A comparison of the observed serum 25OHD concentrations between NHANES III and NHANES 2000–2004 suggests that a decline in measured vitamin D status may have occurred in the population over the past 10–15 years, with age-standardized mean serum 25OHD values observed in NHANES 2000–2004 being roughly 5–20 nmol/L lower than those seen in NHANES III (1988–94) for persons ages 12 years and older. Accounting for confounding from assay differences reduced the difference in serum 25OHD between NHANES III and NHANES 2000–2004 by approximately 10 nmol/L. Thus, most of observed difference in serum 25OHD between NHANES III and NHANES 2000–2004 appears to be an artifact of assay changes rather than an actual decline in serum 25OHD concentrations. However, the remaining difference appears to represent a true decline in vitamin D status of the population since NHANES III.
Our analyses to identify potential biological and behavioral factors that contributed to the actual decline in serum 25OHD values between surveys suggest that changes in BMI, milk intake and sun protection may have played a role, at least in the subgroup of non-Hispanic white adults for whom the analyses were conducted. We focused on these factors because they are related to serum 25OHD and they appeared to have changed in a direction that is consistent with a decline in serum 25OH in this population group. These findings may be relevant for subsequent discussions regarding approaches to address the decline in serum 25OHD. In addition, finding that changes in relevant biological and behavioral factors in the population appear to explain some portion of the serum 25OHD difference that remained between surveys after accounting for confounding factors supports the likelihood that the remaining serum 25OHD difference is real rather than being due to other, uncontrolled confounding factors. More research is needed to examine the relative effects of confounding vs. behavioral and biological factors in other population subgroups. Such analyses were precluded in the present study due to lack of data for some of the relevant factors in other population groups and the need to account for differences between surveys in the race/ethnic composition by season.
The impact of using different assays to measure serum 25OHD has been described previously. Binkley et al (16
) found mean serum 25OHD values varied as much as two-fold when different assay types were used to measure the same set of blood samples. Variations in results from the same method when used in different laboratories have also been described (17
). Our results suggest that changes of about 12% in the same assay over time can also impact serum 25OHD concentrations even when performed in the same laboratory. Standard reference materials (SRMs) for serum 25OHD are currently being developed by the National Institute of Standards and Technology (18
), which should improve agreement between assay methods. It is important to note that the assay adjustment used in the present study was based on the current version of the RIA assay because the NHANES III-era assay version could not be reconstructed. However, without a standard reference material for serum 25OHD, it is not clear which assay version is the best in terms of assessing nutritional status.
In addition to examining trends in serum 25OHD, we looked at serum 25OHD concentrations in the current NHANES by selected demographic characteristics. These data fill an important gap for some groups in whom data have previously been scanty, in particular children, adolescents and pregnant women (19
). In the present study, mean serum 25OHD was highest in children ages 1–
5 years, intermediate in children ages 6–
11 years, and lowest in adolescents ages 12–
19. Weng et al. (21
) found a similar pattern by age in their sample of apparently healthy children and adolescents. Pregnant women had higher serum 25OHD than nonpregnant women of the same age in NHANES 2000–
2004. Recent community-based studies of vitamin D status in pregnant women in the US have reported that low vitamin D status is common in this group, but they have not included comparisons with non-pregnant women (20
It is important to note that statistical adjustments to serum 25OHD data for assay differences or biological and behavior factors that changed between NHANES III and NHANES 2000–
2004 were made in the present study only. The publicly-released serum 25OHD data for NHANES III and NHANES 2000–
2004 available on the NHANES website (www.cdc.gov/nchs/nhanes.htm
) are the observed, unadjusted values. We recommend that researchers who use the publicly-released data to make comparisons between surveys should consider the confounding effects of assay differences and changes in population demographics in their analyses.
This study has limitations. The analyses to assess the relative contribution of confounding factors versus changes in vitamin-D-related biological and behavioral risk factors in the population were limited to adults age 20–59 years due to lack of sun protection data in other age groups. These analyses were further limited to non-Hispanic whites to avoid confounding due to differences in the race/ethnic composition of the sample by season between surveys. A correction factor was needed to account for a shift in the 25OHD assay quality control pools that occurred while the assay comparison study was being conducted. The impact of the biological and behavior factors was also indirectly estimated using regression to predict mean values, so that results depend on the robustness of the underlying models. The models to assess changes in vitamin-D-related factors in the population were limited to factors for which nationally representative data were available, so some potentially important factors could not be considered. For example, dietary intake of vitamin D was only represented indirectly by the milk intake variable in the model because direct estimates of dietary vitamin D intake from food are not available from NHANES 2000–2004. The fact that some of the observed difference in serum 25OHD between NHANES III and NHANES 2003–2004 in men was not explained by our models suggests that additional variables may have played a role.
Other study limitations include the potential nonresponse bias in both NHANES datasets, since not all those who were selected to participate in the survey did so. Nonresponse bias in NHANES is reduced by a nonresponse adjustment factor included in the calculation of the sample weights. However, about 5–10% of those who came to the mobile exam centers did not have serum 25OHD data in the two surveys, and this nonresponse is not addressed by the sample weight adjustments. Finally, some important at-risk groups, such as institutionalized persons and people living in the northern US during the winter, were not included in the NHANES sampling frame by design.
In summary, age standardized mean serum 25OHD concentrations were significantly lower in 2000–2004 than in 1988–1994 in all groups examined when based on observed values. Accounting for assay changes noticeably reduced the difference between surveys. However, mean serum 25OHD remained significantly lower in males (except Mexican Americans) in NHANES 2000–2004 than in NHANES III even after adjusting for assay differences. This remaining difference likely represents a real decline in vitamin D status. Changes in BMI, milk intake and sun protection appeared to contribute to this decline in a subgroup of non-Hispanic white adults. The possibility that trends in overweight, sun protection, and milk intake may continue supports the need to continue monitoring the serum 25OHD status of the population.