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Vive la difference

22 Oct, 2008

Ethnic differences seem to play a major role in a person’s susceptibility to cardiovascular disease. Chrissie Giles talks to Professor Nishi Chaturvedi, who has been at the forefront of this research for over 20 years, about a major new study, SABRE.

Some 20 years ago researchers scouring routine mortality statistics from the UK came across something strange. They found that first-generation migrants of Indian Asian descent had significantly more heart attacks and strokes than white European people. In contrast, African Caribbean people, although also at higher risk of stroke, seemed to be at a significantly lower risk of coronary heart disease – particularly men, whose risk was half that of their European counterparts. Nishi Chaturvedi – now Professor of Clinical Epidemiology at the International Centre for Circulatory Health, part of Imperial College Healthcare NHS Trust – was intrigued.

To try to unpick this apparent ethnic aspect to susceptibility to heart disease, she and her colleagues performed two surveys of 4000 Londoners between 1989 and 1991: the Brent study, which looked at an African Caribbean population, and the Southall study, which looked at an Indian Asian group.

These studies found that, although both ethnic minority groups had higher rates of diabetes than Europeans, the profiles of fats (lipids) in the bloodstream varied. Indian Asians tended to have ‘unhealthy’ blood lipid profiles, which are linked to an increased risk of heart disease, while the African Caribbeans had ‘healthy’ lipid profiles.

Although the variation in lipid profile appeared to be behind the ethnic differences in susceptibility to heart disease, it was not clear why diabetes affected lipid profiles differently in the two groups. The study follow-up confirmed that adverse lipid profiles and diabetes contributed to the high rate of heart disease in Indian Asians, but did not explain it completely.

In addition, the studies also showed that the greater risk of stroke in both ethnic groups was strongly related to their higher rates of diabetes, even though blood pressure, the most important risk factor for stroke, differed substantially. So, in 2008, Professor Chaturvedi and colleagues embarked on SABRE (Southall and Brent Revisited). This study, funded by the British Heart Foundation and us, revisits the original participants from the Southall and Brent studies, a unique and large population that has been followed for a long period of time.

“We’re trying to explore the idea of ‘diabetes toxicity’,” says Professor Chaturvedi. “Why does diabetes seem to be particularly bad for an Indian Asian or African Caribbean in terms of cardiovascular disease?” The researchers have begun seeing the individuals from the original studies again, a process expected to take three years. The Southall and Brent studies captured only mortality data since the baseline studies, so now the researchers are studying the participants’ medical records to see what cardiovascular events have occurred since the first studies. They are also looking at early signs of cardiovascular disease that are not apparent to the individual.

Professor Chaturvedi hopes that understanding these risk factors better will lead to a change in the way that people are diagnosed and treated for cardiovascular diseases. “We know a lot about how to prevent disease occurring in the first place if you’re European, and we know a lot about how to treat people who are at high risk of cardiovascular disease,” she says. However, the story isn’t so clear for Indian Asians and African Caribbeans. “Given the differences in risk, we don’t think the same cut-off points should apply,” she says.

This kind of research should benefit everyone, not just those from the populations studied. For example, understanding how African Caribbean men are ‘protected’ from heart disease could provide information relevant to the health of people in other ethnic groups.

What are the ethical implications of this kind of work? “Sometimes there are criticisms that you’re compartmentalising groups that can’t be compartmentalised,” says Professor Chaturvedi. “People ask, ‘What do you mean by “Indian Asian” or “African Caribbean”? Is this a genetic or a racial thing?’”

She explains that ‘ethnicity’, the definition they use to group individuals, may appear somewhat heterogeneous, being based on a number of factors including lifestyle, religion, language, cultural beliefs and behaviour – which can differ substantially between groups. “Nevertheless, whether you’re talking about a Punjabi Sikh, a Muslim or a Bangladeshi, all those groups share a predisposition to heart disease and diabetes… It does go back to an underlying biological explanation rather than one based on lifestyle,” she says.

“In order to reduce that risk you have to appreciate why it occurs in the first place. This comparative epidemiology will help us to understand that difference.”

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