Original articleSerum 25-hydroxyvitamin D status of adolescents and adults in two seasonal subpopulations from NHANES III
Introduction
Interest in vitamin D status in the U.S. has intensified lately due to the recognition that, despite the fortification of milk with vitamin D in this country, subclinical vitamin D deficiency may be common in certain population subgroups such as medical inpatients or homebound elderly persons.8, 35 Hypovitaminosis D status has been associated with a number of negative skeletal consequences, including secondary hyperparathyroidism, and increased bone turnover, rates of bone loss, and fracture risk.4, 8, 19, 23, 35 Although there have been many community-based studies of vitamin D status in the U.S.,26 data from a representative sample of the population have not been previously available. Furthermore, data on some groups, such as adolescents or individuals living in the southern U.S., are limited.
The inclusion of serum 25-hydroxyvitamin D (25-OHD) levels in the most recently completed National Health and Nutrition Examination Survey, or NHANES III (1988–1994), provides the opportunity to examine vitamin D status in a representative sample of the noninstitutionalized U.S. population for the first time. However, the NHANES III survey design contains an inherent season-latitude structure that affects interpretation of serum 25-OHD levels, because they vary by these factors.37 Specifically, because physical exams, including phlebotomy, in NHANES III were conducted in mobile vans that can be adversely affected by weather, data had to be collected in northern latitudes in summer and in southern latitudes in winter. In the present study, we stratified the representative sample of U.S. adolescents and adults from NHANES III into two seasonal subpopulations to assess vitamin D status while addressing the season-latitude structure of the survey design. Specifically, we examined the prevalence of vitamin D deficiency and insufficiency in the representative sample of adolescents and adults: (a) whose blood was collected during the winter at median latitude 32°N (range 25°–41°N); and (b) whose blood was collected during the summer at median latitude 39°N (range 25°–47°N). These groups may be at lower risk of inadequate vitamin D status due to potentially greater ultraviolet radiation levels present at lower latitudes in winter or during summer months at higher latitudes. In addition, we compared the vitamin D status of two minority groups in the U.S., namely non-Hispanic blacks and Mexican Americans, with that of non-Hispanic whites.
Section snippets
Subjects
NHANES III is the most recently completed survey in a series conducted periodically by the National Center for Health Statistics (NCHS), Centers for Disease Control and Prevention (CDC), to assess the health and nutrition of a large representative sample of the noninstitutionalized, civilian U.S. population. Data were collected via household interviews and direct standardized physical examinations were conducted in specially equipped mobile examination centers.29 NHANES III was designed to
Results
Mean 25-OHD values and prevalence rates of vitamin D insufficiency in the U.S. population stratified by season are shown in Table 1. Less than 1% of the winter/lower latitude subpopulation had serum 25-OHD levels indicative of vitamin D deficiency (e.g., <17.5 nmol/L) (data not shown). However, prevalence rates of values associated with vitamin D insufficiency were more common (Table 1); that is, these prevalence rates ranged from approximately 1%–5% with values <25 nmol/L to 25%–57% with
Discussion
We found little evidence of vitamin D deficiency (e.g., serum 25-OHD <17.5 nmol/L) in the two seasonal subpopulations of noninstitutionalized U.S. adolescents and adults examined in the present study. However, vitamin D insufficiency was common, especially when defined as serum 25-OHD <37.5–<62.5 nmol/L — despite the presumably more favorable UV radiation levels available at lower latitudes in winter or at higher latitudes during summer months. In particular, vitamin D insufficiency was common
Acknowledgements
The authors acknowledge Donna J. LaVoie, (MT)ASCP, and Della B. Twite, (MT)ASCP, for their outstanding efforts in performing the NHANES III vitamin D assays.
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