Age, body mass index and Type 2 diabetes—associations modified by ethnicity
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The aim of this study was to assess the effect of ethnicity on the association between age and body mass index as well as the prevalence of diabetes.
We selected population-based studies carried out after 1980 in the DECODE/A studies representing different ethnic groups: 11 European, 1 Maltese, 3 Indian, 2 Chinese and 3 Japanese surveys. The total numbers of subjects were 14,240 men and 15,129 women who were 30 to 89 years of age. Diabetes was diagnosed according to the 1999 World Health Organization criteria based on a standard 75 g OGTT. Sex-specific prevalence of diabetes by age and BMI was stratified by ethnic group, in particular the interaction of ethnicity on the associations between age/BMI and the prevalence of diabetes.
The prevalence of diabetes was higher in studies from India and Malta compared to Japan, China, and the rest of Europe. The association between BMI and diabetes, adjusted for age, showed noticeable differences between the ethnic groups with an increase in prevalence starting at a BMI between 15 and 20 kg/m2 in the Maltese and Indian populations compared to 25 kg/m2 in Europeans.
The effect of BMI on the age-adjusted prevalence of Type 2 diabetes was modified by ethnicity with considerably lower thresholds in Indian and Maltese subjects compared to those from the rest of Europe. This difference should be reflected in national and international recommendations regarding "optimal" BMI.
KeywordsDiabetes prevalence ethnic BMI age DECODE DECODA
Diabetes Epidemiology Collaborative analysis of Diagnostic criteria in Europe
Diabetes Epidemiology Collaborative analysis of Diagnostic criteria in Asia
World Health Organization
Type 2 diabetes mellitus is among the fastest growing diseases in the world due to a combination of unfavourable changes in modifiable risk factors and a high genetic susceptibility in many populations . The rapid increase of diabetes worldwide is primarily a consequence of aging in most populations and the increase of obesity and physical inactivity .
Cross-sectional [3, 4, 5] and prospective [6, 7, 8] studies have shown an association between age and body mass index and the prevalence and incidence of Type 2 diabetes. These associations have been consistently reported from different ethnic groups.
The prevalence of Type 2 diabetes varies considerably between countries, regions and ethnic groups . Although the variation in prevalence has been associated with regional variations in the distribution of established behavioral and environmental risk factors [3, 4, 5], it has not been fully explained by these factors.
The aim of this study was to assess the effect of ethnicity on the association between age, BMI, and the prevalence of Type 2 diabetes based on the analysis of data from two collaborative studies, "Diabetes Epidemiology: Collaborative analysis of Diagnostic Criteria in Europe (DECODE)" and "Diabetes Epidemiology: Collaborative Analysis of Diagnostic Criteria in Asia (DECODA)" [9, 10].
Subjects and methods
Participants and study design
We invited centres in Europe and Asia, who had done surveys on the prevalence of Type 2 diabetes, using a standard 75-g OGTT with fasting and 2-h glucose measurements, to participate in the DECODE and DECODA studies. The study design and data collection have been described in detail in previous publications [9, 10]. Each centre provided original data including age, sex, date of examination, height, weight, status of known diabetes, glucose concentrations (fasting and 2-h), blood specimen used (venous whole blood, venous plasma, serum or capillary whole blood) and the method of the glucose assay used. BMI was calculated as weight (kg)/height2 (m). Data were sent to the Diabetes and Genetic Epidemiology Unit at the National Public Health Institute in Helsinki, Finland.
Characteristics of survey analysed for this study
Study and ethnic
Numbers (% men)
BMI [Median (range)] (kg/m2)
Chennai (Madras), India
Chennai urban (CUPS), India
Finrisk 87, Finland
Finrisk 92, Finland
Pol-Monica Kracow, Poland
Subjects who had previously been diagnosed with Type 2 diabetes or those who were newly-diagnosed with a fasting plasma glucose over 7.0 mmol/l and/or a 2-h plasma glucose over 11.1 mmol/l after a 75-g OGTT according to the WHO criteria , were considered as having diabetes. For surveys using serum, venous whole blood or capillary whole blood, the corresponding diagnostic cut-points were: 7.0, 6.1 and 6.1 mmol/l (fasting) and 11.1, 10.0 and 11.1 mmol/l (2-h post load).
All analyses were carried out and are reported by sex. The sex-specific mean levels of BMI stratified by age were compared between ethnic groups by ANOVA in 10-year age group strata. The logistic regression model was adopted to estimate the prevalence of diabetes (log odds (diabetes) = intercept + α age + γ ethnicity) with age as a continuous variable and ethnicity classified as European, Maltese, Indian, Chinese and Japanese. The prevalence of diabetes adjusted for age and BMI was estimated using a spline model with age, BMI and ethnicity included as explanatory variables and where age of 45, 55, 60 and 70 years and BMI of 22, 24, 26 and 28 kg/m2 were adopted as knots (for details see equation in Appendix 1). The interaction effect of ethnicity and age/BMI was done by using the chi-squared log likelihood ratio test with nested models. Similar statistical analyses were done to test the interaction effect of sex on the association between age, BMI and prevalence of diabetes within the same ethnic group. All statistical analysis was done by using SPSS for Windows version 11.0. A p value of less than 0.05 was considered to be statistically significant. All participants gave their informed consent to participate in the study according to the declaration of Helsinki and other ethical standards relevant to the time when the data was collected.
