The investigators hypothesize that patients receiving efavirenz will be more likely to have lower 25-hydroxyvitamin D and 1,25 dihydroxyvitamin D3levels based on the fact that efavirenz is an inducer of CYP3A4 and CYP24 enzymes that degrade 25-hydroxyvitamin D and 1,25 dihydroxyvitamin D3, respectively, to inactive metabolites. The investigators speculate that patients on a tenofovir-containing regimen will be more likely to have progression of bone density loss compared to those in the non-tenofovir-containing regimen.