Changes in Age at Diagnosis of Type 2 Diabetes Mellitus in the United States, 1988 to 2000

INTRODUCTION

The prevalence of diagnosed diabetes in the United States is increasing, coincident with increases in the prevalence of obesity and sedentary lifestyle.^1–,³ Among adults in the United States, population estimates of the prevalence of diagnosed diabetes increased from 3.4% in 1976 to 1980, to 5.1% in 1988 to 1994, to 5.9% in 1999 to 2000.^4–,⁶ Rates of diagnosed diabetes for racial and ethnic minorities generally exceeded that of non-Hispanic whites.^4–,⁶

There has been concern that child and adolescent age-groups have also been affected by rising trends in type 2 diabetes mellitus.^7,8 The prevalence of overweight children and adolescents is increasing in the United States.^9,10 Although there have not been US population estimates of the prevalence of type 2 diabetes in youth, an increase in type 2 diabetes has been noted in pediatric diabetes referral centers, as well as in Pima Indians.^11,12 Additionally, there have not been estimates of population trends in age of onset of type 2 diabetes among adults in the United States.

Given the increasing prevalence of diabetes in the United States and the evidence that type 2 diabetes is being diagnosed in younger populations, the onset of type 2 diabetes may be occurring at a much younger age in the US population as a whole. An understanding of this trend will help define the magnitude of this health problem. We report the mean self-reported age at diagnosis of type 2 diabetes and make comparisons between the most recently available population data from the National Health and Nutrition Examination Survey (NHANES 1999–2000), and data from the Third National Health and Nutrition Examination Survey (NHANES III) (1988–1994).

METHODS

RESULTS

The NHANES 1999–2000 contained data on 4,387 persons aged 20 years and older: 47.7% was male; the average age was 45.0 years; 73.6% was non-Hispanic white, 11.4% was non-Hispanic black, and 15.0% was Hispanic. Of these survey participants, we determined that 430 had a diagnosis of type 2 diabetes mellitus. After applying sampling weights, these 430 participants represent 11.6 million persons with type 2 diabetes in the United States in 1999–2000. The NHANES III contained data on 17,010 participants aged 20 years and older: 47.6% was male; the average age was 44.7 years; 79.1% was non-Hispanic white, 11.3% was non-Hispanic was black, and 9.5% was Hispanic. Of these participants, we determined that 1,271 had a diagnosis of type 2 diabetes mellitus. After applying sampling weights, these 1,271 survey participants represent 8.5 million people with type 2 diabetes in the United States in 1988 to 1994.

DISCUSSION

The mean age at diagnosis of type 2 diabetes mellitus in the United States has decreased from 52 years, reported in the years 1988 to 1994, to 46 years, reported in the years 1999 to 2000. There are several plausible explanations that might account for this rather dramatic decrease occurring during a course of a decade. This change may represent earlier onset of type 2 diabetes or earlier detection or a combination of these 2 effects. Whether the decrease in age at diagnosis was due to an overall decrease in actual age of onset among American adults or to earlier detection of type 2 diabetes by physicians is unclear and cannot be determined from these data. Decreasing age at diagnosis could also occur through increased public awareness, leading to earlier physician consultation about recognized symptoms of diabetes, which might reflect better education of the population about their risks for diabetes.

Earlier detection by physicians is almost certainly a factor influencing the decreasing age at diagnosis. The diagnostic criteria for diabetes mellitus changed between the time of the NHANES III (1988–1994) and the NHANES 1999–2000 from the criteria of a fasting plasma glucose ≥140 mg/dL or a 2-hour plasma glucose ≥200 mg/dL to the current American Diabetes Association (ADA) criteria of a fasting plasma glucose of ≥126 mg/dL.^15,16 The institution of yearly updated recommendations for clinical practice and screening from the ADA may play a role in increased detection of diabetes by physicians and other health care clinicians.^16,17

The United States Preventive Services Task Force (USPSTF) has recommended against mass screening of asymptomatic persons for type 2 diabetes, concluding that the evidence for or against mass screening is inconclusive.^18,19 The Centers for Disease Control and Prevention (CDC) Diabetes Cost-Effectiveness Study Group concluded that opportunistic screening of all adults older than the age of 25 years would cost $236,449 per life-year gained and $56,649 per quality-adjusted life-year gained, and it has generally recommended against mass screening for type 2 diabetes in adults.²⁰ The ADA does not recommend general population screening, but it does recommend that screening be considered every 3 years for subgroups such as those aged 45 years and older, as well as increased screening based on family history of type 2 diabetes, minority race/ethnicity, and other risk factors.¹⁷ As suggested by these nationally representative findings of decreasing mean age at diagnosis of type 2 diabetes, these recommendations might need to be reexamined to include clinical screening recommendations for younger age-groups, especially those at high risk.

A few limitations must be acknowledged. The NHANES did not ask participants to differentiate whether they had type 1 or type 2 diabetes mellitus. For our analysis, we needed to differentiate individuals with these 2 very different disease processes to obtain meaningful data. We believe we have created a construct that allows for the categorization of subjects in a manner that minimizes misclassification bias. As a check on our categorization scheme, we looked at the distribution of C peptide levels in those that we categorized as having type 2 diabetes in the 1999–2000 data set. We found that among these participants who had the fasting laboratory test performed (n = 156), 91.7% had a measured C peptide ≥0.50 nmol/L, consistent with type 2 diabetes.²¹ In addition, both the age at diagnosis and the diagnosis itself are self-reported, which may lead to the question of validity of the diagnosis and correct recall of age at diagnosis. Self-reports of diabetes have been used as the basis of many population-based studies and should not be systematically biased in a way differing from NHANES III to NHANES 1999–2000.^2–,⁶ Furthermore, memory for age at the time of diagnosis of other health conditions has been shown to be fairly accurate when compared with medical records.²²

Using the NHANES data set is probably the only existing method to make nationally representative population estimates about age at diagnosis of type 2 diabetes. The observed decrease in the age at diagnosis may reflect population trends in the true age of onset of type 2 diabetes. Alternatively, health care clinicians may be doing a better job at recognizing the subtle symptoms of diabetes. They are also likely to be making earlier diagnoses as a result of more stringent diagnostic criteria. It is further possible that our population may be becoming better educated about these symptoms, leading them to more readily seek appropriate health care. In either case, it is likely that we are detecting type 2 diabetes earlier in the course of the disease, with likely positive health benefits for this population. Identification of factors that can further improve our detection of type 2 diabetes may be a promising area for future research.