1 Division of Epidemiology, Stockholm Centre of Public Health, and Department of Epidemiology, Institute of Environmental Medicine, Karolinska Institutet, Stockholm, Sweden
2 Department of Molecular Medicine, Karolinska Institutet, Stockholm, Sweden
3 Department of Public Health, University of Helsinki, Helsinki, Finland, and Department of Mental Health, National Public Health Institute, Helsinki, Finland
Correspondence: Sofia Carlsson, Division of Epidemiology, Karolinska University Hospital, S-171 76 Stockholm, Sweden. E-mail: sofia.carlsson{at}imm.ki.se
SirsA possible protective effect of coffee consumption on the risk of developing type 2 diabetes was suggested recently in a study by van Dam and Feskens.1 This finding is supported by a Japanese study,2 whereas no protective effect of coffee was reported in a recent Finnish study3 and in Pima Indians.4 This issue is important to study further, especially in view of the prevalent use of coffee. The aim of this study was to explore the association between coffee consumption and type 2 diabetes in a large cohort of Finnish twins, thereby taking hereditary and early childhood factors into account.
The Finnish Twin Cohort was compiled in 1975 and includes all same-sex twins born in Finland before 1958.5 We restricted the study population to subjects 3060 years old at baseline as in the Dutch study (n = 10 652). Information on coffee consumption and other lifestyle factors was collected by questionnaires in 1975 and 1981. By record linkage to national registers in Finland of hospital discharges and prescribed medication, 408 incident cases of type 2 diabetes were identified during 20 years of follow-up (19761995) using the procedures described by Kaprio et al.6 To analyse the association between coffee consumption and type 2 diabetes we calculated relative risks using Cox proportional hazards. Time-dependent analyses were performed using baseline information on coffee and other lifestyle factors together with information from the 1981 follow-up questionnaire. To take the intra-pair correlation into account we included gamma frailty components7 analysed by S-plus.8 In addition, we calculated odds ratios (OR) of type 2 diabetes in coffee discordant twin pairs using conditional logistic regression analyses (SAS PHREG).
The results were compatible with a reduced incidence of type 2 diabetes in subjects consuming 7 cups of coffee per day, compared with subjects consuming
2 cups per day (Table 1). Exclusion of abstainers from the reference category resulted in a relative risk of 0.70 (95% CI: 0.49, 0.99) in subjects consuming
7 cups of coffee per day. When coffee consumption was analysed as a continuous variable, the relative risk was 0.89 (95% CI: 0.73, 1.08) per 5 cups of coffee per day. Analyses of coffee discordant twin pairs suggested a reduced risk of type 2 diabetes in twins with moderate or high coffee intake compared with their low-consuming siblings.
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Overall the association between coffee and diabetes was weaker than in the Dutch study and there was no indication of a doseresponse pattern. It has been suggested that the type of coffee, e.g. boiled versus filter coffee, may have bearing on possible health effects.9 However, in Finland as in The Netherlands, filter coffee is the predominant type. There are also other possible explanations for the somewhat weaker results of the present study. The long follow-up period: 20 years instead of 10 years as in the Dutch study, may have induced misclassification of coffee consumption. Such misclassification would tend to dilute the association between coffee consumption and type 2 diabetes and also distort a possible doseresponse relationship. We tried to minimize this problem by updating exposure information during follow-up. It should also be noted that analyses based only on the first 10 years of follow-up did not change our results. Adjustment for lifestyle factors such as smoking and body mass index strengthened the association between coffee intake and type 2 diabetes. A limitation was that we were not able to take into account whether milk, cream, and/or sugar was added to the coffee. Also, diet could be related to coffee consumption as well as the risk of type 2 diabetes but unfortunately this information was not available. It is possible that more complete adjustment for lifestyle factors would have strengthened the association between coffee consumption and type 2 diabetes. Finally, our cases were identified by record linkage. This method has been shown to detect 80% of known diabetes cases6 but will miss cases treated by diet alone. These cases would have been included in the Dutch study as they investigated self-reported diabetes. Still, van Damm and Feskens did perform separate analyses in medication-treated cases and reported similar results as for the whole cohort.
In conclusion, our results are compatible with a protective effect of coffee consumption on the risk of type 2 diabetes. Future studies are, however, needed to further explore this issue, addressing what the possible underlying mechanisms may be.
Acknowledgments
The Finnish Twin Cohort data collection was partially supported by grants from the Academy of Finland (#51266) and the Sigrid Juselius Foundation. Identification of diabetes cases was partially supported by a grant from the Swedish Council for Working Life and Social Research.
References
1 van Dam RM, Feskens EJ. Coffee consumption and risk of type 2 diabetes mellitus. Lancet 2002;360:147778.[CrossRef][ISI][Medline]
2 Isogawa A, Noda M, Takahshi Y, Kadowaki T, Tsugane S. Coffee consumption and risk of type 2 diabetes mellitus. Lancet 2003;361:70304.[Medline]
3 Reunanen A, Heliovaara M, Aho K. Coffee consumption and risk of type 2 diabetes mellitus. Lancet 2003;361:70203.[Medline]
4 Saremi A, Tullock-Reid M, Knowler W. Coffee consumption and the incidence of type 2 diabetes. Diabetes Care 2003;26:221112.
5 Kaprio J, Sarna S, Koskenvuo M, Rantasalo I. The Finnish Twin Registry: Formation and compilation, questionnaire study, zygosity determination procedures and research program. Prog Clin Biol Res 1978;24B:17984.
6 Kaprio, J, Tuomeilehto J, Koskenvuo M et al. Concordance of type 1 (insulin-dependent) and type 2 (non-insulin-dependent) diabetes mellitus in a population-based cohort of twins in Finland. Diabetologia 1992;35:106067.[ISI][Medline]
7 Thernau TM, Grambsch PM, Pankratz VS. Penalized Survival Models and Frailty. Technical Report #66. 2000.
8 S-PLUS 2000. Guide to Statistics. Vol. 2. Data Analysis Products Division. MathSoft, Seattle, WA.
9 Hammar N, Andersson T, Alfredsson L et al. Association of boiled and filtered coffee with incidence of first nonfatal myocardial infarction- The SHEEP and the VHEEP study. J Int Med 2003;253:17.[CrossRef]