1 Department of Health Sciences Research, Mayo Clinic, Rochester, MN.
2 Department of Medicine, Mayo Clinic, Scottsdale, AZ.
3 SmithKline Beecham Pharmaceuticals, Philadelphia, PA.
4 Olmsted Medical Center, Rochester, MN.
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
diabetes mellitus, non-insulin-dependent; incidence
Abbreviations: NDDG, National Diabetes Data Group; NHIS, National Health Interview Survey
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
To address these limitations, researchers in the San Antonio Heart Study obtained data on glucose levels at baseline and follow-up for Mexican Americans and non-Hispanic Whites residing in San Antonio, Texas. They found a threefold increase in the incidence of type 2 diabetes over the subsequent 78 years between 1979 and 1988, although the trend reached statistical significance only among Mexican Americans (4). Several limitations of the NHIS data were also addressed with retrospective review of laboratory glucose values in the predominately White population of Rochester, Minnesota. The Rochester study applied standardized glycemic criteria for diabetes and found that the age- and sex-adjusted incidence of diabetes increased significantly between 19701974 and 19851989 (5
). Although a comparison of patient characteristics as of the incidence date suggested that the rise in diabetes incidence was not an artifact of earlier detection of milder cases (5
, 6
), the question of whether observed trends in incidence were accompanied by increases in diabetes ascertainment and/or detection remains unanswered.
In the present study, we updated the Rochester incidence rates from 19701989 through the year 1994 to test whether the previously observed increase in diabetes incidence has continued at the same rate or has accelerated, leveled off, or declined. In addition, we examined trends in passive surveillance, i.e., the proportion of all local residents aged 30 years or more who had blood glucose values measured over this time period, which might explain temporal changes in the observed incidence of diabetes.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The resources of the Rochester Epidemiology Project were used to construct the Rochester incident diabetes cohort. As described elsewhere (5, 9
), case assignment was determined through retrospective review of each patient's complete (hospital and ambulatory) medical records by trained nurse abstractors, under the direction of an endocrinologist (P. J. P.). The records were reviewed for all laboratory glucose values (available from 1930 to the present) and for evidence of any antidiabetes medication taken over the entire duration of residence in the community. Methods of ascertainment were consistent throughout the study period. Standardized case criteria were applied using National Diabetes Data Group (NDDG) recommendationsi.e., two consecutive fasting plasma glucose measurements of
140 mg/dl (7.8 mmol/liter) or both 1- and 2-hour glucose measurements of
200 mg/dl (11.1 mmol/liter) obtained during a standard oral glucose tolerance test (10
). Using the method of West (11
), adjustments were made for temporal changes in laboratory methods. Persons who failed to meet the above glycemic criteria but used oral antidiabetes agents or insulin for at least 2 weeks or until death also qualified as cases. The abstractor determined the date on which each individual first met the criteria and verified Rochester residency on that date. Case subjects were required to have been a Rochester resident 1 year prior to diagnosis. The study was approved by the institutional review boards of the Mayo Clinic and the Olmsted Medical Center, and in accordance with a Minnesota state statute (8
), individuals who refused authorization for use of their medical records in research (n = 72) were excluded.
Although laboratory glucose values have been available in the medical records since 1930, they were not available electronically prior to 1987. Because it was not feasible to manually review the hard-copy records of all Rochester residents, the review for all years was limited to potential cases. Potential cases were identified by generating a list from the Rochester Epidemiology Project diagnostic index of all Rochester residents ever assigned any diabetes-related diagnosis (e.g., elevated glucose level, hyperglycemia, impaired glucose tolerance, "rule out diabetes," diabetic retinopathy, etc.) since 1945 (9). This preliminary step recognized that diabetes is a chronic progressive condition and was based on the assumption that individuals whose laboratory glucose values met NDDG criteria would, at some point in their medical history, receive a diagnosis of diabetes or a diabetes-like condition, e.g., "elevated glucose." This assumption was examined in a recent study that identified all 7,192 Rochester residents who died on or after age 30 years during the time period 19791994 (12
); the median duration of medical history available for review was 43 years (interquartile range, 2458 years), and 25 percent (n = 1,788) had been assigned at least one of the diabetes diagnoses under consideration. The likelihood of such a diagnosis did not change as a function of calendar year of death, providing confidence that the proportion of residents considered as potential cases in constructing the Rochester incident diabetes cohort was high and stable over time. Analysis of temporal trends in the present study was limited to adult members of the 19701994 incident cohorti.e., all identified individuals who first met NDDG criteria between January 1, 1970, and December 31, 1994, and who were aged 30 years or more and residing in Rochester at the time they first met the criteria.
