The study evaluates trends in vitamin D insufficiency in the US population from 1988–1994 (NHANES III) to 2001–2004 (NHANES 2001–2004). The mean serum 25-hydroxyvitamin D (25[OH]D) level decreased from 30 ng/mL to 24 ng/mL, with a significant increase in the prevalence of levels below 10 ng/mL (from 2% to 6%) and a corresponding decrease in levels of 30 ng/mL or more (from 45% to 23%). Racial/ethnic disparities persisted, with non-Hispanic blacks showing a sharp rise in insufficiency (from 9% to 29%) and a dramatic drop in levels of 30 ng/mL or more (from 12% to 3%). Age and sex disparities largely equalized, but racial/ethnic differences remained. Current vitamin D supplementation recommendations are considered inadequate to address the growing epidemic of insufficiency. Factors contributing to the decline include reduced sunlight exposure, increased sunscreen use, decreased outdoor activity, and obesity. The study highlights the need for higher vitamin D intake, particularly in winter and at higher latitudes, to improve public health outcomes. The findings underscore the importance of addressing racial/ethnic disparities in vitamin D status to reduce health inequities.The study evaluates trends in vitamin D insufficiency in the US population from 1988–1994 (NHANES III) to 2001–2004 (NHANES 2001–2004). The mean serum 25-hydroxyvitamin D (25[OH]D) level decreased from 30 ng/mL to 24 ng/mL, with a significant increase in the prevalence of levels below 10 ng/mL (from 2% to 6%) and a corresponding decrease in levels of 30 ng/mL or more (from 45% to 23%). Racial/ethnic disparities persisted, with non-Hispanic blacks showing a sharp rise in insufficiency (from 9% to 29%) and a dramatic drop in levels of 30 ng/mL or more (from 12% to 3%). Age and sex disparities largely equalized, but racial/ethnic differences remained. Current vitamin D supplementation recommendations are considered inadequate to address the growing epidemic of insufficiency. Factors contributing to the decline include reduced sunlight exposure, increased sunscreen use, decreased outdoor activity, and obesity. The study highlights the need for higher vitamin D intake, particularly in winter and at higher latitudes, to improve public health outcomes. The findings underscore the importance of addressing racial/ethnic disparities in vitamin D status to reduce health inequities.