To our knowledge, we report the first evaluation of temporal trends in vitamin D status in the US population. The NHANES data demonstrate a marked decrease in mean serum 25(OH)D levels from the 1988–1994 to the 2001–2004 collections and a corresponding increase in vitamin D insufficiency across all demographic strata. These findings have important implications for health disparities and public health.
During 2001 to 2004, only 23% of US adolescents and adults had serum 25(OH)D levels of 30 ng/mL or more, the minimum level that appears necessary for general health benefits.22,26
In particular, these higher 25(OH)D levels have been associated with reduced incidence and improved outcomes in cardiovascular disease,3–5
Furthermore, supplementation appears to mitigate the risk associated with vitamin D insufficiency.14–17
The current analysis describes a much higher prevalence (77% during NHANES 2001–2004) of vitamin D insufficiency in the US population than previously reported.31,32
Changes in population demographics were limited and do not appear to explain the observed increase in prevalence. In addition, NHANES preferentially avoided sampling in northern latitudes during the winter; thus, the true prevalence of vitamin D insufficiency in the US population is likely even higher than we have reported.
(cholecalciferol) is derived from skin exposure to UV-B rays and dietary intake (including supplements). Few foods contain vitamin D2
(ergocalciferol) or vitamin D3
and recommended doses of vitamin D supplementation have not changed significantly in the past 2 decades.18
Therefore,ingestion is,at most, a small component of the marked change in prevalence.
Because exposure to UV-B rays is the primary determinant of vitamin D status in humans, this is more likely the primary cause of the increasing prevalence of vitamin D insufficiency.29
Although widespread campaigns for sunscreen use and sun avoidance, including Healthy People 2010
, have reduced the incidence of skin cancers,34
sunscreen with a sun protection factor of 15 also decreases the synthesis of vitamin D3
Increased sunscreen use with a higher sun protection factor likely contributed to the reported trend of lower 25(OH)D levels. In addition, decreased outdoor activity and obesity have been associated with vitamin D insufficiency.30,36
The increased inactivity and obesity in the US population has likely contributed to the observed rise in vitamin D insufficiency.37
Previously published data suggested that vitamin D insufficiency was more prevalent among older adults (owing to reduction of 7-dehydrocholesterol levels in skin), women (owing to lower outdoor activity), and individuals with darker skin (owing to increased melanin).26,30,32
We found that previous differences by age and sex have equalized, but those by race/ethnicity have remained. The loss of age- and sex-related differences may be secondary to disproportionately greater time indoors (eg, with television, computers, and video games) and less time outdoors among younger compared with older individuals and males compared with females.
Although socioeconomic status, health care access, lifestyle, and cultural factors contribute to racial/ethnic disparities in disease incidence and outcomes,38
vitamin D insufficiency may provide a plausible biological explanation for health differences. In particular, black and, to a lesser extent, Hispanic Americans have a markedly higher prevalence of vitamin D insufficiency and higher incidence and worse outcomes for cardiovascular disease, certain cancers, diabetes mellitus, and renal disease, all of which have been linked to vitamin D insufficiency.22,26,38
Indeed, Wolf et al39
recently reported that, compared with white patients undergoing dialysis, mortality was 16% lower in black patients receiving vitamin D supplements but 35% higher in black patients not receiving supplements. Randomized trials of vitamin D supplementation in diverse cohorts of patients will help to further evaluate this hypothesis.
As recently suggested,22,26–28
our data provide additional evidence that current recommendations18
for vitamin D supplementation (200–600 IU/d) are inadequate to achieve optimal serum 25(OH)D levels in most of the US population. These recommendations were based on older prevalence data, which had a lower prevalence of suboptimal serum 25(OH)D levels, and were focused on skeletal outcomes, which target lower overall levels than are required for optimal general health.19–25
For instance, 400 or 800 IU/d would raise serum 25(OH)D levels by only approximately 4 or 9 ng/mL, respectively,40
which would be inadequate for many Americans based on the present analysis. Furthermore, current recommendations are stratified by age; however, our data suggest that current serum 25(OH)D levels are similar across the age spectrum.
Recommendations based on race/ethnicity (ie, higher levels of supplementation for blacks) would be more consistent with current population-based data. In addition, although not specifically evaluated in this analysis, recommendations by season and latitude (ie, higher supplementation in the winter and at higher latitudes) would likely be more effective.
This study has some potential limitations. First, methodologic explanations for the increased prevalence of vitamin D insufficiency should be considered. However, data were collected by the same group (the National Center for Health Statistics) using similar methods, a similar sampling technique, and the same 25(OH)D assay, which make this unlikely. In addition, nonresponse bias could have some effect on the results, although the survey weights provided by the National Center for Health Statistics attempt to control for this. We have based our serum 25(OH)D thresholds on previous outcome studies; no outcomes were assessed in this analysis. Serum was collected at only 1 point and preferentially collected in northern states in the summer and southern states in the winter. As a result, the data presented most likely represent the best-case scenario; random sampling across all seasons should yield an even higher prevalence of vitamin D insufficiency. Additional potential limitations are that the demographic information was self-reported and that other factors associated with vitamin D insufficiency were not analyzed (eg, season, latitude, estimated sunlight exposure, and diet). However, because determinants of vitamin D insufficiency have been well described, the purpose of our analysis was to examine the prevalence of vitamin D insufficiency in the US population.