Vitamin D plays an important role not only in the pathogenesis of skeletal disorders and calcium homeostasis, but also in the development of several chronic conditions.1,2 It has been suggested that vitamin D deficiency is a risk factor for type 1 diabetes mellitus.3,4 Vitamin D deficiency impairs insulin secretion of pancreatic β-cells4–6 and increases insulin resistance,7 which are major factors in the pathogenesis of type 2 diabetes. Accordingly, low vitamin D might also be a risk factor for type 2 diabetes.8
The epidemiologic evidence of an association between vitamin D deficiency and type 2 diabetes is mainly based on cross-sectional studies.9–14 The only prospective study dealing the effect of vitamin D deficiency on the incidence of type 2 diabetes was based on the intake of vitamin D.15 Vitamin D intake is a weak proxy measure of vitamin D status; tissue vitamin D gives a more reliable picture.16 The incidence of type 2 diabetes in relation to serum vitamin D levels has been investigated in 2 small prospective population studies.17,18 Both studies showed an inverse association between serum vitamin D status and occurrence of type 2 diabetes. In the present study, we further investigated the link between serum vitamin D and type 2 diabetes by pooling the primary data from these 2 Finnish cohorts.
Subjects and Methods
The present study is based on 2 cohorts, the Finnish Mobile Clinic Health Examination Survey,19 carried out in 1973–1976, and the Mini-Finland Health Survey,20 carried out in 1978–1980. The earlier data consisted of 19,518 men and women, aged 20 years and older, from 12 municipalities in different parts of Finland. The later consisted of 8000 persons from 40 geographical areas and was a representative sample of the Finnish population of adults aged 30 years and over. Among those free of type 2 diabetes at baseline, there were 3327 persons 40–69 years of age (in the earlier study) and 4176 persons 40–74 years of age (in the latter study) (Table 1).
Data on education, smoking, leisure time physical activity, previous diseases, and previous and present medication, (eg, for hypertension), were self-reported on a questionnaire or in a health interview.19,21 Subjects classified their leisure-time physical activity as (1) none or little; (2) walking, cycling, or related light activities at least 4 hours per week; (3) ball games, jogging, or related activities at least 3 hours per week; or (4) regular vigorous exercise. Height and weight were measured, and body mass index (BMI) was calculated (kg/m2). Casual blood pressure was measured with the auscultatory method, and blood samples were collected and stored at −20°C. Serum total cholesterol, triglycerides, insulin, and plasma fasting glucose were measured. The serum samples were kept frozen in the same storage at −20°C until 2002 (for the earlier study) and 2003 (for the later study), when the serum vitamin D concentrations (serum 25(OH)D) were determined using the radioimmunoassay (RIA, DiaSorin, MI). The interassay coefficient of variation of the 25-OH-vitamin D measurements was 7.8% at the mean level of 47.3 nmol/L (n = 167). The serum vitamin D levels were higher in men than in women and associated with month of serum measurement. The sex and age-adjusted correlation coefficient for the month and vitamin D association was 0.44 in controls.
Diabetes cases at baseline were identified by information given by the participants and by fasting glucose values.19,20 All previously known cases or persons newly diagnosed with diabetes at baseline were excluded from the analyses. Diabetes cases occurring during the follow-up were identified based on the registry of reimbursement for costs of diabetes medication. According to the Finnish sickness insurance legislation, diabetes patients needing drug treatment are allowed certain drugs free of charge. To get this drug allowance, a certificate must be obtained from the physician in charge, describing the diagnostic criteria applied when the diabetes was diagnosed. The certificate is accepted after confirmation by special advisers at the Social Insurance Institution,22 which maintains a central register of all patients receiving drug reimbursement. Participants in the present study populations were linked to this register with the unique social-security code assigned to each Finnish citizen.
