Subjects and Methods
Healthy volunteers, aged 18 to 50 years, were recruited for endocrine research studies through mass mailings and advertisements in local newspapers and Internet sites.[15,16] Visits occurred between January 2006 and May 2008. Subjects were excluded if they had significant cardiac, hepatic, oncologic, psychiatric disease, or disorders known to affect bone mineral metabolism including hyperthyroidism, hyperparathyroidism, osteomalacia, or Paget's disease. Subjects using medications affecting either vitamin D or bone were also excluded. Multivitamin use and/or vitamin D supplementation up to 2,000 IU daily was allowed.
All blood samples were drawn in the nonfasting state, and measurements were made immediately on fresh samples. Serum 25(OH)D levels were measured using a chemiluminescent immunoassay (LIAISON, DiaSorin, Stillwater, Minnesota) with a detection limit of 2 ng/mL and intra-assay and interassay coefficients of variation of 6% to 8% and 12% to 16%, respectively. Plasma parathyroid hormone (PTH) was measured in a subset of subjects (n = 334) using a two-site radioimmunoassay (RIA) (Nichols Institute Diagnostics, San Clemente, California) with a detection limit of 1 pg/mL and intra-assay and interassay coefficients of variation of 2% to 3% and 6%, respectively. A subset of subjects (n = 406) was assessed for multivitamin use; any use of a multivitamin was coded as "yes". Study protocols were approved by the Human Research Committee of Partners HealthCare Systems (the responsible Institutional Review Board), and all subjects provided written informed consent.
Subjects were classified by sex, self-identified race (American Indian or Alaska Native, Asian, Black or African-American, Native Hawaiian or Other Pacific Islander, and White) and self-identified ethnicity (Hispanic or Latino or not Hispanic or Latino). The season of blood draw was classified as winter (January through March), spring (April through June), summer (July through September), or fall (October through December).
Data are expressed as mean ± standard deviation unless otherwise specified. Unpaired t-test or analysis of variance (ANOVA) was used, as appropriate, to evaluate differences in 25(OH)D levels. Pair-wise comparisons were assessed only if the overall ANOVA was statistically significant; Bonferroni correction for multiple comparisons was used. Two-way ANOVA was used to test for the interaction of season with race and with multivitamin use. Either c2 testing or Fisher exact test was used to compare proportions of subjects with low 25(OH)D levels. The Pearson correlation coefficient for PTH and 25(OH)D was determined. The subset of the cohort in which multivitamin use was assessed was used for regression modeling and was randomly divided into training (n = 305) and validation (n = 101) sets. Logistic regression modeling was used to determine if race, ethnicity, sex, multivitamin use, age, or season predicted the likelihood of a subject having a 25(OH)D level ≤20 (14) or ≤30 ng/mL. Only subjects with race defined as "white," "black," or "Asian" were included in the model. All variables with P<0.1 as well as additional variables considered clinically relevant were retained in the model. Goodness of fit was assessed with the Hosmer-Lemeshow test. Statistical analyses were performed using Stata 11 (College Station, Texas).
Endocr Pract. 2012;18(6):914-923. © 2012 American Association of Clinical Endocrinologists