These recent data indicate that 12.9% of the adult U.S. population aged ≥20 years have diabetes (7.7% previously diagnosed and 5.1% undiagnosed), of which 39.8% is undiagnosed. Another 29.5% of the population is at risk of diabetes based on having pre-diabetes (IFG or IGT). Overall, approximately 40% of the U.S. population has some hyperglycemic condition. The elderly population is particularly susceptible, with about one-third having diagnosed or undiagnosed diabetes and three-quarters having diabetes or pre-diabetes. Minority groups continue to suffer disproportionately, as prevalence of diagnosed and undiagnosed diabetes combined is 70–80% higher in non-Hispanic blacks and Mexican Americans than in non-Hispanic white subjects. But whereas diagnosed diabetes has risen significantly over the last 10–15 years, particularly in non-Hispanic blacks, undiagnosed diabetes as a proportion of total diabetes has remained relatively stable and may be decreasing, particularly in Mexican Americans.
NHANES is unique because its results represent people in the U.S. noninstitutionalized population, and the survey, in contrast with other national surveys, includes a laboratory component that measures FPG. NHANES was particularly unique in 2005–2006 because of inclusion of an OGTT not previously performed since 1988–1994. The FPG is recommended for screening for diabetes and IFG because it is more reproducible and convenient and less costly. FPG and 2-h OGTT glucose, however, measure different physiological phenomena (4
). Two-hour glucose is more sensitive than FPG to detecting glucose defects in the elderly (7
). Whereas 91% of people having diabetes by FPG were likewise classified by 2-h glucose, the 2-h glucose added another 2.6% of diabetes prevalence. The corresponding figure among people aged 40–74 years in NHANES 1988–1994 was 2.0%, which is not significantly different (9
). There was substantial disagreement among those having IFG, in whom 8.5% were diabetic and 60.2% normoglycemic based on 2-h glucose. This explains the doubling of the prevalence of undiagnosed diabetes based on 2-h glucose (aged ≥20 years crude 2.5% by FPG, 4.9% by 2-h glucose, and 5.1% having either) and halving of pre-diabetes (aged ≥20 years crude 25.7% by FPG,13.8% by 2-h glucose, and 29.5% having either).
We note that determination of undiagnosed diabetes and pre-diabetes by either FPG or 2-h glucose was based on a single plasma glucose reading from subjects who self-reported that they fasted appropriately, whereas retesting is suggested for diagnosis in a clinical setting. Consequently, some of the prevalence estimates may be overstated. In addition, the available sample size in 2005–2006 limited the ability to detect differences. Nevertheless, some important differences were detected between groups and over time.
As shown in previous reports (3
), diagnosed diabetes remains more than twice as high in non-Hispanic blacks and Mexican Americans than in non-Hispanic whites, after accounting for differences in age and sex distributions. The racial/ethnic disparity is reflected in prevalence of total diabetes (diagnosed and undiagnosed) and total hyperglycemic conditions. Undiagnosed diabetes was not greater in these groups.
Diagnosed diabetes increased significantly between 1988–1994 and 2005–2006 in all age groups and in both men and women. The rise in prevalence of diagnosed diabetes was particularly prominent in non-Hispanic blacks and was reflected in a rise in total diabetes and total hyperglycemic conditions over time (most prominently in non-Hispanic blacks).
Based on both FPG and 2-h glucoses, almost 40% of total diabetes was undiagnosed. The proportion that was undiagnosed was significantly higher in non-Hispanic whites than in Mexican Americans. The proportion of total diabetes that was undiagnosed tended to decrease between the surveys, but this was statistically significant only among Mexican Americans.
Thus, whereas diagnosed and total diabetes and total hyperglycemic conditions remain disproportionately high in minority groups, it may be that diabetes is being diagnosed more frequently in these groups, both over time and relative to non-Hispanic whites. This was also found when comparing data from 1999–2002 with those from 1988–1994 (3
). More focused screening may be occurring in these groups (19
). Decreases over the past several decades in the proportion of diabetes that is undiagnosed have occurred only among the most obese (20
). We also found that pre-diabetes decreased significantly over time in Mexican Americans.
Overall, almost 30% of the population had pre-diabetes (IFG or IGT), a condition that increases the risk for diabetes and is associated with other cardiovascular risk factors (4
). In 2005–2006, IFG was 70% higher in men than in women, consistent with findings in 1999–2002 (3
). This was reflected in their higher prevalences of total pre-diabetes and total hyperglycemic conditions. No differences by sex, however, were found in the prevalence of diagnosed or undiagnosed diabetes. These observations currently lack explanation. It is encouraging that prevalence of pre-diabetes did not appear to increase between the surveys; this is surprising given the increase in diagnosed diabetes and obesity (10
) over time.
The sheer magnitude of prevalence of hyperglycemic conditions found in 2005–2006 portends all the consequences of diabetes including its myriad of complications and costs both to individuals and to society. The prevalence of diabetes continues to increase over time but appears to be recognized more commonly. Despite some evidence that overweight and obesity may be plateauing in adults (21
) and adolescents (22
), their prevalences remain high; and, even in adolescents, features of insulin resistance are found in the presence of IFG (23
). Lifestyle modification including weight management and increased physical activity should be prescribed and practiced in those with diabetes (24
) and pre-diabetes (25
), particularly in minority groups.