Prevalence of diabetes by age
Interaction between ethnicity, age, BMI and prevalence of Type 2 diabetes
This study confirms the large ethnic, regional and sex-dependent variation in the age-adjusted prevalence of Type 2 diabetes as reviewed by the WHO . This study also shows that this variation cannot be explained by differences in BMI between the populations. We found that ethnicity modified the effect of BMI on the age-adjusted prevalence of Type 2 diabetes. The association between age and the prevalence of diabetes was also modified by ethnicity. Together these observations indicate that ethnicity not only affects the over all susceptibility to diabetes, but also modifies the effect of established risk factors for Type 2 diabetes. Ethnicity is associated with a long list of factors including genetic constitution, living conditions, life style factors and anthropometry. Thus it is not simply the difference in BMI distributions that explain the worldwide variation in the age-standardized prevalence of Type 2 diabetes.
The relative contribution of reduced insulin secretion and reduced insulin sensitivity as contributors to the development of diabetes differs between various ethnic groups. Asian Indians have been shown to be more insulin-resistant than most other ethnic groups . Preferential deposition and distribution of fat in the abdominal region is associated with reduced insulin-mediated glucose disposal, independent of an overall degree of obesity [28, 29, 30]. Anthropometric studies have shown that the distribution of fat differs between ethnic groups, and thus identical distributions of BMI in two populations could still reflect large differences with respect to the accumulation of intra-abdominal fat, and Asian populations have been shown to be more prone to abdominal obesity and low muscle mass [27, 31, 32, 33]. Thus excessive insulin resistance related to intra-abdominal fat deposition and low muscle mass could explain the very high prevalence of diabetes in the Indian populations despite the absence of significant obesity as expressed by BMI.
Indians had a very high prevalence of diabetes adjusted for age, and the risk started to increase at very low levels of BMI. The explanation should probably partly be found through the same explanations as those given for Chinese and Japanese subjects, but in addition to this there could also be an association with the very low birth weight often seen in Indian subjects . It has been suggested that low birth weight as an indicator of intrauterine malnutrition could be an important contributor to the otherwise peculiar prevalence pattern found in Indian subjects . A recent study carried out in India has confirmed the importance of intrauterine and early childhood development for the risk of Type 2 diabetes .
In contrast, insulin deficiency rather than insulin resistance has been reported to play a prominent role in the progression of diabetes in Japanese subjects . Thus the insulin secretory inferiority could partly explain the higher prevalence of diabetes in non-obese Japanese and Chinese populations compared to European populations.
Our finding that the association between BMI and diabetes is modified by ethnicity supports the need for a redefinition of obesity based on geographical region and ethnicity as suggested by the joint enterprise of the Regional Office for the Western Pacific of the WHO, the International Association for the Study of Obesity and the International Obesity Task Force in 2000 . Although BMI is a simple and non-invasive method for assessing obesity and excessive fat stores within a single population, it is less reliable when it comes to comparing body composition between populations [38, 39]. Our findings indicate that BMI is an unreliable measure for studying the association between obesity and insulin resistance or Type 2 diabetes across ethnic groups. Further studies should focus on the potential ethnic differences in the regional fat distribution, and simple and reliable instruments are needed to evaluate the metabolic consequences of visceral fat obesity between geographical regions and ethnic groups. The simple measurement of waist circumference could be a suitable measure.
We found that the overall effect of age on the prevalence of diabetes differed considerably between the ethnic groups even in subjects with the same BMI. Moreover we found that the prevalence started to increase at a higher age in the European cohort than in any other group studied. These observations must, however, be interpreted with some caution as differences in mortality between ethnic groups will contribute to differences in the prevalence of diabetes. The delayed clinical diagnosis in the Chinese and Japanese populations combined with the excess mortality associated with Type 2 diabetes  could explain the lower prevalence of diabetes in the old age groups in some Asian populations.
The increasing prevalence of diabetes worldwide has been labelled an "epidemic of diabetes" and at present the focus is on how this "epidemic" can be prevented. Three different studies have shown that dietary modification and increased physical activity can reduce the 6-year risk of developing diabetes by 40 to 60% in high-risk subjects with IGT [41, 42, 43]. These recent trials have provided guidelines as to who should be targeted in the preventive process. Our data showing that ethnicity has a major effect on the deleterious consequences of obesity with aging suggests that target groups for intervention strategies (based on age and BMI) may have to be defined for each ethnic group separately, and future studies should aim at defining these high risk groups.
Japan Diabetes Foundation and Japan Arteriosclerosis Prevention Fund (Japanese Ministry of Health, Welfare and Labour) supported a fellowship for T. Nakagami. The DECODE/A study has been carried out by the support of grants from Novartis Pharma AG, Basel Switzerland and the Finnish Academy (grants 46558, 76502 and 77618). The DECODE Study was funded by Novo Nordisk, Bagsværd, Denmark.
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