Methods
Study population
The population of the present study consisted of 59 inmates incarcerated at the Fourth Avenue Maricopa County Jail. All male inmates between the ages of 20 and 41 y who were able to read and write in English were eligible for study inclusion. Because one of our a priori hypotheses was to compare circulating 25(OH)D concentrations of those who were newly incarcerated with those who had been at the Fourth Avenue Jail for at least 1 y, we employed two main procedures for study recruitment. For the newly incarcerated group (group 1), men within the eligible age range who presented for a health assessment within 2 mo of incarceration were approached for participation in the study by Maricopa County Correctional Health Services (MCCHS) staff. For those incarcerated >1 y (group 2), any eligible inmate who presented for, or requested, a medical assessment or examination was invited to join the study. When inmates from either group agreed to participate, the consent process was completed.
Ethics
This study was conducted according to the guidelines laid down in the Declaration of Helsinki and all procedures involving human subjects were approved by the University of Arizona Institutional Review Board (Protocol 13-0439) and the MCCHS. Written informed consent was obtained from all participants. Because the proposed study was conducted within a population categorized as vulnerable by the Code of Federal Regulations, we followed all additional procedures required by the University of Arizona Human Subjects Protection Program. We addressed each of criteria outlined in 45 CFR §46.305, and thereafter the study and all related procedures were approved by the university's Institutional Review Board (Protocol 13-0439) and the MCCHS.
Data collection and study procedures
After the informed consent process was completed, including obtaining written informed consent from all participants, data regarding age, race, height and weight, and time of incarceration were collected from each study participant. Additionally, a single blood draw of 10 cc was obtained from each inmate. These samples were collected at the initial health assessment for newly incarcerated inmates, and at the health examination of those inmates who had been in the jail for >1 y. Samples were immediately shipped for analysis of 25(OH)D using the methods described here.
Analysis of vitamin D metabolite levels
Deidentified serum samples from study participants were shipped to Bio-Reference Laboratories, Incorporated (Elmwood Park, NJ, USA) for analysis of 25(OH)D concentrations using a chemiluminescent immunoassay. Bio-Reference Laboratories is a standard commercial laboratory and employs several quality assurance and quality control measures to monitor analytical precision and to identify possible laboratory shifts over time. The laboratory currently holds licenses for testing in New York City, New Jersey, and New York state, as well as an interstate license issued by the U.S. Health Care Financing Administration (HCFA). The laboratory did not retain any data related to this study, and all analyses were conducted in a blinded fashion.
Statistical analyses
Descriptive data for study population characteristics overall and in groups 1 and 2 were calculated with means and SDs for the continuous variables and frequencies and percentages for the categorical variables. For comparisons of participant characteristics and concentrations of 25(OH)D by group, a Student's t test, with log-transformation of 25(OH)D levels, was used; χ2 analyses were employed to compare categorical variables by group. Analysis of variance was used for statistical tests of differences in 25(OH)D concentrations by population characteristics with more than two categories. Additionally, we categorized study participants using the criteria for vitamin D deficiency and inadequacy established by the Endocrine Society [23]. Deficiency was defined as circulating 25(OH)D concentrations 20 and