Serum 25-hydroxyvitamin D status of the US population: 1988-1994 compared with 2000-2004

Abstract

Background: Changes in serum 25-hydroxyvitamin D [25(OH)D] concentrations in the US population have not been described.

Objective: We used data from the National Health and Nutrition Examination Surveys (NHANES) to compare serum 25(OH)D concentrations in the US population in 2000-2004 with those in 1988-1994 and to identify contributing factors.

Design: Serum 25(OH)D was measured with a radioimmunoassay kit in 20 289 participants in NHANES 2000-2004 and in 18 158 participants in NHANES III (1988-1994). Body mass index (BMI) was calculated from measured height and weight. Milk intake and sun protection were assessed by questionnaire. Assay differences were assessed by re-analyzing 150 stored serum specimens from NHANES III with the current assay.

Results: Age-adjusted mean serum 25(OH)D concentrations were 5-20 nmol/L lower in NHANES 2000-2004 than in NHANES III. After adjustment for assay shifts, age-adjusted means in NHANES 2000-2004 remained significantly lower (by 5-9 nmol/L) in most males, but not in most females. In a study subsample, adjustment for the confounding effects of assay differences changed mean serum 25(OH)D concentrations by approximately 10 nmol/L, and adjustment for changes in the factors likely related to real changes in vitamin D status (ie, BMI, milk intake, and sun protection) changed mean serum 25(OH)D concentrations by 1-1.6 nmol/L.

Conclusions: Overall, mean serum 25(OH)D was lower in 2000-2004 than 1988-1994. Assay changes unrelated to changes in vitamin D status accounted for much of the difference in most population groups. In an adult subgroup, combined changes in BMI, milk intake, and sun protection appeared to contribute to a real decline in vitamin D status.

Figure 1

Age-standardized mean serum 25OHD by sex and season of blood collection among persons age 12 years and older: NHANES III (as originally assayed and as predicted if current assay used) versus NHANES 2000–2004 NHW = Non-Hispanic white NHB = Non-Hispanic black MA = Mexican * P < 0.05 comparing NHANES III to NHANES 2000-2004 based on t-tests **Comparison for November-March limited to Non-Hispanic whites only due to significant different in proportion of nonwhites between NHANES III and NHANES 2000-2004 for the November-March sample.

Figure 1

Age-standardized mean serum 25OHD by sex and season of blood collection among persons age 12 years and older: NHANES III (as originally assayed and as predicted if current assay used) versus NHANES 2000–2004 NHW = Non-Hispanic white NHB = Non-Hispanic black MA = Mexican * P < 0.05 comparing NHANES III to NHANES 2000-2004 based on t-tests **Comparison for November-March limited to Non-Hispanic whites only due to significant different in proportion of nonwhites between NHANES III and NHANES 2000-2004 for the November-March sample.

Figure 2

Mean age- and sex-adjusted serum 25OHD by selected variables for non-Hispanic whites ages 20–59 years: NHANES 2003–2004 P values from overall F test for variable from linear regression.

Figure 3

Observed and predicted age-standardized mean serum 25OHD by sex among non-Hispanic whites age 20–59 years examined in April-October: NHANES III (1988–1994) versus NHANES 2003–2004 * Calculated as NHANES III mean minus NHANES 2003-04 mean ** Mean predicted for NHANES III 1988-1994 if current RIA was used. *** Mean predicted for NHANES III 1988-1994 assuming current RIA was used and mean predicted for NHANES 2003-2004 if mean BMI and milk consumption rates from NHANES III and sun protective behavior rates from 1992 NHIS applied.

Figure 3

Observed and predicted age-standardized mean serum 25OHD by sex among non-Hispanic whites age 20–59 years examined in April-October: NHANES III (1988–1994) versus NHANES 2003–2004 * Calculated as NHANES III mean minus NHANES 2003-04 mean ** Mean predicted for NHANES III 1988-1994 if current RIA was used. *** Mean predicted for NHANES III 1988-1994 assuming current RIA was used and mean predicted for NHANES 2003-2004 if mean BMI and milk consumption rates from NHANES III and sun protective behavior rates from 1992 NHIS applied.

Figure 3

Observed and predicted age-standardized mean serum 25OHD by sex among non-Hispanic whites age 20–59 years examined in April-October: NHANES III (1988–1994) versus NHANES 2003–2004 * Calculated as NHANES III mean minus NHANES 2003-04 mean ** Mean predicted for NHANES III 1988-1994 if current RIA was used. *** Mean predicted for NHANES III 1988-1994 assuming current RIA was used and mean predicted for NHANES 2003-2004 if mean BMI and milk consumption rates from NHANES III and sun protective behavior rates from 1992 NHIS applied.