Follow-up time was defined as the number of days from the baseline examination to the dates of type 2 diabetes occurrence, death, or withdrawal (ie, end of follow-up), whichever came first. The follow-up varied from 17 years for the later cohort to 22 years for the earlier cohort (Table 1). During the follow-up period, the number of incident type 2 diabetes cases identified from a nationwide registry was 230 for the earlier cohort and 182 for the later cohort (Table 1). All medical certificates of these cases were checked, and every case met the World Health Organization diagnostic criteria for type 2 diabetes mellitus.23
A nested case-control design was adopted for both sets of data. Two controls per case were selected from the earlier cohort and 3 per case from the later cohort by individual matching for sex, age and municipality. Controls were drawn from the same municipality as the type 2 diabetes cases, and age was matched using nearest available matching. The differences in age between cases and controls varied from 0 to 2 years for 95% of the patients and from 3 to 5 in 5%. Length of follow-up was controlled for. The group at risk from which controls were selected comprised all persons who were free of type 2 diabetes until the date of diabetes diagnosis of the case. Matching for municipality also controlled for the month of baseline examination (ie, for possible seasonal variation in exposure to vitamin D coming from the sunlight) and for duration of storage of serum samples.24
The conditional logistic model25 was used to assess the association between vitamin D intake and type 2 diabetes risk in the single subcohorts. To avoid assumptions of the shape of the relationship between vitamin D exposure and type 2 diabetes incidence in the statistical analyses, relative odds (odds ratios [ORs]) were estimated for quartiles of vitamin D. Two-sided 95% confidence intervals (CIs) were estimated. The P value for trend was calculated by including vitamin D as a continuous variable in the model.
We defined 2 main models, one of which included serum vitamin D and age only, and another that also included the a priori potential confounding factors of body mass index, physical exercise, smoking, and education. Modification of the effect of different risk factors on the association between vitamin D and type 2 diabetes incidence was explored by including an interaction term between the vitamin D variable as a continuous variable and the potential effect-modifying factor (ie, sex, age, season, hypertension, body mass index, and the factors serum cholesterol and blood pressure) as a categorical variable.
The pooling methodology is described in more detail elsewhere26 and only briefly described here. The subcohort-specific logs of relative odds were combined, weighting them by the inverse of their variance in a random-effects model.27 The P value for test of trend was based on a Wald test of the pooled estimates. Pooled P value for test of interaction was obtained using the squared Wald statistic in which the squared pooled estimate of the interaction coefficient was divided by its variance and referring the Wald statistic to a χ2 distribution with 1 degree of freedom. Heterogeneity among the study-specific relative odds was tested using the asymptotic DerSimonian and Laird Q statistic.27 The potential modification of the effect of exposure (heterogeneity) due to sex was tested by the Wald test.28 The calculations were performed using SAS (version 9.1; SAS Institute, Cary, NC).
An inverse association between age-adjusted serum vitamin D and type 2 diabetes incidence was found in the pooled population of individuals (Table 2). The age-adjusted relative odds (OR) of the disease comparing the highest with the lowest quartiles of the serum concentration of vitamin D was 0.60 (CI = 0.37–0.96; P for trend = 0.06; P for heterogeneity = 0.46) (Fig. 1). The pooled relative odds are apparently an appropriate summary of the data since testing for heterogeneity among substudies did not indicate significant differences (P for heterogeneity = 0.46). After further adjustment for the potential confounding factors of body mass index, physical activity, smoking, and education, the OR was unchanged (OR = 0.60; CI = 0.25–1.48; P for trend = 0.38; P for heterogeneity = 0.08). The increase in heterogeneity was due to different effects in men and women for adjustment for body mass index. This was eliminated by stratifying the analysis by sex. The adjusted relative odds were 0.28 (CI = 0.10–0.81; P for trend <0.001; P for heterogeneity = 0.44) for men and 1.14 (CI = 0.60–2.17; P for trend = 0.89; P for heterogeneity = 0.64) for women (Table 3). Further adjustment for serum cholesterol and blood pressure or exclusion of the cases occurring during the first 5 years of follow-up did not notably alter the results. Inclusion of an interaction term between vitamin D and age, body mass index, serum cholesterol, blood pressure, and season did not notably alter the results (data not shown).
High serum concentration of vitamin D was related to a reduced incidence of type 2 diabetes in our study based on pooling 2 nested case-control studies. Men in the highest quartile of serum vitamin D had an 82% lower risk compared with those in the lowest quartile after adjustment for body mass index, physical activity, smoking, and education. Further adjustment for the intermediate factors serum cholesterol and blood pressure did not change the results.
