The number of deaths attributable to diabetes calculated in this study is three to four times greater than those given in the conventional international statistical reports largely based on diabetes given as an underlying cause on death certificates 1 . The number of excess deaths attributable to diabetes is similar in magnitude to those reported for HIV/AIDS in the year 2002 1 .
The higher proportion of excess deaths in females compared to males is explained by their lower background mortality levels, and the larger increase in the absolute risks of dying in women compared with men if diabetic, in almost all age groups. Although diabetes is often perceived as a disease of affluent countries, the proportion of all deaths that are attributable to diabetes in developing countries is not negligible. This issue of considerable preventable mortality due to diabetes has been recognized for type 1 diabetes 2 , but the vast majority of persons have type 2 diabetes.
A potential source of error in this study is that relative risks of dying in persons with diabetes, compared to those without diabetes, were obtained from studies conducted in a relatively small number of countries, most of them developed. Because little data from developing countries has been published in a format suitable for this study, unpublished data from the DECODE and DECODA studies have been used (Jaakko Tuomilehto, personal communication). This is unlikely to have overestimated the burden of diabetes mortality in developing countries, as available studies from Mauritius and Brazil show that the risk of death is about three times higher in persons with known diabetes, compared to individuals with normal blood glucose, or the general population 3 4 . This is consistent with the sparse information available from low-income countries indicating a poor prognosis for persons with diabetes 5 .
The latest validated available country-specific mortality rates were for the year 2001 and it is possible that the overall number of deaths in each country has not been accurately estimated because these mortality rates, rather than those for the year 2007, were applied to the estimated population size in the year 2007. It is unlikely, however, that the country-specific adult mortality rates have changed substantially since 2001.
The age and sex-specific prevalence of diabetes by country used in this study was estimated from population-based surveys. However, population-based studies of diabetes prevalence have not been conducted for more than one-third of the countries of the world, so the diabetes prevalence for many countries was obtained by extrapolation, and error is possible. The prevalence estimates include both diagnosed and undiagnosed diabetes.
The relative risks of dying were derived from cohort studies of patients with diabetes. The proportion of undiagnosed diabetes varies between populations, but is rarely lower than 30%, and is often higher than 50%, even in developed countries. The relative risk of dying may not be the same for diagnosed and undiagnosed diabetes, but published data from the DECODE study show that mortality in people with undiagnosed diabetes is as high as in people with previously diagnosed diabetes 6 . Moreover, people with a lesser degree of hyperglycaemia, impaired glucose tolerance (IGT), have a 40% increased mortality, regardless whether they progress to diabetes or not 7 .
One of the reasons for non-diagnosis of diabetes could be lack of symptoms, which could reflect a milder metabolic disturbance and possibly a better prognosis. If so, the calculated 3.8 million deaths could be an overestimate of the true number. The results of the DECODE study indicate that risk of death is not significantly different between previously and newly diagnosed persons with diabetes, when unrecognized diabetes is defined by the two-hour post-load glucose value rather than by fasting glucose 6 .
Although these mortality estimates are unlikely to be accurate, given the assumptions on which the calculations are based, they do provide a more realistic estimate of diabetes-attributable mortality than currently exist. The number of deaths related to hyperglycaemia would likely be even higher if mortality attributable to IGT were taken into account 8 .
1.World Health Organization The World Health Report 2004. Geneva, Switzerland: World Health Organization; 2004
2.Matsushima,M. LaPorte,R.E. Maruyama,M. Shimizu,K. Nishimura,R. Tajima,N. Geographic variation in mortality among individuals with youth-onset diabetes mellitus across the world. DERI Mortality Study Group. Diabetes Epidemiology Research International. Diabetologia.1997; 40(2): 212-216
3.Salles,G.F. Bloch,K.V. Cardoso,C.R. Mortality and predictors of mortality in a cohort of Brazilian type 2 diabetic patients. Diabetes Care.2004; 27(6): 1299-1305
4.Shaw,J.E. Hodge,A.M. de Courten,M. Chitson,P. Zimmet,P.Z. Isolated post-challenge hyperglycaemia confirmed as a risk factor for mortality.. Diabetologia.1999; 42(9): 1050-1054
5.Lester,F.T. Clinical features, complications and mortality in type 2 (non-insulin dependent) diabetic patients in Addis Abeba, Ethiopia, 1976-1990. Ethiop Med J.1993; 31(2): 109-126
6.The DECODE study group Is the current definition for diabetes relevant to mortality risk from all causes and cardiovascular and noncardiovascular diseases?. Diabetes Care.2003; 26(3): 688-696
7.Qiao,Q. Jousilahti,P. Eriksson,J. Tuomilehto,J Predictive properties of impaired glucose tolerance for cardiovascular risk are not explained by the development of overt diabetes during follow-up. Diabetes Care.2003; 26(10): 2910-2914
8.The DECODE study group Glucose tolerance and mortality: comparison of WHO and American Diabetes Association diagnostic criteria. European Diabetes Epidemiology Group. Diabetes Epidemiology: Collaborative analysis Of Diagnostic criteria in Europe. Lancet.1999; 354(9179): 617-21