Characteristics of the incident cases were summarized for successive quinquennia (19701974, 19751979, 19801984, 19851989, and 19901994) using descriptive statistics. Incidence rates were calculated for each sex and age group (3044, 4554, 5564, 6574, 7584, and 85 years) for successive quinquennia, with newly diagnosed cases used as the numerator and Rochester person-years at risk as the denominator. The denominator was based on decennial census data with linear interpolation for intercensus years (13
). Incidence rates were age- and sex-adjusted by the direct method to the White population of the United States as of 1980. Poisson regression was used to evaluate associations between incidence and age, sex, and calendar year of incidence. The significance of two-way interaction terms and higher-order polynomials was examined. To determine whether incidence of diabetes in the latest quinquennium increased at a faster rate, continued to increase at the same rate, leveled off, or declined, we added calendar period (19701989 vs. 19901994) to the final model as an independent variable. If the period variable contributed significantly to the model, this would indicate that there was a change in the incidence trend in the last quinquennium compared with the earlier time period.
To address whether temporal trends in diabetes incidence were accompanied by changes in surveillance, we used Poisson regression to test for a significant effect of calendar year on the percentage of local residents who had a blood glucose measurement in each year during the period 19871994. As we noted above, laboratory glucose values were not available electronically for the entire population prior to 1987. For comparison purposes, we determined whether there was an increase in diabetes incidence over this same time period by limiting the Poisson regression analysis of incidence described above to the calendar years 19871994 and testing for a significant effect of calendar year.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
Figure 2 shows the age-adjusted incidence of diabetes in both males and females for the time period 19871994, along with the age-adjusted proportions of local residents who had at least one blood glucose measurement in each year during the same time period. Over this time period, there was no significant effect of calendar year (p = 0.181), adjusted for age and sex, on the percentage of local residents with at least one laboratory glucose measurement taken at the Mayo Clinic or the Olmsted Medical Center. The percentage of local residents with at least one blood glucose measurement in each year averaged approximately 37 percent annually for males and 44 percent annually for females. In each year, 90 percent of individuals with at least one blood glucose determination had three or fewer measurements per year; the proportion with only one value was unchanged over time (i.e., 71 percent, 69 percent, 66 percent, 69 percent, 70 percent, 69 percent, 65 percent, and 70 percent, successively, in the years 19871994). However, during the same time period, there was a significant effect of calendar year (p = 0.033), adjusted for age and sex, on diabetes incidence. This indicates that while surveillance did not change over this time period, the incidence of diabetes increased significantly.
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The recent rise in diabetes incidence in Rochester between 1970 and 1994 may have two possible explanations. One is an increase in surveillance over this time period. A second explanation is that there was an actual increase in the rate of diabetes, probably due to changes in environmental factors (e.g., obesity). The first possibility is improbable, since the proportion of individuals receiving at least one blood glucose measurement each year did not increase. Thus, increased surveillance is an unlikely explanation for the observed increase in diabetes incidence. Instead, a possible explanation for the rise in diabetes incidence may be the marked increase in obesity in the general population, which has been observed in a number of studies (4, 14
, 15
). In a previous article (16
), we provided data on body mass index (weight (kg)/height (m)2) from a prospective survey of Rochester residents aged
45 years in 1986. A subsequent survey of Rochester residents was performed in 1996. In both surveys, weight and height were measured in a sample of the population identified by random digit dialing. There was a significant increase in mean body mass index between 1986 and 1996 for both male and female residents. The difference was most apparent in the younger age groups (Dr. Irene Meissner, Mayo Clinic, personal communication, 2001). These observations regarding temporal increases in body mass index within the Rochester population generally are consistent with our recent report of temporal increases in body mass index among members of the 19701989 Rochester diabetes cohort, as well as with the age-specific temporal increases in the rate of new cases of diabetes observed in this population (see figure 1).
Other than the studies in Rochester, there have been few opportunities, using standardized glycemic criteria, to examine trends in the incidence of diabetes in this country. One such study in San Antonio, Texas, found an increase in the incidence of type 2 diabetes from 1979 through 1988 in Mexican Americans and non-Hispanic Whites who participated in the San Antonio Heart Study (4). Participants in the study were aged 2564 years and were randomly selected from low-, middle-, and high-income neighborhoods in San Antonio. Participants who were nondiabetic at baseline and returned for follow-up 78 years later were examined for diabetes. Standardized criteria for identifying cases of diabetes were used at both examinations. The percentage of individuals who developed type 2 diabetes in the subsequent 7- to 8-year follow-up period increased significantly, from 5.7 percent for Mexican Americans first enrolled in 1979 to 15.7 percent for those first enrolled in 1988. In non-Hispanic Whites, there was a borderline-significant increase from 2.6 percent to 9.4 percent over the same time period. This trend remained significant for Mexican Americans and borderline-significant for non-Hispanic Whites after adjustment for a number of baseline risk factors, including obesity.
