Background Vitamin D insufficiency is associated with suboptimal health. The prevalence of vitamin D insufficiency may be rising, but population-based trends are uncertain. We sought to evaluate US population trends in vitamin D insufficiency. Methods We compared serum 25-hydroxyvitamin D (25[OH]D) levels from the Third National Health and Nutrition Examination Survey (NHANES III), collected during 1988 through 1994, with NHANES data collected from 2001 through 2004 (NHANES 2001–2004). Complete data were available for 18 883 participants in NHANES III and 13 369 participants in NHANES 2001–2004. Results The mean serum 25(OH)D level was 30 (95% confidence interval [CI], 29–30) ng/mL during NHANES III and decreased to 24 (23–25) ng/mL during NHANES 2001–2004. Accordingly, the prevalence of 25(OH)D levels of less than 10 ng/mL increased from 2% (95% CI, 2%–2%) to 6% (5%–8%), and 25(OH)D levels of 30 ng/mL or more decreased from 45% (43%–47%) to 23% (20%–26%). The prevalence of 25(OH)D levels of less than 10 ng/mL in non-Hispanic blacks rose from 9% during NHANES III to 29% during NHANES 2001–2004, with a corresponding decrease in the prevalence of levels of 30 ng/mL or more from 12% to 3%. Differences by age strata (mean serum 25[OH]D levels ranging from 28–32 ng/mL) and sex (28 ng/mL for women and 32 ng/mL for men) during NHANES III equalized during NHANES 2001–2004 (24 vs 24 ng/mL for age and 24 vs 24 ng/mL for sex). Conclusions National data demonstrate a marked decrease in serum 25(OH)D levels from the 1988–1994 to the 2001–2004 NHANES data collections. Racial/ethnic differences have persisted and may have important implications for known health disparities. Current recommendations for vitamin D supplementation are inadequate to address the growing epidemic of vitamin D insufficiency.

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