Cardiovascular Disease and Diabetes

Diabetes, both type 1 and type 2, is increasing in prevalence and it is estimated that three million individuals in the UK will have type 2 disease by 2010 (Gale, 2002; Fisher, 2003). Overall the numbers of people with type 2 far exceed those with type 1 and, in addition, they are usually middle aged or elderly and often present with concomitant CVD risk factors. However, comments such as 'Diabetes mellitus, and particularly non-insulin dependent diabetes mellitus increases the risk for all manifestations of vascular disease' (Laakso, 1998) and 'CVD complications occur more often in patients with NIDDM than in patients with IDDM' (Laakso and Lehto, 1997) can easily be misconstrued. Epidemiological studies measure outcome in a number of different ways. While absolute numbers can be counted, other measurements, adjusted for the size of the group, are more commonly used. For example, a rate (of an event) can be calculated as the number of such events per 100 000 people per year. Another commonly used epidemiological measure is the standardised mortality ratio (SMR), which is calculated as the number of observed deaths in the study population compared with the number of deaths that would be expected if general population rates, allowing for the size and age distribution of the study group, were applied. Once the smaller numbers and younger age distribution of people with type 1 diabetes have been taken into account, comparisons can be made.

A direct comparison of all-cause mortality, matched for age, calendar period and country, was made in the WHO Multinational Study (Head and Fuller, 1990). They studied mortality among 4740 diabetic men and women, aged 35-55 years, from 10 centres around the world and they calculated age-adjusted death rates, by centre, separately for type 1 and type 2 diabetes. Death rates for patients with type 1 diabetes were almost always higher than for the corresponding type 2 group.

Standardised mortality ratios, which take into account the underlying mortality in the general population, can also be compared. All-cause mortality in middle-aged and elderly patients with type 2 diabetes is generally 2-4 times higher than the mortality in the general population (Manson etal., 1991; Moss etal., 1991; Muggeo etal., 1995).

The Diabetes UK Cohort Study (Laing etal., 1999a), of patients with type 1 diabetes, reported an overall SMR of 2.7 for men and 4.0 for women. However, at younger ages in type 1 studies the SMRs for all cause mortality are higher, partly reflecting the much lower mortality in the general population in this age group, with SMRs for the under 40s from Pittsburgh being 5.0 for men and 9.3 for women (Dorman etal., 1984). The Diabetes UK Cohort Study reported all-cause SMRs of 3.7 for men and 4.9 for women in the 30-39 age group. In both studies the relative risk of death was higher in the women than men.

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