The diabetes case criteria in both the San Antonio Heart Study and the present investigation were standardized over time, although we did not prospectively examine the population as was done in the San Antonio study. These two studies differ from almost all other reports of secular trends in diabetes incidence, in which case identification is based on a self-report of having received a diagnosis of diabetes from a clinician in the past year. Data from the sequential NHIS surveys revealed little change between 1968 and 1992 in the annual rate of noninstitutionalized US adults aged 18 years reporting having received a first diagnosis of diabetes in the previous year (2
). These trends differ from a subsequent NHIS report that revealed a significant increase in both crude and age-adjusted diabetes incidence rates for the time period 19801994 (17
). The disparate trends could reflect either a sudden increase in diabetes prevalence in 19921994 or a failure to detect an increase in the earlier NHIS because of the 1979 introduction of more stringent NDDG recommendations for assigning a clinical diagnosis of diabetes (9
, 19
).
The extent to which the present study findings are generalizable to the entire US population is limited by the fact that the Rochester population was 95 percent White during the years of the study. Our study was also limited by the fact that, to minimize misclassification and to ensure a standardized definition throughout the study period, the criteria we employed for ascertaining incident diabetes cases were intentionally strict. As a result, individuals whose glucose values never met NDDG criteria but did meet more recently introduced American Diabetes Association criteria (18) would have been excluded. Individuals who never had a diagnosis suggestive of diabetes in the Rochester Epidemiology Project diagnostic index during their entire period of residence in the community but who would have qualified if prospectively screened also were not included. Cross-sectional studies that prospectively screen community-dwelling persons suggest that a substantial proportion of such individuals qualify as diabetic on the basis of oral glucose tolerance testing or fasting glucose testing, even though they report no prior diagnosis of diabetes (1
). However, because diabetes is a chronic, progressive disease, many such individuals ultimately come to clinical attention, as was demonstrated by our study of the proportion of deceased Rochester residents with any mention of diabetes or diabetes-like conditions in their medical history (12
). In addition, a significant (3545 percent) and consistent proportion of Rochester residents had at least one blood glucose determination each year.
While a large proportion of Rochester residents have a blood glucose measurement every few years, the increase in diabetes incidence observed in the most recent quinquennium, 19901994, could still be an underestimate. With each update of the Rochester diabetes cohort, we discover new cases that, on the basis of review of the patient's glucose values, actually qualified as cases in earlier time periods. This discrepancy results from the fact that potential cases for each update of the Rochester diabetes cohort are identified from the list of residents with any prior clinical mention of diabetes or diabetes-like conditions through the end of the study period (see Materials and Methods). Thus, with each update, some individuals who have qualified as diabetes cases on the basis of glycemic criteria but have not yet been assigned a clinical diagnosis are missing from the list of potential cases.
For example, in the most recent update prior to this one (5), we reported that the numbers of cases identified for the time periods 19701974, 19751979, 19801984, and 19851989 were 270, 288, 334, and 437, respectively. While the present update detected a relatively small number of previously unidentified cases (i.e., three, two, and two) for the periods 19701974, 19751979, and 19801984, respectively, 23 new cases were identified for the time period 19851989. The 19851989 age- and sex-adjusted annual incidence rate reported in the previous update was 303 per 100,000 population (95 percent confidence interval: 274, 332), as compared with the 316 per 100,000 (95 percent confidence interval: 286, 345) reported here. Thus, it is likely that the next update of the cohort through 1999 will identify additional cases for the time period 19901994 and that the apparent downturn from 1993 to 1994 (figure 2) is partly an artifact of our method of case identification.
In conclusion, we observed a significant increase in the incidence of diabetes among residents of Rochester, Minnesota, aged 30 years or more during the period 19701994. This increase accelerated in the last quinquennium and was seen in both sexes, most dramatically in younger adults. An increase in diabetes surveillance is an unlikely explanation for this increased incidence. These results, combined with those of other studies (4, 5
), provide further (albeit indirect) evidence that the epidemic of obesity in the United States is contributing to an epidemic of diabetes. The implications of a further increase in diabetes incidence are likely to be substantial, including increases in the incidence of cardiovascular disease and other complications related to diabetes.
![]() |
ACKNOWLEDGMENTS |
---|
The authors acknowledge the contributions of the nurse abstractors on this project, Kristine Otto-Higgins and Sue Helling.
![]() |
NOTES |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|