Southall And Brent REvisited: cohort profile of SABRE, a UK population-based comparison of cardiovascular disease and diabetes in people of European, Indian Asian and African Caribbean origins

Abstract

Comparisons of chronic disease in migrant and host populations have provided valuable aetiological insights, and furthered the understanding of balance between genetic and environmental influences on the disease. The UK is home to several migrant groups, the largest and longest established being of Indian Asian and African Caribbean origins. One of the earliest and largest waves of Indian Asian migration to the UK followed the 1947 partition of India. As these migrants reached middle age in the 1970s, it became increasingly apparent, both in the UK and elsewhere overseas, that this population was at greatly increased risk of cardiovascular disease (CVD), both coronary heart disease (CHD) and stroke, compared with host populations.1–6 Initial exploration of conventional risk factors, such as diet, total cholesterol, smoking and blood pressure, showed that these differed markedly between ethnic subgroups and were often more favourable than the host UK population, and therefore these factors could not account for the excess CVD risk observed in all Indian Asian subgroups (such as Indians of Gujarati/Hindu Punjabi/Sikh Punjabi origin, Pakistanis and Bangladeshis). Notably, a greater prevalence of diabetes was found in all Indian Asian subgroups, which linked with the observation of greater fasting hyperinsulinaemia, led us to hypothesize that insulin resistance, associated with upper body fat distribution, could underpin the excess diabetes, hypertriglyceridaemia and CHD in Indian Asians.3 This hypothesis was tested in the Southall study, a large population-based inter-ethnic cross-sectional study of middle-aged men and women carried out between 1988 and 1990.7 A large number of migrants of Black African descent from the Caribbean and West Africa also arrived in the UK during the post–Second World War period, encouraged by the UK government in an effort to fill the many vacancies for key workers.8 As these African Caribbean migrants were also reaching middle age in the 1980s, an intriguing paradox was emerging in the UK, which had been noted previously in similar populations in the USA and Caribbean. Despite high rates of diabetes, people of African and African Caribbean descent had relatively low CHD risks compared with White European ethnic origin populations.6,9–11 In stark contrast, stroke-related mortality was doubled and mortality from hypertensive disease was increased between 4-fold (men) and 7-fold (women) in Caribbean-born migrants compared with the general UK population.10 The latter finding was especially puzzling, as previous population-based studies did not provide unequivocal evidence that blood pressure or the prevalence of hypertension were elevated in people of Black African descent.12–14 Exploration of the diabetes/CHD paradox and definitive exploration of ethnic differences in resting and ambulatory blood pressure formed the basis of the Brent population-based comparison of African Caribbean and European middle-aged men and women.15 This cross-sectional comparison followed the successful completion of the Southall study in 1990. The Southall and Brent studies were performed according to identical protocols by the same team, using the same equipments and hospital laboratories. The geographical areas of the baseline studies are contiguous and the two studies were conducted consecutively. In order to improve statistical power, the two baseline data sets have now been amalgamated. Together, the Southall and Brent studies form the basis of the largest UK population-based tri-ethnic cohort. Cross-sectional analysis of the Southall and Brent studies yielded important information on disease prevalence and associations (see below), but could only hint at causality. In addition, first-generation migrants as a cohort are now reaching pensionable age, where disease burden is anticipated to be high, yet little data exist for this age group. In particular, heart failure, as a consequence of CHD, diabetes and hypertension, a rarity in middle-aged individuals, now becomes especially prevalent, causing more than one-third of all deaths in older age.16 Finally, interim mortality follow-up analyses of this cohort showed striking ethnic differences in the effects of dysglycaemia on CVD mortality (see What Has It Found? section), adding urgency to the need for follow-up study of cardiometabolic health and the mechanisms underlying ethnic differences. The Southall And Brent REvisited (SABRE) study was initiated in 2008 as a 20-year follow-up of the two baseline studies. Ethics committee approval was obtained for both baseline studies as well as for SABRE. Broadly, the baseline studies were set up to examine, in cross section, the associations between components of the insulin resistance or ‘metabolic’ syndrome in each of the three ethnic groups and to determine how these associations might underlie the higher rates of CHD in Indian Asians and the higher rates of hypertensive target organ damage and stroke in Afri