased risk of heart attack and stroke, and there seem to be roles for socioeconomic status, risk factors and migration itself.
In the UK, one study has shown higher reported “psychosocial adversity” in South Asians who had suffered heart attacks, compared with UK whites, in terms of greater chronic stress, in the form of financial strain, residential crowding, family conflict, social deprivation and discrimination, despite larger social networks. These effects were largely independent of socioeconomic status. Linguistic and cultural barriers have been previously cited as potential causes of higher rates of heart attacks in South Asians, but it is not as simple as that, as Hindus and Sikhs were more likely to seek help for chest pain than their white European counterparts in one study.
Lifestyle is also likely to be very important and many differences seem to manifest in early childhood, such as lower physical activity in South Asians and higher saturated fat intake in Indians.
Last year, I wrote about research looking at ethnicity and risk of stroke in the UK and the USA. As I said then, the research continues to describe differences in traditional and non-traditional risk factors, but not so much has been done to design and implement interventions to reduce the variations in health due to ethnicity. The interplay of environmental factors and ethnicity in cardiovascular disease is no less complex than the interplay of genes and environment. Encouragingly, the discourse about the best policies to tackle health disparities related to ethnicity is well underway on both sides of the pond.
The Pubmed Index and lessons from ethnicity and stroke
I have started using what I have termed the “Pubmed index” to show trends on a particular topic in medical research. In essence, after an unrestricted search in Pubmed to find how many articles mention my search terms, I look at how many hits I get by restricting the search to 10, 5 and 1 years, to find out how much of that research has been done recently, and how “hot” the topic is. “Ethnicity” and “stroke” gives 2411 references, of which 1719 hits are in the last 10 years, 1067 in the last 5 years, and 182 in the last year. In other words, most of the work on ethnicity and stroke has been done in the last 10 years, and the topic is still being researched.
This makes sense because 10-15 years ago, data from the UK and North America showed that there were differences between black and white populations in stroke. For example, there was an excess of haemorrhagic stroke in black populations. In America, this ethnic group had reduced access to stroke services and higher mortality rates, leading to the so-called “stroke belt” in the South-eastern region of the US. The research base for racial or ethnic disparities in stroke has increased across ethnic groups, including Hispanics and South Asians. More recently, with epidemiologic transition and rising prevalence of chronic diseases, data is showing differences in stroke subtypes and risk factors between black and white populations in sub-Saharan Africa.
Ethnicity research has faced hurdles, partly because inadequate definitions and partly due to concerns about political correctness, and ethnic minorities are still under-represented in clinical trials in stroke. With increasing migration within and between populations, a greater consensus is required regarding future studies of ethnic disparities. More comparisons need to be made between ethnic groups as “immigrant populations” versus the ethnic groups in “home populations”, to consider what the effects of migration on disease in its own right are.
Prospective studies which follow populations over time, like the REGARDS(REasons for Geographic And Racial Differences in Stroke) Study in the US, are a preferred methodology than retrospective studies. There has been surprisingly little research about ethnicity in relation to other “non-modifiable risk factors”, such as family history or genome-wide scans, which will help in characterising how ethnicity is associated with risk factors, behaviour, treatment and disease outcomes.
This week, the BMJ published a study online showing that black patients and patients from higher socioeconomic groups were more likely to be admitted to stroke units than white patients and poorer patients in an area of London. These results are interesting because historically data about ethnic disparities, regardless of the country or the setting, has invariably shown that ethnic minorities have less access to services. A rapid response to the article, rightly concludes that we need to know more accurately what is happening overall in the population, rather than having isolated studies. As unwarranted variations in population medicine are increasingly studied and linked with changes in health policy, proactive, rather than reactive studies of ethnicity are required. In other words, it is not enough to just describe variations and disparities, we need to move towards explanations and potential actions which can reduce disparities, and need better population-wide surveillance.