These data suggest that vitamin D may provide protection against type 2 diabetes mellitus. As far as we know, only one previous prospective study on this topic has been published.15 That study, which also reported an association, was based on vitamin D intake. Intake does not include variation in vitamin D due to sunlight.
The effect of vitamin D on pancreatic β-cells and subsequent insulin release is mediated through vitamin D receptor, and thus the major focus of the association of vitamin D deficiency and diabetes mellitus has been the function of this receptor in humans and experimental animals.6,8 Polymorphism of this receptor leading to impaired function has been observed especially in type 1 diabetes, but observations have also included individuals with type 2 diabetes.29 Vitamin D is thought to promote insulin secretion by increasing the cytosolic calcium concentration in β-cells. The effect of insufficient vitamin D supply on insulin resistance has not been investigated as closely as the effects on insulin secretion. However, according to a recent metabolic study using a hyperglycemic clamp technique on healthy persons,7 serum vitamin D concentration is negatively correlated with first-phase insulin response, which indicates insulin resistance. Furthermore, treatment of women with type 2 diabetes by vitamin D supplements decreases insulin resistance.30 In cross-sectional study of the third National Health and Nutrition Examination Surveys (NHANES), serum vitamin D concentration was inversely correlated with newly detected diabetes and Homeostasis Model Assessment index for insulin resistance, but not with Homeostasis Model Assessment index for β-cell function14 and, in a separate study, with metabolic syndrome.31 Thus, according to these observations, vitamin D deficiency seems to be associated with the 2 central components of type 2 diabetes pathogenesis: impaired insulin secretion and peripheral insulin resistance.
The strengths of the present study are the prospective study design; the use of serum vitamin D concentration covering the total amount of the vitamin available (due both to diet and to sunlight); the pooled data giving greater power to detect an association; and better ability to investigate potential effect modification.
There are some methodologic factors that may have suppressed the true associations or have caused artificial associations. First, we could not obtain a comprehensive picture of vitamin D exposure. The baseline examination was carried out during different parts of the year and vitamin D levels vary strongly by sunlight exposure. No measurements were carried out during July, thus depressing the seasonal variation in vitamin D values. Study of the interaction between serum vitamin D concentration and season (sunny vs. dark period) confirmed a lack of bias due to season, however. Another potential source of bias is the long storage time of the serum samples before serum vitamin D determinations. The few studies published so far have given contradictory results, with a reduction in vitamin D concentration during 11 months’ storage of plasma samples at −18°C32 and only a slight change after 22 months’ storage.33
Furthermore, possible changes in dietary habits during the 22-year follow-up period may have altered the vitamin D status over time and thus have weakened the association between vitamin D status and type 2 diabetes incidence. For example, fish intake (and, accordingly, vitamin D intake) has increased during the follow-up period.34 Second, since our outcome variable included patients receiving reimbursed medication for treatment of type 2 diabetes, all individuals treating their disease with dietary changes are included in the control group. This will cause conservative estimates of the strength of association between vitamin D status and type 2 diabetes risk. Third, a more detailed study of the complex patterns of different nutrients in the diet would be necessary to obtain a deeper understanding of the benefits of vitamin D, and therefore the lack of information on dietary factors may have oversimplified our interpretations. Fourth, our study population (especially the women) had low levels of vitamin D, which may be due to the low intake of fish and short period of sun in Finland; therefore, the findings of this study cannot be generalized to populations with high levels of vitamin D. Fifth, although the main nondietary risk factors of type 2 diabetes were adjusted for in this study,35 there could nonetheless be uncontrolled residual confounding, especially due to dietary factors but also unmeasured nondietary factors. Finally, the reduced risk could be associated with a healthy lifestyle or some other beneficial factor related to the sources of vitamin D.
In conclusion, the results of the present pooled study support the hypothesis that low vitamin D status predicts development of type 2 diabetes. Since the results may be due to confounding by dietary and lifestyle factors, further studies are needed before firm conclusions can be made about the role of vitamin D in diabetes prevention.
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